[v9,1/3] drm: Add writeback connector type

Message ID 20180524164103.8378-2-Liviu.Dudau@arm.com
State New
Headers show
Series
  • drm: Introduce writeback connectors
Related show

Commit Message

Liviu Dudau May 24, 2018, 4:41 p.m.
From: Brian Starkey <brian.starkey@arm.com>

Writeback connectors represent writeback engines which can write the
CRTC output to a memory framebuffer. Add a writeback connector type and
related support functions.

Drivers should initialize a writeback connector with
drm_writeback_connector_init() which takes care of setting up all the
writeback-specific details on top of the normal functionality of
drm_connector_init().

Writeback connectors have a WRITEBACK_FB_ID property, used to set the
output framebuffer, and a WRITEBACK_PIXEL_FORMATS blob used to expose the
supported writeback formats to userspace.

When a framebuffer is attached to a writeback connector with the
WRITEBACK_FB_ID property, it is used only once (for the commit in which
it was included), and userspace can never read back the value of
WRITEBACK_FB_ID. WRITEBACK_FB_ID can only be set if the connector is
attached to a CRTC.

Changes since v1:
 - Added drm_writeback.c + documentation
 - Added helper to initialize writeback connector in one go
 - Added core checks
 - Squashed into a single commit
 - Dropped the client cap
 - Writeback framebuffers are no longer persistent

Changes since v2:
 Daniel Vetter:
 - Subclass drm_connector to drm_writeback_connector
 - Relax check to allow CRTC to be set without an FB
 - Add some writeback_ prefixes
 - Drop PIXEL_FORMATS_SIZE property, as it was unnecessary
 Gustavo Padovan:
 - Add drm_writeback_job to handle writeback signalling centrally

Changes since v3:
 - Rebased
 - Rename PIXEL_FORMATS -> WRITEBACK_PIXEL_FORMATS

Chances since v4:
 - Embed a drm_encoder inside the drm_writeback_connector to
   reduce the amount of boilerplate code required from the drivers
   that are using it.

Changes since v5:
 - Added Rob Clark's atomic_commit() vfunc to connector helper
   funcs, so that writeback jobs are commited from atomic helpers
 - Updated create_writeback_properties() signature to return an
   error code rather than a boolean false for failure.
 - Free writeback job with the connector state rather than when
   doing the cleanup_work()

Changes since v7:
 - fix extraneous use of out_fence that is only introduced in a
   subsequent patch.

Changes since v8:
 - whitespace changes pull from subsequent patch

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Brian Starkey <brian.starkey@arm.com>
[rebased and fixed conflicts]
Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
[rebased and added atomic_commit() vfunc for writeback jobs]
Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
---
 Documentation/gpu/drm-kms.rst            |   9 +
 drivers/gpu/drm/Makefile                 |   2 +-
 drivers/gpu/drm/drm_atomic.c             | 128 ++++++++++++
 drivers/gpu/drm/drm_atomic_helper.c      |  30 +++
 drivers/gpu/drm/drm_connector.c          |   4 +-
 drivers/gpu/drm/drm_writeback.c          | 255 +++++++++++++++++++++++
 include/drm/drm_atomic.h                 |   3 +
 include/drm/drm_connector.h              |  13 ++
 include/drm/drm_mode_config.h            |  15 ++
 include/drm/drm_modeset_helper_vtables.h |  11 +
 include/drm/drm_writeback.h              |  90 ++++++++
 include/uapi/drm/drm_mode.h              |   1 +
 12 files changed, 559 insertions(+), 2 deletions(-)
 create mode 100644 drivers/gpu/drm/drm_writeback.c
 create mode 100644 include/drm/drm_writeback.h

Comments

Liviu Dudau June 7, 2018, 9:26 a.m. | #1
On Fri, May 25, 2018 at 04:32:38PM +0100, Brian Starkey wrote:
> On Thu, May 24, 2018 at 05:41:01PM +0100, Liviu Dudau wrote:
> > From: Brian Starkey <brian.starkey@arm.com>
> > 
> > Writeback connectors represent writeback engines which can write the
> > CRTC output to a memory framebuffer. Add a writeback connector type and
> > related support functions.
> > 
> > Drivers should initialize a writeback connector with
> > drm_writeback_connector_init() which takes care of setting up all the
> > writeback-specific details on top of the normal functionality of
> > drm_connector_init().
> > 
> > Writeback connectors have a WRITEBACK_FB_ID property, used to set the
> > output framebuffer, and a WRITEBACK_PIXEL_FORMATS blob used to expose the
> > supported writeback formats to userspace.
> > 
> > When a framebuffer is attached to a writeback connector with the
> > WRITEBACK_FB_ID property, it is used only once (for the commit in which
> > it was included), and userspace can never read back the value of
> > WRITEBACK_FB_ID. WRITEBACK_FB_ID can only be set if the connector is
> > attached to a CRTC.
> > 
> > Changes since v1:
> > - Added drm_writeback.c + documentation
> > - Added helper to initialize writeback connector in one go
> > - Added core checks
> > - Squashed into a single commit
> > - Dropped the client cap
> > - Writeback framebuffers are no longer persistent
> > 
> > Changes since v2:
> > Daniel Vetter:
> > - Subclass drm_connector to drm_writeback_connector
> > - Relax check to allow CRTC to be set without an FB
> > - Add some writeback_ prefixes
> > - Drop PIXEL_FORMATS_SIZE property, as it was unnecessary
> > Gustavo Padovan:
> > - Add drm_writeback_job to handle writeback signalling centrally
> > 
> > Changes since v3:
> > - Rebased
> > - Rename PIXEL_FORMATS -> WRITEBACK_PIXEL_FORMATS
> > 
> > Chances since v4:
> > - Embed a drm_encoder inside the drm_writeback_connector to
> >   reduce the amount of boilerplate code required from the drivers
> >   that are using it.
> > 
> > Changes since v5:
> > - Added Rob Clark's atomic_commit() vfunc to connector helper
> >   funcs, so that writeback jobs are commited from atomic helpers
> > - Updated create_writeback_properties() signature to return an
> >   error code rather than a boolean false for failure.
> > - Free writeback job with the connector state rather than when
> >   doing the cleanup_work()
> > 
> > Changes since v7:
> > - fix extraneous use of out_fence that is only introduced in a
> >   subsequent patch.
> > 
> > Changes since v8:
> > - whitespace changes pull from subsequent patch
> > 
> > Reviewed-by: Eric Anholt <eric@anholt.net>
> > Signed-off-by: Brian Starkey <brian.starkey@arm.com>
> > [rebased and fixed conflicts]
> > Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
> > [rebased and added atomic_commit() vfunc for writeback jobs]
> > Signed-off-by: Rob Clark <robdclark@gmail.com>
> > Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
> > ---
> > Documentation/gpu/drm-kms.rst            |   9 +
> > drivers/gpu/drm/Makefile                 |   2 +-
> > drivers/gpu/drm/drm_atomic.c             | 128 ++++++++++++
> > drivers/gpu/drm/drm_atomic_helper.c      |  30 +++
> > drivers/gpu/drm/drm_connector.c          |   4 +-
> > drivers/gpu/drm/drm_writeback.c          | 255 +++++++++++++++++++++++
> > include/drm/drm_atomic.h                 |   3 +
> > include/drm/drm_connector.h              |  13 ++
> > include/drm/drm_mode_config.h            |  15 ++
> > include/drm/drm_modeset_helper_vtables.h |  11 +
> > include/drm/drm_writeback.h              |  90 ++++++++
> > include/uapi/drm/drm_mode.h              |   1 +
> > 12 files changed, 559 insertions(+), 2 deletions(-)
> > create mode 100644 drivers/gpu/drm/drm_writeback.c
> > create mode 100644 include/drm/drm_writeback.h
> > 
> > diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
> > index 1dffd1ac4cd44..809d403087f95 100644
> > --- a/Documentation/gpu/drm-kms.rst
> > +++ b/Documentation/gpu/drm-kms.rst
> > @@ -373,6 +373,15 @@ Connector Functions Reference
> > .. kernel-doc:: drivers/gpu/drm/drm_connector.c
> >    :export:
> > 
> > +Writeback Connectors
> > +--------------------
> > +
> > +.. kernel-doc:: drivers/gpu/drm/drm_writeback.c
> > +  :doc: overview
> > +
> > +.. kernel-doc:: drivers/gpu/drm/drm_writeback.c
> > +  :export:
> > +
> > Encoder Abstraction
> > ===================
> > 
> > diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> > index ef9f3dab287fd..69c13517ea3a6 100644
> > --- a/drivers/gpu/drm/Makefile
> > +++ b/drivers/gpu/drm/Makefile
> > @@ -18,7 +18,7 @@ drm-y       :=	drm_auth.o drm_bufs.o drm_cache.o \
> > 		drm_encoder.o drm_mode_object.o drm_property.o \
> > 		drm_plane.o drm_color_mgmt.o drm_print.o \
> > 		drm_dumb_buffers.o drm_mode_config.o drm_vblank.o \
> > -		drm_syncobj.o drm_lease.o
> > +		drm_syncobj.o drm_lease.o drm_writeback.o
> > 
> > drm-$(CONFIG_DRM_LIB_RANDOM) += lib/drm_random.o
> > drm-$(CONFIG_DRM_VM) += drm_vm.o
> > diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> > index 895741e9cd7db..e32a93931c0dc 100644
> > --- a/drivers/gpu/drm/drm_atomic.c
> > +++ b/drivers/gpu/drm/drm_atomic.c
> > @@ -30,6 +30,7 @@
> > #include <drm/drm_atomic.h>
> > #include <drm/drm_mode.h>
> > #include <drm/drm_print.h>
> > +#include <drm/drm_writeback.h>
> > #include <linux/sync_file.h>
> > 
> > #include "drm_crtc_internal.h"
> > @@ -676,6 +677,45 @@ static void drm_atomic_crtc_print_state(struct drm_printer *p,
> > 		crtc->funcs->atomic_print_state(p, state);
> > }
> > 
> > +/**
> > + * drm_atomic_connector_check - check connector state
> > + * @connector: connector to check
> > + * @state: connector state to check
> > + *
> > + * Provides core sanity checks for connector state.
> > + *
> > + * RETURNS:
> > + * Zero on success, error code on failure
> > + */
> > +static int drm_atomic_connector_check(struct drm_connector *connector,
> > +		struct drm_connector_state *state)
> > +{
> > +	struct drm_crtc_state *crtc_state;
> > +	struct drm_writeback_job *writeback_job = state->writeback_job;
> > +
> > +	if ((connector->connector_type != DRM_MODE_CONNECTOR_WRITEBACK) || !writeback_job)
> > +		return 0;
> > +
> > +	if (writeback_job->fb && !state->crtc) {
> > +		DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] framebuffer without CRTC\n",
> > +				 connector->base.id, connector->name);
> > +		return -EINVAL;
> > +	}
> > +
> > +	if (state->crtc)
> > +		crtc_state = drm_atomic_get_existing_crtc_state(state->state,
> > +								state->crtc);
> > +
> > +	if (writeback_job->fb && !crtc_state->active) {
> > +		DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] has framebuffer, but [CRTC:%d] is off\n",
> > +				 connector->base.id, connector->name,
> > +				 state->crtc->base.id);
> > +		return -EINVAL;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > /**
> >  * drm_atomic_get_plane_state - get plane state
> >  * @state: global atomic state object
> > @@ -1286,6 +1326,12 @@ static int drm_atomic_connector_set_property(struct drm_connector *connector,
> > 			return -EINVAL;
> > 		}
> > 		state->content_protection = val;
> > +	} else if (property == config->writeback_fb_id_property) {
> > +		struct drm_framebuffer *fb = drm_framebuffer_lookup(dev, NULL, val);
> > +		int ret = drm_atomic_set_writeback_fb_for_connector(state, fb);
> > +		if (fb)
> > +			drm_framebuffer_unreference(fb);
> > +		return ret;
> > 	} else if (connector->funcs->atomic_set_property) {
> > 		return connector->funcs->atomic_set_property(connector,
> > 				state, property, val);
> > @@ -1367,6 +1413,9 @@ drm_atomic_connector_get_property(struct drm_connector *connector,
> > 		*val = state->scaling_mode;
> > 	} else if (property == connector->content_protection_property) {
> > 		*val = state->content_protection;
> > +	} else if (property == config->writeback_fb_id_property) {
> > +		/* Writeback framebuffer is one-shot, write and forget */
> > +		*val = 0;
> > 	} else if (connector->funcs->atomic_get_property) {
> > 		return connector->funcs->atomic_get_property(connector,
> > 				state, property, val);
> > @@ -1584,6 +1633,74 @@ drm_atomic_set_crtc_for_connector(struct drm_connector_state *conn_state,
> > }
> > EXPORT_SYMBOL(drm_atomic_set_crtc_for_connector);
> > 
> > +/*
> > + * drm_atomic_get_writeback_job - return or allocate a writeback job
> > + * @conn_state: Connector state to get the job for
> > + *
> > + * Writeback jobs have a different lifetime to the atomic state they are
> > + * associated with. This convenience function takes care of allocating a job
> > + * if there isn't yet one associated with the connector state, otherwise
> > + * it just returns the existing job.
> > + *
> > + * Returns: The writeback job for the given connector state
> > + */
> > +static struct drm_writeback_job *
> > +drm_atomic_get_writeback_job(struct drm_connector_state *conn_state)
> > +{
> > +	WARN_ON(conn_state->connector->connector_type != DRM_MODE_CONNECTOR_WRITEBACK);
> > +
> > +	if (!conn_state->writeback_job)
> > +		conn_state->writeback_job =
> > +			kzalloc(sizeof(*conn_state->writeback_job), GFP_KERNEL);
> > +
> > +	return conn_state->writeback_job;
> > +}
> > +
> > +/**
> > + * drm_atomic_set_writeback_fb_for_connector - set writeback framebuffer
> > + * @conn_state: atomic state object for the connector
> > + * @fb: fb to use for the connector
> > + *
> > + * This is used to set the framebuffer for a writeback connector, which outputs
> > + * to a buffer instead of an actual physical connector.
> > + * Changing the assigned framebuffer requires us to grab a reference to the new
> > + * fb and drop the reference to the old fb, if there is one. This function
> > + * takes care of all these details besides updating the pointer in the
> > + * state object itself.
> > + *
> > + * Note: The only way conn_state can already have an fb set is if the commit
> > + * sets the property more than once.
> > + *
> > + * See also: drm_writeback_connector_init()
> > + *
> > + * Returns: 0 on success
> > + */
> > +int drm_atomic_set_writeback_fb_for_connector(
> > +		struct drm_connector_state *conn_state,
> > +		struct drm_framebuffer *fb)
> > +{
> > +	struct drm_writeback_job *job =
> > +		drm_atomic_get_writeback_job(conn_state);
> > +	if (!job)
> > +		return -ENOMEM;
> > +
> > +	if (job->fb)
> > +		drm_framebuffer_unreference(job->fb);
> > +	if (fb)
> > +		drm_framebuffer_reference(fb);
> > +	job->fb = fb;
> > +
> > +	if (fb)
> > +		DRM_DEBUG_ATOMIC("Set [FB:%d] for connector state %p\n",
> > +				 fb->base.id, conn_state);
> > +	else
> > +		DRM_DEBUG_ATOMIC("Set [NOFB] for connector state %p\n",
> > +				 conn_state);
> > +
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL(drm_atomic_set_writeback_fb_for_connector);
> > +
> > /**
> >  * drm_atomic_add_affected_connectors - add connectors for crtc
> >  * @state: atomic state
> > @@ -1702,6 +1819,8 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
> > 	struct drm_plane_state *plane_state;
> > 	struct drm_crtc *crtc;
> > 	struct drm_crtc_state *crtc_state;
> > +	struct drm_connector *conn;
> > +	struct drm_connector_state *conn_state;
> > 	int i, ret = 0;
> > 
> > 	DRM_DEBUG_ATOMIC("checking %p\n", state);
> > @@ -1724,6 +1843,15 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
> > 		}
> > 	}
> > 
> > +	for_each_new_connector_in_state(state, conn, conn_state, i) {
> > +		ret = drm_atomic_connector_check(conn, conn_state);
> > +		if (ret) {
> > +			DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] atomic core check failed\n",
> > +					 conn->base.id, conn->name);
> > +			return ret;
> > +		}
> > +	}
> > +
> > 	if (config->funcs->atomic_check) {
> > 		ret = config->funcs->atomic_check(state->dev, state);
> > 
> > diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> > index 130da5195f3b6..5fd12666f7d7a 100644
> > --- a/drivers/gpu/drm/drm_atomic_helper.c
> > +++ b/drivers/gpu/drm/drm_atomic_helper.c
> > @@ -30,6 +30,7 @@
> > #include <drm/drm_plane_helper.h>
> > #include <drm/drm_crtc_helper.h>
> > #include <drm/drm_atomic_helper.h>
> > +#include <drm/drm_writeback.h>
> > #include <linux/dma-fence.h>
> > 
> > #include "drm_crtc_helper_internal.h"
> > @@ -1172,6 +1173,25 @@ void drm_atomic_helper_commit_modeset_disables(struct drm_device *dev,
> > }
> > EXPORT_SYMBOL(drm_atomic_helper_commit_modeset_disables);
> > 
> > +static void drm_atomic_helper_commit_writebacks(struct drm_device *dev,
> > +						struct drm_atomic_state *old_state)
> > +{
> > +	struct drm_connector *connector;
> > +	struct drm_connector_state *new_conn_state;
> > +	int i;
> > +
> > +	for_each_new_connector_in_state(old_state, connector, new_conn_state, i) {
> > +		const struct drm_connector_helper_funcs *funcs;
> > +
> > +		funcs = connector->helper_private;
> > +
> > +		if (new_conn_state->writeback_job && new_conn_state->writeback_job->fb) {
> > +			WARN_ON(connector->connector_type != DRM_MODE_CONNECTOR_WRITEBACK);
> > +			funcs->atomic_commit(connector, new_conn_state->writeback_job);
> > +		}
> > +	}
> > +}
> > +
> > /**
> >  * drm_atomic_helper_commit_modeset_enables - modeset commit to enable outputs
> >  * @dev: DRM device
> > @@ -1251,6 +1271,8 @@ void drm_atomic_helper_commit_modeset_enables(struct drm_device *dev,
> > 
> > 		drm_bridge_enable(encoder->bridge);
> > 	}
> > +
> > +	drm_atomic_helper_commit_writebacks(dev, old_state);
> > }
> > EXPORT_SYMBOL(drm_atomic_helper_commit_modeset_enables);
> > 
> > @@ -3660,6 +3682,9 @@ __drm_atomic_helper_connector_duplicate_state(struct drm_connector *connector,
> > 	if (state->crtc)
> > 		drm_connector_get(connector);
> > 	state->commit = NULL;
> > +
> > +	/* Don't copy over a writeback job, they are used only once */
> > +	state->writeback_job = NULL;
> > }
> > EXPORT_SYMBOL(__drm_atomic_helper_connector_duplicate_state);
> > 
> > @@ -3789,6 +3814,11 @@ __drm_atomic_helper_connector_destroy_state(struct drm_connector_state *state)
> > 
> > 	if (state->commit)
> > 		drm_crtc_commit_put(state->commit);
> > +
> > +	if (state->writeback_job) {
> > +		drm_writeback_cleanup_job(state->writeback_job);
> > +		kfree(state->writeback_job);
> > +	}
> 
> I don't think this will work properly. After calling
> drm_writeback_queue_job, the driver is meant to set
> state->writeback_job to NULL, and then the writeback queue was meant
> to take full ownership of the job (and free it on the
> drm_writeback_signal_completion path).
> 
> So, if drivers do that - by the time we get here, the driver will have
> set the pointer to NULL, and we'll leak the job (because it's no
> longer freed on the signal path).
> 
> If drivers don't set it to NULL after queueing, then this can race
> with the drm_writeback_signal_completion() path and do nasty things.
> 
> If there's no way to free the job on the signal_completion() path,
> then we'll probably need to refcount the writeback_job, and add a lock
> to it to protect the job->fb.
> 
> I was trying originally to avoid the refcount, but tbh the fact that
> drivers had to set the pointer to NULL after calling
> drm_writeback_queue_job is a little ugly and a little fragile, so
> perhaps it's the best thing to do.

Hi Brian,

I've got back from my holiday and had another look at the code when I've
realised that all this is probably not needed, as I don't think that
your initial assumption that drm_framebuffer_unreference() can sleep is
true. That means we can drop the framebuffer reference in the
drm_writeback_signal_completion() safely and we don't need all the
cleanup jobs.

I'm going to update the patch and also switch to
drm_framebuffer_{get,put} functions and send the new series today.

Best regards,
Liviu

> 
> -Brian
> 
> > }
> > EXPORT_SYMBOL(__drm_atomic_helper_connector_destroy_state);
> > 
> > diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> > index 9b9ba5d5ec0cb..d031e46ebffcf 100644
> > --- a/drivers/gpu/drm/drm_connector.c
> > +++ b/drivers/gpu/drm/drm_connector.c
> > @@ -87,6 +87,7 @@ static struct drm_conn_prop_enum_list drm_connector_enum_list[] = {
> > 	{ DRM_MODE_CONNECTOR_VIRTUAL, "Virtual" },
> > 	{ DRM_MODE_CONNECTOR_DSI, "DSI" },
> > 	{ DRM_MODE_CONNECTOR_DPI, "DPI" },
> > +	{ DRM_MODE_CONNECTOR_WRITEBACK, "Writeback" },
> > };
> > 
> > void drm_connector_ida_init(void)
> > @@ -249,7 +250,8 @@ int drm_connector_init(struct drm_device *dev,
> > 	config->num_connector++;
> > 	spin_unlock_irq(&config->connector_list_lock);
> > 
> > -	if (connector_type != DRM_MODE_CONNECTOR_VIRTUAL)
> > +	if (connector_type != DRM_MODE_CONNECTOR_VIRTUAL &&
> > +	    connector_type != DRM_MODE_CONNECTOR_WRITEBACK)
> > 		drm_object_attach_property(&connector->base,
> > 					      config->edid_property,
> > 					      0);
> > diff --git a/drivers/gpu/drm/drm_writeback.c b/drivers/gpu/drm/drm_writeback.c
> > new file mode 100644
> > index 0000000000000..1141325b004ab
> > --- /dev/null
> > +++ b/drivers/gpu/drm/drm_writeback.c
> > @@ -0,0 +1,255 @@
> > +/*
> > + * (C) COPYRIGHT 2016 ARM Limited. All rights reserved.
> > + * Author: Brian Starkey <brian.starkey@arm.com>
> > + *
> > + * This program is free software and is provided to you under the terms of the
> > + * GNU General Public License version 2 as published by the Free Software
> > + * Foundation, and any use by you of this program is subject to the terms
> > + * of such GNU licence.
> > + */
> > +
> > +#include <drm/drm_crtc.h>
> > +#include <drm/drm_modeset_helper_vtables.h>
> > +#include <drm/drm_property.h>
> > +#include <drm/drm_writeback.h>
> > +#include <drm/drmP.h>
> > +
> > +/**
> > + * DOC: overview
> > + *
> > + * Writeback connectors are used to expose hardware which can write the output
> > + * from a CRTC to a memory buffer. They are used and act similarly to other
> > + * types of connectors, with some important differences:
> > + *  - Writeback connectors don't provide a way to output visually to the user.
> > + *  - Writeback connectors should always report as "disconnected" (so that
> > + *    clients which don't understand them will ignore them).
> > + *  - Writeback connectors don't have EDID.
> > + *
> > + * A framebuffer may only be attached to a writeback connector when the
> > + * connector is attached to a CRTC. The WRITEBACK_FB_ID property which sets the
> > + * framebuffer applies only to a single commit (see below). A framebuffer may
> > + * not be attached while the CRTC is off.
> > + *
> > + * Writeback connectors have some additional properties, which userspace
> > + * can use to query and control them:
> > + *
> > + *  "WRITEBACK_FB_ID":
> > + *	Write-only object property storing a DRM_MODE_OBJECT_FB: it stores the
> > + *	framebuffer to be written by the writeback connector. This property is
> > + *	similar to the FB_ID property on planes, but will always read as zero
> > + *	and is not preserved across commits.
> > + *	Userspace must set this property to an output buffer every time it
> > + *	wishes the buffer to get filled.
> > + *
> > + *  "WRITEBACK_PIXEL_FORMATS":
> > + *	Immutable blob property to store the supported pixel formats table. The
> > + *	data is an array of u32 DRM_FORMAT_* fourcc values.
> > + *	Userspace can use this blob to find out what pixel formats are supported
> > + *	by the connector's writeback engine.
> > + */
> > +
> > +static int create_writeback_properties(struct drm_device *dev)
> > +{
> > +	struct drm_property *prop;
> > +
> > +	if (!dev->mode_config.writeback_fb_id_property) {
> > +		prop = drm_property_create_object(dev, DRM_MODE_PROP_ATOMIC,
> > +						  "WRITEBACK_FB_ID",
> > +						  DRM_MODE_OBJECT_FB);
> > +		if (!prop)
> > +			return -ENOMEM;
> > +		dev->mode_config.writeback_fb_id_property = prop;
> > +	}
> > +
> > +	if (!dev->mode_config.writeback_pixel_formats_property) {
> > +		prop = drm_property_create(dev, DRM_MODE_PROP_BLOB |
> > +					   DRM_MODE_PROP_ATOMIC |
> > +					   DRM_MODE_PROP_IMMUTABLE,
> > +					   "WRITEBACK_PIXEL_FORMATS", 0);
> > +		if (!prop)
> > +			return -ENOMEM;
> > +		dev->mode_config.writeback_pixel_formats_property = prop;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +static const struct drm_encoder_funcs drm_writeback_encoder_funcs = {
> > +	.destroy = drm_encoder_cleanup,
> > +};
> > +
> > +/**
> > + * drm_writeback_connector_init - Initialize a writeback connector and its properties
> > + * @dev: DRM device
> > + * @wb_connector: Writeback connector to initialize
> > + * @con_funcs: Connector funcs vtable
> > + * @enc_helper_funcs: Encoder helper funcs vtable to be used by the internal encoder
> > + * @formats: Array of supported pixel formats for the writeback engine
> > + * @n_formats: Length of the formats array
> > + *
> > + * This function creates the writeback-connector-specific properties if they
> > + * have not been already created, initializes the connector as
> > + * type DRM_MODE_CONNECTOR_WRITEBACK, and correctly initializes the property
> > + * values. It will also create an internal encoder associated with the
> > + * drm_writeback_connector and set it to use the @enc_helper_funcs vtable for
> > + * the encoder helper.
> > + *
> > + * Drivers should always use this function instead of drm_connector_init() to
> > + * set up writeback connectors.
> > + *
> > + * Returns: 0 on success, or a negative error code
> > + */
> > +int drm_writeback_connector_init(struct drm_device *dev,
> > +				 struct drm_writeback_connector *wb_connector,
> > +				 const struct drm_connector_funcs *con_funcs,
> > +				 const struct drm_encoder_helper_funcs *enc_helper_funcs,
> > +				 const u32 *formats, int n_formats)
> > +{
> > +	struct drm_property_blob *blob;
> > +	struct drm_connector *connector = &wb_connector->base;
> > +	struct drm_mode_config *config = &dev->mode_config;
> > +	int ret = create_writeback_properties(dev);
> > +
> > +	if (ret != 0)
> > +		return ret;
> > +
> > +	blob = drm_property_create_blob(dev, n_formats * sizeof(*formats),
> > +					formats);
> > +	if (IS_ERR(blob))
> > +		return PTR_ERR(blob);
> > +
> > +	drm_encoder_helper_add(&wb_connector->encoder, enc_helper_funcs);
> > +	ret = drm_encoder_init(dev, &wb_connector->encoder,
> > +			       &drm_writeback_encoder_funcs,
> > +			       DRM_MODE_ENCODER_VIRTUAL, NULL);
> > +	if (ret)
> > +		goto fail;
> > +
> > +	connector->interlace_allowed = 0;
> > +
> > +	ret = drm_connector_init(dev, connector, con_funcs,
> > +				 DRM_MODE_CONNECTOR_WRITEBACK);
> > +	if (ret)
> > +		goto connector_fail;
> > +
> > +	ret = drm_mode_connector_attach_encoder(connector,
> > +						&wb_connector->encoder);
> > +	if (ret)
> > +		goto attach_fail;
> > +
> > +	INIT_LIST_HEAD(&wb_connector->job_queue);
> > +	spin_lock_init(&wb_connector->job_lock);
> > +
> > +	drm_object_attach_property(&connector->base,
> > +				   config->writeback_fb_id_property, 0);
> > +
> > +	drm_object_attach_property(&connector->base,
> > +				   config->writeback_pixel_formats_property,
> > +				   blob->base.id);
> > +	wb_connector->pixel_formats_blob_ptr = blob;
> > +
> > +	return 0;
> > +
> > +attach_fail:
> > +	drm_connector_cleanup(connector);
> > +connector_fail:
> > +	drm_encoder_cleanup(&wb_connector->encoder);
> > +fail:
> > +	drm_property_unreference_blob(blob);
> > +	return ret;
> > +}
> > +EXPORT_SYMBOL(drm_writeback_connector_init);
> > +
> > +/**
> > + * drm_writeback_queue_job - Queue a writeback job for later signalling
> > + * @wb_connector: The writeback connector to queue a job on
> > + * @job: The job to queue
> > + *
> > + * This function adds a job to the job_queue for a writeback connector. It
> > + * should be considered to take ownership of the writeback job, and so any other
> > + * references to the job must be cleared after calling this function.
> > + *
> > + * Drivers must ensure that for a given writeback connector, jobs are queued in
> > + * exactly the same order as they will be completed by the hardware (and
> > + * signaled via drm_writeback_signal_completion).
> > + *
> > + * For every call to drm_writeback_queue_job() there must be exactly one call to
> > + * drm_writeback_signal_completion()
> > + *
> > + * See also: drm_writeback_signal_completion()
> > + */
> > +void drm_writeback_queue_job(struct drm_writeback_connector *wb_connector,
> > +			     struct drm_writeback_job *job)
> > +{
> > +	unsigned long flags;
> > +
> > +	spin_lock_irqsave(&wb_connector->job_lock, flags);
> > +	list_add_tail(&job->list_entry, &wb_connector->job_queue);
> > +	spin_unlock_irqrestore(&wb_connector->job_lock, flags);
> > +}
> > +EXPORT_SYMBOL(drm_writeback_queue_job);
> > +
> > +/**
> > + * drm_writeback_cleanup_job - Cleanup and free a writeback job
> > + * @job: The writeback job to free
> > + *
> > + * Drops any references held by the writeback job, and frees the structure.
> > + */
> > +void drm_writeback_cleanup_job(struct drm_writeback_job *job)
> > +{
> > +	if (job->fb)
> > +		drm_framebuffer_unreference(job->fb);
> > +}
> > +EXPORT_SYMBOL(drm_writeback_cleanup_job);
> > +
> > +/*
> > + * @cleanup_work: deferred cleanup of a writeback job
> > + *
> > + * The job cannot be cleaned up directly in drm_writeback_signal_completion,
> > + * because it may be called in interrupt context. Dropping the framebuffer
> > + * reference can sleep, and so the cleanup is deferred to a workqueue.
> > + */
> > +static void cleanup_work(struct work_struct *work)
> > +{
> > +	struct drm_writeback_job *job = container_of(work,
> > +						     struct drm_writeback_job,
> > +						     cleanup_work);
> > +	drm_writeback_cleanup_job(job);
> > +}
> > +
> > +/**
> > + * drm_writeback_signal_completion - Signal the completion of a writeback job
> > + * @wb_connector: The writeback connector whose job is complete
> > + *
> > + * Drivers should call this to signal the completion of a previously queued
> > + * writeback job. It should be called as soon as possible after the hardware
> > + * has finished writing, and may be called from interrupt context.
> > + * It is the driver's responsibility to ensure that for a given connector, the
> > + * hardware completes writeback jobs in the same order as they are queued.
> > + *
> > + * Unless the driver is holding its own reference to the framebuffer, it must
> > + * not be accessed after calling this function.
> > + *
> > + * See also: drm_writeback_queue_job()
> > + */
> > +void
> > +drm_writeback_signal_completion(struct drm_writeback_connector *wb_connector)
> > +{
> > +	unsigned long flags;
> > +	struct drm_writeback_job *job;
> > +
> > +	spin_lock_irqsave(&wb_connector->job_lock, flags);
> > +	job = list_first_entry_or_null(&wb_connector->job_queue,
> > +				       struct drm_writeback_job,
> > +				       list_entry);
> > +	if (job)
> > +		list_del(&job->list_entry);
> > +	spin_unlock_irqrestore(&wb_connector->job_lock, flags);
> > +
> > +	if (WARN_ON(!job))
> > +		return;
> > +
> > +	INIT_WORK(&job->cleanup_work, cleanup_work);
> > +	queue_work(system_long_wq, &job->cleanup_work);
> > +}
> > +EXPORT_SYMBOL(drm_writeback_signal_completion);
> > diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
> > index a57a8aa90ffb7..a5e904105db3a 100644
> > --- a/include/drm/drm_atomic.h
> > +++ b/include/drm/drm_atomic.h
> > @@ -594,6 +594,9 @@ void drm_atomic_set_fence_for_plane(struct drm_plane_state *plane_state,
> > int __must_check
> > drm_atomic_set_crtc_for_connector(struct drm_connector_state *conn_state,
> > 				  struct drm_crtc *crtc);
> > +int drm_atomic_set_writeback_fb_for_connector(
> > +		struct drm_connector_state *conn_state,
> > +		struct drm_framebuffer *fb);
> > int __must_check
> > drm_atomic_add_affected_connectors(struct drm_atomic_state *state,
> > 				   struct drm_crtc *crtc);
> > diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
> > index 675cc3f8cf852..60c0ffb947d5e 100644
> > --- a/include/drm/drm_connector.h
> > +++ b/include/drm/drm_connector.h
> > @@ -429,6 +429,19 @@ struct drm_connector_state {
> > 	 * protection. This is most commonly used for HDCP.
> > 	 */
> > 	unsigned int content_protection;
> > +
> > +	/**
> > +	 * @writeback_job: Writeback job for writeback connectors
> > +	 *
> > +	 * Holds the framebuffer for a writeback connector. As the writeback
> > +	 * completion may be asynchronous to the normal commit cycle, the
> > +	 * writeback job lifetime is managed separately from the normal atomic
> > +	 * state by this object.
> > +	 *
> > +	 * See also: drm_writeback_queue_job() and
> > +	 * drm_writeback_signal_completion()
> > +	 */
> > +	struct drm_writeback_job *writeback_job;
> > };
> > 
> > /**
> > diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> > index 33b3a96d66d0a..9ca32bf7bf183 100644
> > --- a/include/drm/drm_mode_config.h
> > +++ b/include/drm/drm_mode_config.h
> > @@ -779,6 +779,21 @@ struct drm_mode_config {
> > 	 */
> > 	struct drm_property *panel_orientation_property;
> > 
> > +	/**
> > +	 * @writeback_fb_id_property: Property for writeback connectors, storing
> > +	 * the ID of the output framebuffer.
> > +	 * See also: drm_writeback_connector_init()
> > +	 */
> > +	struct drm_property *writeback_fb_id_property;
> > +
> > +	/**
> > +	 * @writeback_pixel_formats_property: Property for writeback connectors,
> > +	 * storing an array of the supported pixel formats for the writeback
> > +	 * engine (read-only).
> > +	 * See also: drm_writeback_connector_init()
> > +	 */
> > +	struct drm_property *writeback_pixel_formats_property;
> > +
> > 	/* dumb ioctl parameters */
> > 	uint32_t preferred_depth, prefer_shadow;
> > 
> > diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
> > index 35e2a3a79fc56..3b289773297c2 100644
> > --- a/include/drm/drm_modeset_helper_vtables.h
> > +++ b/include/drm/drm_modeset_helper_vtables.h
> > @@ -974,6 +974,17 @@ struct drm_connector_helper_funcs {
> > 	 */
> > 	int (*atomic_check)(struct drm_connector *connector,
> > 			    struct drm_connector_state *state);
> > +
> > +	/**
> > +	 * @atomic_commit:
> > +	 *
> > +	 * This hook is to be used by drivers implementing writeback connectors
> > +	 * that need a point when to commit the writeback job to the hardware.
> > +	 *
> > +	 * This callback is used by the atomic modeset helpers.
> > +	 */
> > +	void (*atomic_commit)(struct drm_connector *connector,
> > +			      struct drm_writeback_job *writeback_job);
> > };
> > 
> > /**
> > diff --git a/include/drm/drm_writeback.h b/include/drm/drm_writeback.h
> > new file mode 100644
> > index 0000000000000..499c5c7b0a54e
> > --- /dev/null
> > +++ b/include/drm/drm_writeback.h
> > @@ -0,0 +1,90 @@
> > +/*
> > + * (C) COPYRIGHT 2016 ARM Limited. All rights reserved.
> > + * Author: Brian Starkey <brian.starkey@arm.com>
> > + *
> > + * This program is free software and is provided to you under the terms of the
> > + * GNU General Public License version 2 as published by the Free Software
> > + * Foundation, and any use by you of this program is subject to the terms
> > + * of such GNU licence.
> > + */
> > +
> > +#ifndef __DRM_WRITEBACK_H__
> > +#define __DRM_WRITEBACK_H__
> > +#include <drm/drm_connector.h>
> > +#include <drm/drm_encoder.h>
> > +#include <linux/workqueue.h>
> > +
> > +struct drm_writeback_connector {
> > +	struct drm_connector base;
> > +
> > +	/**
> > +	 * @encoder: Internal encoder used by the connector to fulfill
> > +	 * the DRM framework requirements. The users of the
> > +	 * @drm_writeback_connector control the behaviour of the @encoder
> > +	 * by passing the @enc_funcs parameter to drm_writeback_connector_init()
> > +	 * function.
> > +	 */
> > +	struct drm_encoder encoder;
> > +
> > +	/**
> > +	 * @pixel_formats_blob_ptr:
> > +	 *
> > +	 * DRM blob property data for the pixel formats list on writeback
> > +	 * connectors
> > +	 * See also drm_writeback_connector_init()
> > +	 */
> > +	struct drm_property_blob *pixel_formats_blob_ptr;
> > +
> > +	/** @job_lock: Protects job_queue */
> > +	spinlock_t job_lock;
> > +
> > +	/**
> > +	 * @job_queue:
> > +	 *
> > +	 * Holds a list of a connector's writeback jobs; the last item is the
> > +	 * most recent. The first item may be either waiting for the hardware
> > +	 * to begin writing, or currently being written.
> > +	 *
> > +	 * See also: drm_writeback_queue_job() and
> > +	 * drm_writeback_signal_completion()
> > +	 */
> > +	struct list_head job_queue;
> > +};
> > +
> > +struct drm_writeback_job {
> > +	/**
> > +	 * @cleanup_work:
> > +	 *
> > +	 * Used to allow drm_writeback_signal_completion to defer dropping the
> > +	 * framebuffer reference to a workqueue.
> > +	 */
> > +	struct work_struct cleanup_work;
> > +
> > +	/**
> > +	 * @list_entry:
> > +	 *
> > +	 * List item for the connector's @job_queue
> > +	 */
> > +	struct list_head list_entry;
> > +
> > +	/**
> > +	 * @fb:
> > +	 *
> > +	 * Framebuffer to be written to by the writeback connector. Do not set
> > +	 * directly, use drm_atomic_set_writeback_fb_for_connector()
> > +	 */
> > +	struct drm_framebuffer *fb;
> > +};
> > +
> > +int drm_writeback_connector_init(struct drm_device *dev,
> > +				 struct drm_writeback_connector *wb_connector,
> > +				 const struct drm_connector_funcs *con_funcs,
> > +				 const struct drm_encoder_helper_funcs *enc_helper_funcs,
> > +				 const u32 *formats, int n_formats);
> > +
> > +void drm_writeback_queue_job(struct drm_writeback_connector *wb_connector,
> > +			     struct drm_writeback_job *job);
> > +
> > +void drm_writeback_cleanup_job(struct drm_writeback_job *job);
> > +void drm_writeback_signal_completion(struct drm_writeback_connector *wb_connector);
> > +#endif
> > diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> > index 4b3a1bb58e680..3e6541f0400b8 100644
> > --- a/include/uapi/drm/drm_mode.h
> > +++ b/include/uapi/drm/drm_mode.h
> > @@ -344,6 +344,7 @@ enum drm_mode_subconnector {
> > #define DRM_MODE_CONNECTOR_VIRTUAL      15
> > #define DRM_MODE_CONNECTOR_DSI		16
> > #define DRM_MODE_CONNECTOR_DPI		17
> > +#define DRM_MODE_CONNECTOR_WRITEBACK	18
> > 
> > struct drm_mode_get_connector {
> > 
> > -- 
> > 2.17.0
> >
Brian Starkey June 7, 2018, 11:05 p.m. | #2
On Thu, Jun 07, 2018 at 10:26:39AM +0100, Liviu Dudau wrote:
>On Fri, May 25, 2018 at 04:32:38PM +0100, Brian Starkey wrote:
>> On Thu, May 24, 2018 at 05:41:01PM +0100, Liviu Dudau wrote:
>> > From: Brian Starkey <brian.starkey@arm.com>
>> >
>> > Writeback connectors represent writeback engines which can write the
>> > CRTC output to a memory framebuffer. Add a writeback connector type and
>> > related support functions.
>> >
>> > Drivers should initialize a writeback connector with
>> > drm_writeback_connector_init() which takes care of setting up all the
>> > writeback-specific details on top of the normal functionality of
>> > drm_connector_init().
>> >
>> > Writeback connectors have a WRITEBACK_FB_ID property, used to set the
>> > output framebuffer, and a WRITEBACK_PIXEL_FORMATS blob used to expose the
>> > supported writeback formats to userspace.
>> >
>> > When a framebuffer is attached to a writeback connector with the
>> > WRITEBACK_FB_ID property, it is used only once (for the commit in which
>> > it was included), and userspace can never read back the value of
>> > WRITEBACK_FB_ID. WRITEBACK_FB_ID can only be set if the connector is
>> > attached to a CRTC.
>> >
>> > Changes since v1:
>> > - Added drm_writeback.c + documentation
>> > - Added helper to initialize writeback connector in one go
>> > - Added core checks
>> > - Squashed into a single commit
>> > - Dropped the client cap
>> > - Writeback framebuffers are no longer persistent
>> >
>> > Changes since v2:
>> > Daniel Vetter:
>> > - Subclass drm_connector to drm_writeback_connector
>> > - Relax check to allow CRTC to be set without an FB
>> > - Add some writeback_ prefixes
>> > - Drop PIXEL_FORMATS_SIZE property, as it was unnecessary
>> > Gustavo Padovan:
>> > - Add drm_writeback_job to handle writeback signalling centrally
>> >
>> > Changes since v3:
>> > - Rebased
>> > - Rename PIXEL_FORMATS -> WRITEBACK_PIXEL_FORMATS
>> >
>> > Chances since v4:
>> > - Embed a drm_encoder inside the drm_writeback_connector to
>> >   reduce the amount of boilerplate code required from the drivers
>> >   that are using it.
>> >
>> > Changes since v5:
>> > - Added Rob Clark's atomic_commit() vfunc to connector helper
>> >   funcs, so that writeback jobs are commited from atomic helpers
>> > - Updated create_writeback_properties() signature to return an
>> >   error code rather than a boolean false for failure.
>> > - Free writeback job with the connector state rather than when
>> >   doing the cleanup_work()
>> >
>> > Changes since v7:
>> > - fix extraneous use of out_fence that is only introduced in a
>> >   subsequent patch.
>> >
>> > Changes since v8:
>> > - whitespace changes pull from subsequent patch
>> >
>> > Reviewed-by: Eric Anholt <eric@anholt.net>
>> > Signed-off-by: Brian Starkey <brian.starkey@arm.com>
>> > [rebased and fixed conflicts]
>> > Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
>> > [rebased and added atomic_commit() vfunc for writeback jobs]
>> > Signed-off-by: Rob Clark <robdclark@gmail.com>
>> > Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
>> > ---
>> > Documentation/gpu/drm-kms.rst            |   9 +
>> > drivers/gpu/drm/Makefile                 |   2 +-
>> > drivers/gpu/drm/drm_atomic.c             | 128 ++++++++++++
>> > drivers/gpu/drm/drm_atomic_helper.c      |  30 +++
>> > drivers/gpu/drm/drm_connector.c          |   4 +-
>> > drivers/gpu/drm/drm_writeback.c          | 255 +++++++++++++++++++++++
>> > include/drm/drm_atomic.h                 |   3 +
>> > include/drm/drm_connector.h              |  13 ++
>> > include/drm/drm_mode_config.h            |  15 ++
>> > include/drm/drm_modeset_helper_vtables.h |  11 +
>> > include/drm/drm_writeback.h              |  90 ++++++++
>> > include/uapi/drm/drm_mode.h              |   1 +
>> > 12 files changed, 559 insertions(+), 2 deletions(-)
>> > create mode 100644 drivers/gpu/drm/drm_writeback.c
>> > create mode 100644 include/drm/drm_writeback.h
>> >
>> > diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
>> > index 1dffd1ac4cd44..809d403087f95 100644
>> > --- a/Documentation/gpu/drm-kms.rst
>> > +++ b/Documentation/gpu/drm-kms.rst
>> > @@ -373,6 +373,15 @@ Connector Functions Reference
>> > .. kernel-doc:: drivers/gpu/drm/drm_connector.c
>> >    :export:
>> >
>> > +Writeback Connectors
>> > +--------------------
>> > +
>> > +.. kernel-doc:: drivers/gpu/drm/drm_writeback.c
>> > +  :doc: overview
>> > +
>> > +.. kernel-doc:: drivers/gpu/drm/drm_writeback.c
>> > +  :export:
>> > +
>> > Encoder Abstraction
>> > ===================
>> >
>> > diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
>> > index ef9f3dab287fd..69c13517ea3a6 100644
>> > --- a/drivers/gpu/drm/Makefile
>> > +++ b/drivers/gpu/drm/Makefile
>> > @@ -18,7 +18,7 @@ drm-y       :=	drm_auth.o drm_bufs.o drm_cache.o \
>> > 		drm_encoder.o drm_mode_object.o drm_property.o \
>> > 		drm_plane.o drm_color_mgmt.o drm_print.o \
>> > 		drm_dumb_buffers.o drm_mode_config.o drm_vblank.o \
>> > -		drm_syncobj.o drm_lease.o
>> > +		drm_syncobj.o drm_lease.o drm_writeback.o
>> >
>> > drm-$(CONFIG_DRM_LIB_RANDOM) += lib/drm_random.o
>> > drm-$(CONFIG_DRM_VM) += drm_vm.o
>> > diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
>> > index 895741e9cd7db..e32a93931c0dc 100644
>> > --- a/drivers/gpu/drm/drm_atomic.c
>> > +++ b/drivers/gpu/drm/drm_atomic.c
>> > @@ -30,6 +30,7 @@
>> > #include <drm/drm_atomic.h>
>> > #include <drm/drm_mode.h>
>> > #include <drm/drm_print.h>
>> > +#include <drm/drm_writeback.h>
>> > #include <linux/sync_file.h>
>> >
>> > #include "drm_crtc_internal.h"
>> > @@ -676,6 +677,45 @@ static void drm_atomic_crtc_print_state(struct drm_printer *p,
>> > 		crtc->funcs->atomic_print_state(p, state);
>> > }
>> >
>> > +/**
>> > + * drm_atomic_connector_check - check connector state
>> > + * @connector: connector to check
>> > + * @state: connector state to check
>> > + *
>> > + * Provides core sanity checks for connector state.
>> > + *
>> > + * RETURNS:
>> > + * Zero on success, error code on failure
>> > + */
>> > +static int drm_atomic_connector_check(struct drm_connector *connector,
>> > +		struct drm_connector_state *state)
>> > +{
>> > +	struct drm_crtc_state *crtc_state;
>> > +	struct drm_writeback_job *writeback_job = state->writeback_job;
>> > +
>> > +	if ((connector->connector_type != DRM_MODE_CONNECTOR_WRITEBACK) || !writeback_job)
>> > +		return 0;
>> > +
>> > +	if (writeback_job->fb && !state->crtc) {
>> > +		DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] framebuffer without CRTC\n",
>> > +				 connector->base.id, connector->name);
>> > +		return -EINVAL;
>> > +	}
>> > +
>> > +	if (state->crtc)
>> > +		crtc_state = drm_atomic_get_existing_crtc_state(state->state,
>> > +								state->crtc);
>> > +
>> > +	if (writeback_job->fb && !crtc_state->active) {
>> > +		DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] has framebuffer, but [CRTC:%d] is off\n",
>> > +				 connector->base.id, connector->name,
>> > +				 state->crtc->base.id);
>> > +		return -EINVAL;
>> > +	}
>> > +
>> > +	return 0;
>> > +}
>> > +
>> > /**
>> >  * drm_atomic_get_plane_state - get plane state
>> >  * @state: global atomic state object
>> > @@ -1286,6 +1326,12 @@ static int drm_atomic_connector_set_property(struct drm_connector *connector,
>> > 			return -EINVAL;
>> > 		}
>> > 		state->content_protection = val;
>> > +	} else if (property == config->writeback_fb_id_property) {
>> > +		struct drm_framebuffer *fb = drm_framebuffer_lookup(dev, NULL, val);
>> > +		int ret = drm_atomic_set_writeback_fb_for_connector(state, fb);
>> > +		if (fb)
>> > +			drm_framebuffer_unreference(fb);
>> > +		return ret;
>> > 	} else if (connector->funcs->atomic_set_property) {
>> > 		return connector->funcs->atomic_set_property(connector,
>> > 				state, property, val);
>> > @@ -1367,6 +1413,9 @@ drm_atomic_connector_get_property(struct drm_connector *connector,
>> > 		*val = state->scaling_mode;
>> > 	} else if (property == connector->content_protection_property) {
>> > 		*val = state->content_protection;
>> > +	} else if (property == config->writeback_fb_id_property) {
>> > +		/* Writeback framebuffer is one-shot, write and forget */
>> > +		*val = 0;
>> > 	} else if (connector->funcs->atomic_get_property) {
>> > 		return connector->funcs->atomic_get_property(connector,
>> > 				state, property, val);
>> > @@ -1584,6 +1633,74 @@ drm_atomic_set_crtc_for_connector(struct drm_connector_state *conn_state,
>> > }
>> > EXPORT_SYMBOL(drm_atomic_set_crtc_for_connector);
>> >
>> > +/*
>> > + * drm_atomic_get_writeback_job - return or allocate a writeback job
>> > + * @conn_state: Connector state to get the job for
>> > + *
>> > + * Writeback jobs have a different lifetime to the atomic state they are
>> > + * associated with. This convenience function takes care of allocating a job
>> > + * if there isn't yet one associated with the connector state, otherwise
>> > + * it just returns the existing job.
>> > + *
>> > + * Returns: The writeback job for the given connector state
>> > + */
>> > +static struct drm_writeback_job *
>> > +drm_atomic_get_writeback_job(struct drm_connector_state *conn_state)
>> > +{
>> > +	WARN_ON(conn_state->connector->connector_type != DRM_MODE_CONNECTOR_WRITEBACK);
>> > +
>> > +	if (!conn_state->writeback_job)
>> > +		conn_state->writeback_job =
>> > +			kzalloc(sizeof(*conn_state->writeback_job), GFP_KERNEL);
>> > +
>> > +	return conn_state->writeback_job;
>> > +}
>> > +
>> > +/**
>> > + * drm_atomic_set_writeback_fb_for_connector - set writeback framebuffer
>> > + * @conn_state: atomic state object for the connector
>> > + * @fb: fb to use for the connector
>> > + *
>> > + * This is used to set the framebuffer for a writeback connector, which outputs
>> > + * to a buffer instead of an actual physical connector.
>> > + * Changing the assigned framebuffer requires us to grab a reference to the new
>> > + * fb and drop the reference to the old fb, if there is one. This function
>> > + * takes care of all these details besides updating the pointer in the
>> > + * state object itself.
>> > + *
>> > + * Note: The only way conn_state can already have an fb set is if the commit
>> > + * sets the property more than once.
>> > + *
>> > + * See also: drm_writeback_connector_init()
>> > + *
>> > + * Returns: 0 on success
>> > + */
>> > +int drm_atomic_set_writeback_fb_for_connector(
>> > +		struct drm_connector_state *conn_state,
>> > +		struct drm_framebuffer *fb)
>> > +{
>> > +	struct drm_writeback_job *job =
>> > +		drm_atomic_get_writeback_job(conn_state);
>> > +	if (!job)
>> > +		return -ENOMEM;
>> > +
>> > +	if (job->fb)
>> > +		drm_framebuffer_unreference(job->fb);
>> > +	if (fb)
>> > +		drm_framebuffer_reference(fb);
>> > +	job->fb = fb;
>> > +
>> > +	if (fb)
>> > +		DRM_DEBUG_ATOMIC("Set [FB:%d] for connector state %p\n",
>> > +				 fb->base.id, conn_state);
>> > +	else
>> > +		DRM_DEBUG_ATOMIC("Set [NOFB] for connector state %p\n",
>> > +				 conn_state);
>> > +
>> > +	return 0;
>> > +}
>> > +EXPORT_SYMBOL(drm_atomic_set_writeback_fb_for_connector);
>> > +
>> > /**
>> >  * drm_atomic_add_affected_connectors - add connectors for crtc
>> >  * @state: atomic state
>> > @@ -1702,6 +1819,8 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
>> > 	struct drm_plane_state *plane_state;
>> > 	struct drm_crtc *crtc;
>> > 	struct drm_crtc_state *crtc_state;
>> > +	struct drm_connector *conn;
>> > +	struct drm_connector_state *conn_state;
>> > 	int i, ret = 0;
>> >
>> > 	DRM_DEBUG_ATOMIC("checking %p\n", state);
>> > @@ -1724,6 +1843,15 @@ int drm_atomic_check_only(struct drm_atomic_state *state)
>> > 		}
>> > 	}
>> >
>> > +	for_each_new_connector_in_state(state, conn, conn_state, i) {
>> > +		ret = drm_atomic_connector_check(conn, conn_state);
>> > +		if (ret) {
>> > +			DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] atomic core check failed\n",
>> > +					 conn->base.id, conn->name);
>> > +			return ret;
>> > +		}
>> > +	}
>> > +
>> > 	if (config->funcs->atomic_check) {
>> > 		ret = config->funcs->atomic_check(state->dev, state);
>> >
>> > diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
>> > index 130da5195f3b6..5fd12666f7d7a 100644
>> > --- a/drivers/gpu/drm/drm_atomic_helper.c
>> > +++ b/drivers/gpu/drm/drm_atomic_helper.c
>> > @@ -30,6 +30,7 @@
>> > #include <drm/drm_plane_helper.h>
>> > #include <drm/drm_crtc_helper.h>
>> > #include <drm/drm_atomic_helper.h>
>> > +#include <drm/drm_writeback.h>
>> > #include <linux/dma-fence.h>
>> >
>> > #include "drm_crtc_helper_internal.h"
>> > @@ -1172,6 +1173,25 @@ void drm_atomic_helper_commit_modeset_disables(struct drm_device *dev,
>> > }
>> > EXPORT_SYMBOL(drm_atomic_helper_commit_modeset_disables);
>> >
>> > +static void drm_atomic_helper_commit_writebacks(struct drm_device *dev,
>> > +						struct drm_atomic_state *old_state)
>> > +{
>> > +	struct drm_connector *connector;
>> > +	struct drm_connector_state *new_conn_state;
>> > +	int i;
>> > +
>> > +	for_each_new_connector_in_state(old_state, connector, new_conn_state, i) {
>> > +		const struct drm_connector_helper_funcs *funcs;
>> > +
>> > +		funcs = connector->helper_private;
>> > +
>> > +		if (new_conn_state->writeback_job && new_conn_state->writeback_job->fb) {
>> > +			WARN_ON(connector->connector_type != DRM_MODE_CONNECTOR_WRITEBACK);
>> > +			funcs->atomic_commit(connector, new_conn_state->writeback_job);
>> > +		}
>> > +	}
>> > +}
>> > +
>> > /**
>> >  * drm_atomic_helper_commit_modeset_enables - modeset commit to enable outputs
>> >  * @dev: DRM device
>> > @@ -1251,6 +1271,8 @@ void drm_atomic_helper_commit_modeset_enables(struct drm_device *dev,
>> >
>> > 		drm_bridge_enable(encoder->bridge);
>> > 	}
>> > +
>> > +	drm_atomic_helper_commit_writebacks(dev, old_state);
>> > }
>> > EXPORT_SYMBOL(drm_atomic_helper_commit_modeset_enables);
>> >
>> > @@ -3660,6 +3682,9 @@ __drm_atomic_helper_connector_duplicate_state(struct drm_connector *connector,
>> > 	if (state->crtc)
>> > 		drm_connector_get(connector);
>> > 	state->commit = NULL;
>> > +
>> > +	/* Don't copy over a writeback job, they are used only once */
>> > +	state->writeback_job = NULL;
>> > }
>> > EXPORT_SYMBOL(__drm_atomic_helper_connector_duplicate_state);
>> >
>> > @@ -3789,6 +3814,11 @@ __drm_atomic_helper_connector_destroy_state(struct drm_connector_state *state)
>> >
>> > 	if (state->commit)
>> > 		drm_crtc_commit_put(state->commit);
>> > +
>> > +	if (state->writeback_job) {
>> > +		drm_writeback_cleanup_job(state->writeback_job);
>> > +		kfree(state->writeback_job);
>> > +	}
>>
>> I don't think this will work properly. After calling
>> drm_writeback_queue_job, the driver is meant to set
>> state->writeback_job to NULL, and then the writeback queue was meant
>> to take full ownership of the job (and free it on the
>> drm_writeback_signal_completion path).
>>
>> So, if drivers do that - by the time we get here, the driver will have
>> set the pointer to NULL, and we'll leak the job (because it's no
>> longer freed on the signal path).
>>
>> If drivers don't set it to NULL after queueing, then this can race
>> with the drm_writeback_signal_completion() path and do nasty things.
>>
>> If there's no way to free the job on the signal_completion() path,
>> then we'll probably need to refcount the writeback_job, and add a lock
>> to it to protect the job->fb.
>>
>> I was trying originally to avoid the refcount, but tbh the fact that
>> drivers had to set the pointer to NULL after calling
>> drm_writeback_queue_job is a little ugly and a little fragile, so
>> perhaps it's the best thing to do.
>
>Hi Brian,
>
>I've got back from my holiday and had another look at the code when I've
>realised that all this is probably not needed, as I don't think that
>your initial assumption that drm_framebuffer_unreference() can sleep is
>true. That means we can drop the framebuffer reference in the
>drm_writeback_signal_completion() safely and we don't need all the
>cleanup jobs.

I still think it can.

drm_framebuffer_unreference() drops a kref. The framebuffer's kref
release function is set to drm_framebuffer_free(), which calls
drm_mode_object_unregister() (which can sleep), and ->destroy, which
must call drm_framebuffer_cleanup() (which can sleep).

>
>I'm going to update the patch and also switch to
>drm_framebuffer_{get,put} functions and send the new series today.

Yeah I just noticed and was going to suggest that :-)

Cheers,
-Brian

>
>Best regards, Liviu

>
>>
>> -Brian
>>
>> > }
>> > EXPORT_SYMBOL(__drm_atomic_helper_connector_destroy_state);
>> >
>> > diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
>> > index 9b9ba5d5ec0cb..d031e46ebffcf 100644
>> > --- a/drivers/gpu/drm/drm_connector.c
>> > +++ b/drivers/gpu/drm/drm_connector.c
>> > @@ -87,6 +87,7 @@ static struct drm_conn_prop_enum_list drm_connector_enum_list[] = {
>> > 	{ DRM_MODE_CONNECTOR_VIRTUAL, "Virtual" },
>> > 	{ DRM_MODE_CONNECTOR_DSI, "DSI" },
>> > 	{ DRM_MODE_CONNECTOR_DPI, "DPI" },
>> > +	{ DRM_MODE_CONNECTOR_WRITEBACK, "Writeback" },
>> > };
>> >
>> > void drm_connector_ida_init(void)
>> > @@ -249,7 +250,8 @@ int drm_connector_init(struct drm_device *dev,
>> > 	config->num_connector++;
>> > 	spin_unlock_irq(&config->connector_list_lock);
>> >
>> > -	if (connector_type != DRM_MODE_CONNECTOR_VIRTUAL)
>> > +	if (connector_type != DRM_MODE_CONNECTOR_VIRTUAL &&
>> > +	    connector_type != DRM_MODE_CONNECTOR_WRITEBACK)
>> > 		drm_object_attach_property(&connector->base,
>> > 					      config->edid_property,
>> > 					      0);
>> > diff --git a/drivers/gpu/drm/drm_writeback.c b/drivers/gpu/drm/drm_writeback.c
>> > new file mode 100644
>> > index 0000000000000..1141325b004ab
>> > --- /dev/null
>> > +++ b/drivers/gpu/drm/drm_writeback.c
>> > @@ -0,0 +1,255 @@
>> > +/*
>> > + * (C) COPYRIGHT 2016 ARM Limited. All rights reserved.
>> > + * Author: Brian Starkey <brian.starkey@arm.com>
>> > + *
>> > + * This program is free software and is provided to you under the terms of the
>> > + * GNU General Public License version 2 as published by the Free Software
>> > + * Foundation, and any use by you of this program is subject to the terms
>> > + * of such GNU licence.
>> > + */
>> > +
>> > +#include <drm/drm_crtc.h>
>> > +#include <drm/drm_modeset_helper_vtables.h>
>> > +#include <drm/drm_property.h>
>> > +#include <drm/drm_writeback.h>
>> > +#include <drm/drmP.h>
>> > +
>> > +/**
>> > + * DOC: overview
>> > + *
>> > + * Writeback connectors are used to expose hardware which can write the output
>> > + * from a CRTC to a memory buffer. They are used and act similarly to other
>> > + * types of connectors, with some important differences:
>> > + *  - Writeback connectors don't provide a way to output visually to the user.
>> > + *  - Writeback connectors should always report as "disconnected" (so that
>> > + *    clients which don't understand them will ignore them).
>> > + *  - Writeback connectors don't have EDID.
>> > + *
>> > + * A framebuffer may only be attached to a writeback connector when the
>> > + * connector is attached to a CRTC. The WRITEBACK_FB_ID property which sets the
>> > + * framebuffer applies only to a single commit (see below). A framebuffer may
>> > + * not be attached while the CRTC is off.
>> > + *
>> > + * Writeback connectors have some additional properties, which userspace
>> > + * can use to query and control them:
>> > + *
>> > + *  "WRITEBACK_FB_ID":
>> > + *	Write-only object property storing a DRM_MODE_OBJECT_FB: it stores the
>> > + *	framebuffer to be written by the writeback connector. This property is
>> > + *	similar to the FB_ID property on planes, but will always read as zero
>> > + *	and is not preserved across commits.
>> > + *	Userspace must set this property to an output buffer every time it
>> > + *	wishes the buffer to get filled.
>> > + *
>> > + *  "WRITEBACK_PIXEL_FORMATS":
>> > + *	Immutable blob property to store the supported pixel formats table. The
>> > + *	data is an array of u32 DRM_FORMAT_* fourcc values.
>> > + *	Userspace can use this blob to find out what pixel formats are supported
>> > + *	by the connector's writeback engine.
>> > + */
>> > +
>> > +static int create_writeback_properties(struct drm_device *dev)
>> > +{
>> > +	struct drm_property *prop;
>> > +
>> > +	if (!dev->mode_config.writeback_fb_id_property) {
>> > +		prop = drm_property_create_object(dev, DRM_MODE_PROP_ATOMIC,
>> > +						  "WRITEBACK_FB_ID",
>> > +						  DRM_MODE_OBJECT_FB);
>> > +		if (!prop)
>> > +			return -ENOMEM;
>> > +		dev->mode_config.writeback_fb_id_property = prop;
>> > +	}
>> > +
>> > +	if (!dev->mode_config.writeback_pixel_formats_property) {
>> > +		prop = drm_property_create(dev, DRM_MODE_PROP_BLOB |
>> > +					   DRM_MODE_PROP_ATOMIC |
>> > +					   DRM_MODE_PROP_IMMUTABLE,
>> > +					   "WRITEBACK_PIXEL_FORMATS", 0);
>> > +		if (!prop)
>> > +			return -ENOMEM;
>> > +		dev->mode_config.writeback_pixel_formats_property = prop;
>> > +	}
>> > +
>> > +	return 0;
>> > +}
>> > +
>> > +static const struct drm_encoder_funcs drm_writeback_encoder_funcs = {
>> > +	.destroy = drm_encoder_cleanup,
>> > +};
>> > +
>> > +/**
>> > + * drm_writeback_connector_init - Initialize a writeback connector and its properties
>> > + * @dev: DRM device
>> > + * @wb_connector: Writeback connector to initialize
>> > + * @con_funcs: Connector funcs vtable
>> > + * @enc_helper_funcs: Encoder helper funcs vtable to be used by the internal encoder
>> > + * @formats: Array of supported pixel formats for the writeback engine
>> > + * @n_formats: Length of the formats array
>> > + *
>> > + * This function creates the writeback-connector-specific properties if they
>> > + * have not been already created, initializes the connector as
>> > + * type DRM_MODE_CONNECTOR_WRITEBACK, and correctly initializes the property
>> > + * values. It will also create an internal encoder associated with the
>> > + * drm_writeback_connector and set it to use the @enc_helper_funcs vtable for
>> > + * the encoder helper.
>> > + *
>> > + * Drivers should always use this function instead of drm_connector_init() to
>> > + * set up writeback connectors.
>> > + *
>> > + * Returns: 0 on success, or a negative error code
>> > + */
>> > +int drm_writeback_connector_init(struct drm_device *dev,
>> > +				 struct drm_writeback_connector *wb_connector,
>> > +				 const struct drm_connector_funcs *con_funcs,
>> > +				 const struct drm_encoder_helper_funcs *enc_helper_funcs,
>> > +				 const u32 *formats, int n_formats)
>> > +{
>> > +	struct drm_property_blob *blob;
>> > +	struct drm_connector *connector = &wb_connector->base;
>> > +	struct drm_mode_config *config = &dev->mode_config;
>> > +	int ret = create_writeback_properties(dev);
>> > +
>> > +	if (ret != 0)
>> > +		return ret;
>> > +
>> > +	blob = drm_property_create_blob(dev, n_formats * sizeof(*formats),
>> > +					formats);
>> > +	if (IS_ERR(blob))
>> > +		return PTR_ERR(blob);
>> > +
>> > +	drm_encoder_helper_add(&wb_connector->encoder, enc_helper_funcs);
>> > +	ret = drm_encoder_init(dev, &wb_connector->encoder,
>> > +			       &drm_writeback_encoder_funcs,
>> > +			       DRM_MODE_ENCODER_VIRTUAL, NULL);
>> > +	if (ret)
>> > +		goto fail;
>> > +
>> > +	connector->interlace_allowed = 0;
>> > +
>> > +	ret = drm_connector_init(dev, connector, con_funcs,
>> > +				 DRM_MODE_CONNECTOR_WRITEBACK);
>> > +	if (ret)
>> > +		goto connector_fail;
>> > +
>> > +	ret = drm_mode_connector_attach_encoder(connector,
>> > +						&wb_connector->encoder);
>> > +	if (ret)
>> > +		goto attach_fail;
>> > +
>> > +	INIT_LIST_HEAD(&wb_connector->job_queue);
>> > +	spin_lock_init(&wb_connector->job_lock);
>> > +
>> > +	drm_object_attach_property(&connector->base,
>> > +				   config->writeback_fb_id_property, 0);
>> > +
>> > +	drm_object_attach_property(&connector->base,
>> > +				   config->writeback_pixel_formats_property,
>> > +				   blob->base.id);
>> > +	wb_connector->pixel_formats_blob_ptr = blob;
>> > +
>> > +	return 0;
>> > +
>> > +attach_fail:
>> > +	drm_connector_cleanup(connector);
>> > +connector_fail:
>> > +	drm_encoder_cleanup(&wb_connector->encoder);
>> > +fail:
>> > +	drm_property_unreference_blob(blob);
>> > +	return ret;
>> > +}
>> > +EXPORT_SYMBOL(drm_writeback_connector_init);
>> > +
>> > +/**
>> > + * drm_writeback_queue_job - Queue a writeback job for later signalling
>> > + * @wb_connector: The writeback connector to queue a job on
>> > + * @job: The job to queue
>> > + *
>> > + * This function adds a job to the job_queue for a writeback connector. It
>> > + * should be considered to take ownership of the writeback job, and so any other
>> > + * references to the job must be cleared after calling this function.
>> > + *
>> > + * Drivers must ensure that for a given writeback connector, jobs are queued in
>> > + * exactly the same order as they will be completed by the hardware (and
>> > + * signaled via drm_writeback_signal_completion).
>> > + *
>> > + * For every call to drm_writeback_queue_job() there must be exactly one call to
>> > + * drm_writeback_signal_completion()
>> > + *
>> > + * See also: drm_writeback_signal_completion()
>> > + */
>> > +void drm_writeback_queue_job(struct drm_writeback_connector *wb_connector,
>> > +			     struct drm_writeback_job *job)
>> > +{
>> > +	unsigned long flags;
>> > +
>> > +	spin_lock_irqsave(&wb_connector->job_lock, flags);
>> > +	list_add_tail(&job->list_entry, &wb_connector->job_queue);
>> > +	spin_unlock_irqrestore(&wb_connector->job_lock, flags);
>> > +}
>> > +EXPORT_SYMBOL(drm_writeback_queue_job);
>> > +
>> > +/**
>> > + * drm_writeback_cleanup_job - Cleanup and free a writeback job
>> > + * @job: The writeback job to free
>> > + *
>> > + * Drops any references held by the writeback job, and frees the structure.
>> > + */
>> > +void drm_writeback_cleanup_job(struct drm_writeback_job *job)
>> > +{
>> > +	if (job->fb)
>> > +		drm_framebuffer_unreference(job->fb);
>> > +}
>> > +EXPORT_SYMBOL(drm_writeback_cleanup_job);
>> > +
>> > +/*
>> > + * @cleanup_work: deferred cleanup of a writeback job
>> > + *
>> > + * The job cannot be cleaned up directly in drm_writeback_signal_completion,
>> > + * because it may be called in interrupt context. Dropping the framebuffer
>> > + * reference can sleep, and so the cleanup is deferred to a workqueue.
>> > + */
>> > +static void cleanup_work(struct work_struct *work)
>> > +{
>> > +	struct drm_writeback_job *job = container_of(work,
>> > +						     struct drm_writeback_job,
>> > +						     cleanup_work);
>> > +	drm_writeback_cleanup_job(job);
>> > +}
>> > +
>> > +/**
>> > + * drm_writeback_signal_completion - Signal the completion of a writeback job
>> > + * @wb_connector: The writeback connector whose job is complete
>> > + *
>> > + * Drivers should call this to signal the completion of a previously queued
>> > + * writeback job. It should be called as soon as possible after the hardware
>> > + * has finished writing, and may be called from interrupt context.
>> > + * It is the driver's responsibility to ensure that for a given connector, the
>> > + * hardware completes writeback jobs in the same order as they are queued.
>> > + *
>> > + * Unless the driver is holding its own reference to the framebuffer, it must
>> > + * not be accessed after calling this function.
>> > + *
>> > + * See also: drm_writeback_queue_job()
>> > + */
>> > +void
>> > +drm_writeback_signal_completion(struct drm_writeback_connector *wb_connector)
>> > +{
>> > +	unsigned long flags;
>> > +	struct drm_writeback_job *job;
>> > +
>> > +	spin_lock_irqsave(&wb_connector->job_lock, flags);
>> > +	job = list_first_entry_or_null(&wb_connector->job_queue,
>> > +				       struct drm_writeback_job,
>> > +				       list_entry);
>> > +	if (job)
>> > +		list_del(&job->list_entry);
>> > +	spin_unlock_irqrestore(&wb_connector->job_lock, flags);
>> > +
>> > +	if (WARN_ON(!job))
>> > +		return;
>> > +
>> > +	INIT_WORK(&job->cleanup_work, cleanup_work);
>> > +	queue_work(system_long_wq, &job->cleanup_work);
>> > +}
>> > +EXPORT_SYMBOL(drm_writeback_signal_completion);
>> > diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
>> > index a57a8aa90ffb7..a5e904105db3a 100644
>> > --- a/include/drm/drm_atomic.h
>> > +++ b/include/drm/drm_atomic.h
>> > @@ -594,6 +594,9 @@ void drm_atomic_set_fence_for_plane(struct drm_plane_state *plane_state,
>> > int __must_check
>> > drm_atomic_set_crtc_for_connector(struct drm_connector_state *conn_state,
>> > 				  struct drm_crtc *crtc);
>> > +int drm_atomic_set_writeback_fb_for_connector(
>> > +		struct drm_connector_state *conn_state,
>> > +		struct drm_framebuffer *fb);
>> > int __must_check
>> > drm_atomic_add_affected_connectors(struct drm_atomic_state *state,
>> > 				   struct drm_crtc *crtc);
>> > diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
>> > index 675cc3f8cf852..60c0ffb947d5e 100644
>> > --- a/include/drm/drm_connector.h
>> > +++ b/include/drm/drm_connector.h
>> > @@ -429,6 +429,19 @@ struct drm_connector_state {
>> > 	 * protection. This is most commonly used for HDCP.
>> > 	 */
>> > 	unsigned int content_protection;
>> > +
>> > +	/**
>> > +	 * @writeback_job: Writeback job for writeback connectors
>> > +	 *
>> > +	 * Holds the framebuffer for a writeback connector. As the writeback
>> > +	 * completion may be asynchronous to the normal commit cycle, the
>> > +	 * writeback job lifetime is managed separately from the normal atomic
>> > +	 * state by this object.
>> > +	 *
>> > +	 * See also: drm_writeback_queue_job() and
>> > +	 * drm_writeback_signal_completion()
>> > +	 */
>> > +	struct drm_writeback_job *writeback_job;
>> > };
>> >
>> > /**
>> > diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
>> > index 33b3a96d66d0a..9ca32bf7bf183 100644
>> > --- a/include/drm/drm_mode_config.h
>> > +++ b/include/drm/drm_mode_config.h
>> > @@ -779,6 +779,21 @@ struct drm_mode_config {
>> > 	 */
>> > 	struct drm_property *panel_orientation_property;
>> >
>> > +	/**
>> > +	 * @writeback_fb_id_property: Property for writeback connectors, storing
>> > +	 * the ID of the output framebuffer.
>> > +	 * See also: drm_writeback_connector_init()
>> > +	 */
>> > +	struct drm_property *writeback_fb_id_property;
>> > +
>> > +	/**
>> > +	 * @writeback_pixel_formats_property: Property for writeback connectors,
>> > +	 * storing an array of the supported pixel formats for the writeback
>> > +	 * engine (read-only).
>> > +	 * See also: drm_writeback_connector_init()
>> > +	 */
>> > +	struct drm_property *writeback_pixel_formats_property;
>> > +
>> > 	/* dumb ioctl parameters */
>> > 	uint32_t preferred_depth, prefer_shadow;
>> >
>> > diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
>> > index 35e2a3a79fc56..3b289773297c2 100644
>> > --- a/include/drm/drm_modeset_helper_vtables.h
>> > +++ b/include/drm/drm_modeset_helper_vtables.h
>> > @@ -974,6 +974,17 @@ struct drm_connector_helper_funcs {
>> > 	 */
>> > 	int (*atomic_check)(struct drm_connector *connector,
>> > 			    struct drm_connector_state *state);
>> > +
>> > +	/**
>> > +	 * @atomic_commit:
>> > +	 *
>> > +	 * This hook is to be used by drivers implementing writeback connectors
>> > +	 * that need a point when to commit the writeback job to the hardware.
>> > +	 *
>> > +	 * This callback is used by the atomic modeset helpers.
>> > +	 */
>> > +	void (*atomic_commit)(struct drm_connector *connector,
>> > +			      struct drm_writeback_job *writeback_job);
>> > };
>> >
>> > /**
>> > diff --git a/include/drm/drm_writeback.h b/include/drm/drm_writeback.h
>> > new file mode 100644
>> > index 0000000000000..499c5c7b0a54e
>> > --- /dev/null
>> > +++ b/include/drm/drm_writeback.h
>> > @@ -0,0 +1,90 @@
>> > +/*
>> > + * (C) COPYRIGHT 2016 ARM Limited. All rights reserved.
>> > + * Author: Brian Starkey <brian.starkey@arm.com>
>> > + *
>> > + * This program is free software and is provided to you under the terms of the
>> > + * GNU General Public License version 2 as published by the Free Software
>> > + * Foundation, and any use by you of this program is subject to the terms
>> > + * of such GNU licence.
>> > + */
>> > +
>> > +#ifndef __DRM_WRITEBACK_H__
>> > +#define __DRM_WRITEBACK_H__
>> > +#include <drm/drm_connector.h>
>> > +#include <drm/drm_encoder.h>
>> > +#include <linux/workqueue.h>
>> > +
>> > +struct drm_writeback_connector {
>> > +	struct drm_connector base;
>> > +
>> > +	/**
>> > +	 * @encoder: Internal encoder used by the connector to fulfill
>> > +	 * the DRM framework requirements. The users of the
>> > +	 * @drm_writeback_connector control the behaviour of the @encoder
>> > +	 * by passing the @enc_funcs parameter to drm_writeback_connector_init()
>> > +	 * function.
>> > +	 */
>> > +	struct drm_encoder encoder;
>> > +
>> > +	/**
>> > +	 * @pixel_formats_blob_ptr:
>> > +	 *
>> > +	 * DRM blob property data for the pixel formats list on writeback
>> > +	 * connectors
>> > +	 * See also drm_writeback_connector_init()
>> > +	 */
>> > +	struct drm_property_blob *pixel_formats_blob_ptr;
>> > +
>> > +	/** @job_lock: Protects job_queue */
>> > +	spinlock_t job_lock;
>> > +
>> > +	/**
>> > +	 * @job_queue:
>> > +	 *
>> > +	 * Holds a list of a connector's writeback jobs; the last item is the
>> > +	 * most recent. The first item may be either waiting for the hardware
>> > +	 * to begin writing, or currently being written.
>> > +	 *
>> > +	 * See also: drm_writeback_queue_job() and
>> > +	 * drm_writeback_signal_completion()
>> > +	 */
>> > +	struct list_head job_queue;
>> > +};
>> > +
>> > +struct drm_writeback_job {
>> > +	/**
>> > +	 * @cleanup_work:
>> > +	 *
>> > +	 * Used to allow drm_writeback_signal_completion to defer dropping the
>> > +	 * framebuffer reference to a workqueue.
>> > +	 */
>> > +	struct work_struct cleanup_work;
>> > +
>> > +	/**
>> > +	 * @list_entry:
>> > +	 *
>> > +	 * List item for the connector's @job_queue
>> > +	 */
>> > +	struct list_head list_entry;
>> > +
>> > +	/**
>> > +	 * @fb:
>> > +	 *
>> > +	 * Framebuffer to be written to by the writeback connector. Do not set
>> > +	 * directly, use drm_atomic_set_writeback_fb_for_connector()
>> > +	 */
>> > +	struct drm_framebuffer *fb;
>> > +};
>> > +
>> > +int drm_writeback_connector_init(struct drm_device *dev,
>> > +				 struct drm_writeback_connector *wb_connector,
>> > +				 const struct drm_connector_funcs *con_funcs,
>> > +				 const struct drm_encoder_helper_funcs *enc_helper_funcs,
>> > +				 const u32 *formats, int n_formats);
>> > +
>> > +void drm_writeback_queue_job(struct drm_writeback_connector *wb_connector,
>> > +			     struct drm_writeback_job *job);
>> > +
>> > +void drm_writeback_cleanup_job(struct drm_writeback_job *job);
>> > +void drm_writeback_signal_completion(struct drm_writeback_connector *wb_connector);
>> > +#endif
>> > diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
>> > index 4b3a1bb58e680..3e6541f0400b8 100644
>> > --- a/include/uapi/drm/drm_mode.h
>> > +++ b/include/uapi/drm/drm_mode.h
>> > @@ -344,6 +344,7 @@ enum drm_mode_subconnector {
>> > #define DRM_MODE_CONNECTOR_VIRTUAL      15
>> > #define DRM_MODE_CONNECTOR_DSI		16
>> > #define DRM_MODE_CONNECTOR_DPI		17
>> > +#define DRM_MODE_CONNECTOR_WRITEBACK	18
>> >
>> > struct drm_mode_get_connector {
>> >
>> > --
>> > 2.17.0
>> >
>
>-- 
>====================
>| I would like to |
>| fix the world,  |
>| but they're not |
>| giving me the   |
> \ source code!  /
>  ---------------
>    ¯\_(ツ)_/¯

Patch

diff --git a/Documentation/gpu/drm-kms.rst b/Documentation/gpu/drm-kms.rst
index 1dffd1ac4cd44..809d403087f95 100644
--- a/Documentation/gpu/drm-kms.rst
+++ b/Documentation/gpu/drm-kms.rst
@@ -373,6 +373,15 @@  Connector Functions Reference
 .. kernel-doc:: drivers/gpu/drm/drm_connector.c
    :export:
 
+Writeback Connectors
+--------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_writeback.c
+  :doc: overview
+
+.. kernel-doc:: drivers/gpu/drm/drm_writeback.c
+  :export:
+
 Encoder Abstraction
 ===================
 
diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
index ef9f3dab287fd..69c13517ea3a6 100644
--- a/drivers/gpu/drm/Makefile
+++ b/drivers/gpu/drm/Makefile
@@ -18,7 +18,7 @@  drm-y       :=	drm_auth.o drm_bufs.o drm_cache.o \
 		drm_encoder.o drm_mode_object.o drm_property.o \
 		drm_plane.o drm_color_mgmt.o drm_print.o \
 		drm_dumb_buffers.o drm_mode_config.o drm_vblank.o \
-		drm_syncobj.o drm_lease.o
+		drm_syncobj.o drm_lease.o drm_writeback.o
 
 drm-$(CONFIG_DRM_LIB_RANDOM) += lib/drm_random.o
 drm-$(CONFIG_DRM_VM) += drm_vm.o
diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
index 895741e9cd7db..e32a93931c0dc 100644
--- a/drivers/gpu/drm/drm_atomic.c
+++ b/drivers/gpu/drm/drm_atomic.c
@@ -30,6 +30,7 @@ 
 #include <drm/drm_atomic.h>
 #include <drm/drm_mode.h>
 #include <drm/drm_print.h>
+#include <drm/drm_writeback.h>
 #include <linux/sync_file.h>
 
 #include "drm_crtc_internal.h"
@@ -676,6 +677,45 @@  static void drm_atomic_crtc_print_state(struct drm_printer *p,
 		crtc->funcs->atomic_print_state(p, state);
 }
 
+/**
+ * drm_atomic_connector_check - check connector state
+ * @connector: connector to check
+ * @state: connector state to check
+ *
+ * Provides core sanity checks for connector state.
+ *
+ * RETURNS:
+ * Zero on success, error code on failure
+ */
+static int drm_atomic_connector_check(struct drm_connector *connector,
+		struct drm_connector_state *state)
+{
+	struct drm_crtc_state *crtc_state;
+	struct drm_writeback_job *writeback_job = state->writeback_job;
+
+	if ((connector->connector_type != DRM_MODE_CONNECTOR_WRITEBACK) || !writeback_job)
+		return 0;
+
+	if (writeback_job->fb && !state->crtc) {
+		DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] framebuffer without CRTC\n",
+				 connector->base.id, connector->name);
+		return -EINVAL;
+	}
+
+	if (state->crtc)
+		crtc_state = drm_atomic_get_existing_crtc_state(state->state,
+								state->crtc);
+
+	if (writeback_job->fb && !crtc_state->active) {
+		DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] has framebuffer, but [CRTC:%d] is off\n",
+				 connector->base.id, connector->name,
+				 state->crtc->base.id);
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 /**
  * drm_atomic_get_plane_state - get plane state
  * @state: global atomic state object
@@ -1286,6 +1326,12 @@  static int drm_atomic_connector_set_property(struct drm_connector *connector,
 			return -EINVAL;
 		}
 		state->content_protection = val;
+	} else if (property == config->writeback_fb_id_property) {
+		struct drm_framebuffer *fb = drm_framebuffer_lookup(dev, NULL, val);
+		int ret = drm_atomic_set_writeback_fb_for_connector(state, fb);
+		if (fb)
+			drm_framebuffer_unreference(fb);
+		return ret;
 	} else if (connector->funcs->atomic_set_property) {
 		return connector->funcs->atomic_set_property(connector,
 				state, property, val);
@@ -1367,6 +1413,9 @@  drm_atomic_connector_get_property(struct drm_connector *connector,
 		*val = state->scaling_mode;
 	} else if (property == connector->content_protection_property) {
 		*val = state->content_protection;
+	} else if (property == config->writeback_fb_id_property) {
+		/* Writeback framebuffer is one-shot, write and forget */
+		*val = 0;
 	} else if (connector->funcs->atomic_get_property) {
 		return connector->funcs->atomic_get_property(connector,
 				state, property, val);
@@ -1584,6 +1633,74 @@  drm_atomic_set_crtc_for_connector(struct drm_connector_state *conn_state,
 }
 EXPORT_SYMBOL(drm_atomic_set_crtc_for_connector);
 
+/*
+ * drm_atomic_get_writeback_job - return or allocate a writeback job
+ * @conn_state: Connector state to get the job for
+ *
+ * Writeback jobs have a different lifetime to the atomic state they are
+ * associated with. This convenience function takes care of allocating a job
+ * if there isn't yet one associated with the connector state, otherwise
+ * it just returns the existing job.
+ *
+ * Returns: The writeback job for the given connector state
+ */
+static struct drm_writeback_job *
+drm_atomic_get_writeback_job(struct drm_connector_state *conn_state)
+{
+	WARN_ON(conn_state->connector->connector_type != DRM_MODE_CONNECTOR_WRITEBACK);
+
+	if (!conn_state->writeback_job)
+		conn_state->writeback_job =
+			kzalloc(sizeof(*conn_state->writeback_job), GFP_KERNEL);
+
+	return conn_state->writeback_job;
+}
+
+/**
+ * drm_atomic_set_writeback_fb_for_connector - set writeback framebuffer
+ * @conn_state: atomic state object for the connector
+ * @fb: fb to use for the connector
+ *
+ * This is used to set the framebuffer for a writeback connector, which outputs
+ * to a buffer instead of an actual physical connector.
+ * Changing the assigned framebuffer requires us to grab a reference to the new
+ * fb and drop the reference to the old fb, if there is one. This function
+ * takes care of all these details besides updating the pointer in the
+ * state object itself.
+ *
+ * Note: The only way conn_state can already have an fb set is if the commit
+ * sets the property more than once.
+ *
+ * See also: drm_writeback_connector_init()
+ *
+ * Returns: 0 on success
+ */
+int drm_atomic_set_writeback_fb_for_connector(
+		struct drm_connector_state *conn_state,
+		struct drm_framebuffer *fb)
+{
+	struct drm_writeback_job *job =
+		drm_atomic_get_writeback_job(conn_state);
+	if (!job)
+		return -ENOMEM;
+
+	if (job->fb)
+		drm_framebuffer_unreference(job->fb);
+	if (fb)
+		drm_framebuffer_reference(fb);
+	job->fb = fb;
+
+	if (fb)
+		DRM_DEBUG_ATOMIC("Set [FB:%d] for connector state %p\n",
+				 fb->base.id, conn_state);
+	else
+		DRM_DEBUG_ATOMIC("Set [NOFB] for connector state %p\n",
+				 conn_state);
+
+	return 0;
+}
+EXPORT_SYMBOL(drm_atomic_set_writeback_fb_for_connector);
+
 /**
  * drm_atomic_add_affected_connectors - add connectors for crtc
  * @state: atomic state
@@ -1702,6 +1819,8 @@  int drm_atomic_check_only(struct drm_atomic_state *state)
 	struct drm_plane_state *plane_state;
 	struct drm_crtc *crtc;
 	struct drm_crtc_state *crtc_state;
+	struct drm_connector *conn;
+	struct drm_connector_state *conn_state;
 	int i, ret = 0;
 
 	DRM_DEBUG_ATOMIC("checking %p\n", state);
@@ -1724,6 +1843,15 @@  int drm_atomic_check_only(struct drm_atomic_state *state)
 		}
 	}
 
+	for_each_new_connector_in_state(state, conn, conn_state, i) {
+		ret = drm_atomic_connector_check(conn, conn_state);
+		if (ret) {
+			DRM_DEBUG_ATOMIC("[CONNECTOR:%d:%s] atomic core check failed\n",
+					 conn->base.id, conn->name);
+			return ret;
+		}
+	}
+
 	if (config->funcs->atomic_check) {
 		ret = config->funcs->atomic_check(state->dev, state);
 
diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index 130da5195f3b6..5fd12666f7d7a 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -30,6 +30,7 @@ 
 #include <drm/drm_plane_helper.h>
 #include <drm/drm_crtc_helper.h>
 #include <drm/drm_atomic_helper.h>
+#include <drm/drm_writeback.h>
 #include <linux/dma-fence.h>
 
 #include "drm_crtc_helper_internal.h"
@@ -1172,6 +1173,25 @@  void drm_atomic_helper_commit_modeset_disables(struct drm_device *dev,
 }
 EXPORT_SYMBOL(drm_atomic_helper_commit_modeset_disables);
 
+static void drm_atomic_helper_commit_writebacks(struct drm_device *dev,
+						struct drm_atomic_state *old_state)
+{
+	struct drm_connector *connector;
+	struct drm_connector_state *new_conn_state;
+	int i;
+
+	for_each_new_connector_in_state(old_state, connector, new_conn_state, i) {
+		const struct drm_connector_helper_funcs *funcs;
+
+		funcs = connector->helper_private;
+
+		if (new_conn_state->writeback_job && new_conn_state->writeback_job->fb) {
+			WARN_ON(connector->connector_type != DRM_MODE_CONNECTOR_WRITEBACK);
+			funcs->atomic_commit(connector, new_conn_state->writeback_job);
+		}
+	}
+}
+
 /**
  * drm_atomic_helper_commit_modeset_enables - modeset commit to enable outputs
  * @dev: DRM device
@@ -1251,6 +1271,8 @@  void drm_atomic_helper_commit_modeset_enables(struct drm_device *dev,
 
 		drm_bridge_enable(encoder->bridge);
 	}
+
+	drm_atomic_helper_commit_writebacks(dev, old_state);
 }
 EXPORT_SYMBOL(drm_atomic_helper_commit_modeset_enables);
 
@@ -3660,6 +3682,9 @@  __drm_atomic_helper_connector_duplicate_state(struct drm_connector *connector,
 	if (state->crtc)
 		drm_connector_get(connector);
 	state->commit = NULL;
+
+	/* Don't copy over a writeback job, they are used only once */
+	state->writeback_job = NULL;
 }
 EXPORT_SYMBOL(__drm_atomic_helper_connector_duplicate_state);
 
@@ -3789,6 +3814,11 @@  __drm_atomic_helper_connector_destroy_state(struct drm_connector_state *state)
 
 	if (state->commit)
 		drm_crtc_commit_put(state->commit);
+
+	if (state->writeback_job) {
+		drm_writeback_cleanup_job(state->writeback_job);
+		kfree(state->writeback_job);
+	}
 }
 EXPORT_SYMBOL(__drm_atomic_helper_connector_destroy_state);
 
diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
index 9b9ba5d5ec0cb..d031e46ebffcf 100644
--- a/drivers/gpu/drm/drm_connector.c
+++ b/drivers/gpu/drm/drm_connector.c
@@ -87,6 +87,7 @@  static struct drm_conn_prop_enum_list drm_connector_enum_list[] = {
 	{ DRM_MODE_CONNECTOR_VIRTUAL, "Virtual" },
 	{ DRM_MODE_CONNECTOR_DSI, "DSI" },
 	{ DRM_MODE_CONNECTOR_DPI, "DPI" },
+	{ DRM_MODE_CONNECTOR_WRITEBACK, "Writeback" },
 };
 
 void drm_connector_ida_init(void)
@@ -249,7 +250,8 @@  int drm_connector_init(struct drm_device *dev,
 	config->num_connector++;
 	spin_unlock_irq(&config->connector_list_lock);
 
-	if (connector_type != DRM_MODE_CONNECTOR_VIRTUAL)
+	if (connector_type != DRM_MODE_CONNECTOR_VIRTUAL &&
+	    connector_type != DRM_MODE_CONNECTOR_WRITEBACK)
 		drm_object_attach_property(&connector->base,
 					      config->edid_property,
 					      0);
diff --git a/drivers/gpu/drm/drm_writeback.c b/drivers/gpu/drm/drm_writeback.c
new file mode 100644
index 0000000000000..1141325b004ab
--- /dev/null
+++ b/drivers/gpu/drm/drm_writeback.c
@@ -0,0 +1,255 @@ 
+/*
+ * (C) COPYRIGHT 2016 ARM Limited. All rights reserved.
+ * Author: Brian Starkey <brian.starkey@arm.com>
+ *
+ * This program is free software and is provided to you under the terms of the
+ * GNU General Public License version 2 as published by the Free Software
+ * Foundation, and any use by you of this program is subject to the terms
+ * of such GNU licence.
+ */
+
+#include <drm/drm_crtc.h>
+#include <drm/drm_modeset_helper_vtables.h>
+#include <drm/drm_property.h>
+#include <drm/drm_writeback.h>
+#include <drm/drmP.h>
+
+/**
+ * DOC: overview
+ *
+ * Writeback connectors are used to expose hardware which can write the output
+ * from a CRTC to a memory buffer. They are used and act similarly to other
+ * types of connectors, with some important differences:
+ *  - Writeback connectors don't provide a way to output visually to the user.
+ *  - Writeback connectors should always report as "disconnected" (so that
+ *    clients which don't understand them will ignore them).
+ *  - Writeback connectors don't have EDID.
+ *
+ * A framebuffer may only be attached to a writeback connector when the
+ * connector is attached to a CRTC. The WRITEBACK_FB_ID property which sets the
+ * framebuffer applies only to a single commit (see below). A framebuffer may
+ * not be attached while the CRTC is off.
+ *
+ * Writeback connectors have some additional properties, which userspace
+ * can use to query and control them:
+ *
+ *  "WRITEBACK_FB_ID":
+ *	Write-only object property storing a DRM_MODE_OBJECT_FB: it stores the
+ *	framebuffer to be written by the writeback connector. This property is
+ *	similar to the FB_ID property on planes, but will always read as zero
+ *	and is not preserved across commits.
+ *	Userspace must set this property to an output buffer every time it
+ *	wishes the buffer to get filled.
+ *
+ *  "WRITEBACK_PIXEL_FORMATS":
+ *	Immutable blob property to store the supported pixel formats table. The
+ *	data is an array of u32 DRM_FORMAT_* fourcc values.
+ *	Userspace can use this blob to find out what pixel formats are supported
+ *	by the connector's writeback engine.
+ */
+
+static int create_writeback_properties(struct drm_device *dev)
+{
+	struct drm_property *prop;
+
+	if (!dev->mode_config.writeback_fb_id_property) {
+		prop = drm_property_create_object(dev, DRM_MODE_PROP_ATOMIC,
+						  "WRITEBACK_FB_ID",
+						  DRM_MODE_OBJECT_FB);
+		if (!prop)
+			return -ENOMEM;
+		dev->mode_config.writeback_fb_id_property = prop;
+	}
+
+	if (!dev->mode_config.writeback_pixel_formats_property) {
+		prop = drm_property_create(dev, DRM_MODE_PROP_BLOB |
+					   DRM_MODE_PROP_ATOMIC |
+					   DRM_MODE_PROP_IMMUTABLE,
+					   "WRITEBACK_PIXEL_FORMATS", 0);
+		if (!prop)
+			return -ENOMEM;
+		dev->mode_config.writeback_pixel_formats_property = prop;
+	}
+
+	return 0;
+}
+
+static const struct drm_encoder_funcs drm_writeback_encoder_funcs = {
+	.destroy = drm_encoder_cleanup,
+};
+
+/**
+ * drm_writeback_connector_init - Initialize a writeback connector and its properties
+ * @dev: DRM device
+ * @wb_connector: Writeback connector to initialize
+ * @con_funcs: Connector funcs vtable
+ * @enc_helper_funcs: Encoder helper funcs vtable to be used by the internal encoder
+ * @formats: Array of supported pixel formats for the writeback engine
+ * @n_formats: Length of the formats array
+ *
+ * This function creates the writeback-connector-specific properties if they
+ * have not been already created, initializes the connector as
+ * type DRM_MODE_CONNECTOR_WRITEBACK, and correctly initializes the property
+ * values. It will also create an internal encoder associated with the
+ * drm_writeback_connector and set it to use the @enc_helper_funcs vtable for
+ * the encoder helper.
+ *
+ * Drivers should always use this function instead of drm_connector_init() to
+ * set up writeback connectors.
+ *
+ * Returns: 0 on success, or a negative error code
+ */
+int drm_writeback_connector_init(struct drm_device *dev,
+				 struct drm_writeback_connector *wb_connector,
+				 const struct drm_connector_funcs *con_funcs,
+				 const struct drm_encoder_helper_funcs *enc_helper_funcs,
+				 const u32 *formats, int n_formats)
+{
+	struct drm_property_blob *blob;
+	struct drm_connector *connector = &wb_connector->base;
+	struct drm_mode_config *config = &dev->mode_config;
+	int ret = create_writeback_properties(dev);
+
+	if (ret != 0)
+		return ret;
+
+	blob = drm_property_create_blob(dev, n_formats * sizeof(*formats),
+					formats);
+	if (IS_ERR(blob))
+		return PTR_ERR(blob);
+
+	drm_encoder_helper_add(&wb_connector->encoder, enc_helper_funcs);
+	ret = drm_encoder_init(dev, &wb_connector->encoder,
+			       &drm_writeback_encoder_funcs,
+			       DRM_MODE_ENCODER_VIRTUAL, NULL);
+	if (ret)
+		goto fail;
+
+	connector->interlace_allowed = 0;
+
+	ret = drm_connector_init(dev, connector, con_funcs,
+				 DRM_MODE_CONNECTOR_WRITEBACK);
+	if (ret)
+		goto connector_fail;
+
+	ret = drm_mode_connector_attach_encoder(connector,
+						&wb_connector->encoder);
+	if (ret)
+		goto attach_fail;
+
+	INIT_LIST_HEAD(&wb_connector->job_queue);
+	spin_lock_init(&wb_connector->job_lock);
+
+	drm_object_attach_property(&connector->base,
+				   config->writeback_fb_id_property, 0);
+
+	drm_object_attach_property(&connector->base,
+				   config->writeback_pixel_formats_property,
+				   blob->base.id);
+	wb_connector->pixel_formats_blob_ptr = blob;
+
+	return 0;
+
+attach_fail:
+	drm_connector_cleanup(connector);
+connector_fail:
+	drm_encoder_cleanup(&wb_connector->encoder);
+fail:
+	drm_property_unreference_blob(blob);
+	return ret;
+}
+EXPORT_SYMBOL(drm_writeback_connector_init);
+
+/**
+ * drm_writeback_queue_job - Queue a writeback job for later signalling
+ * @wb_connector: The writeback connector to queue a job on
+ * @job: The job to queue
+ *
+ * This function adds a job to the job_queue for a writeback connector. It
+ * should be considered to take ownership of the writeback job, and so any other
+ * references to the job must be cleared after calling this function.
+ *
+ * Drivers must ensure that for a given writeback connector, jobs are queued in
+ * exactly the same order as they will be completed by the hardware (and
+ * signaled via drm_writeback_signal_completion).
+ *
+ * For every call to drm_writeback_queue_job() there must be exactly one call to
+ * drm_writeback_signal_completion()
+ *
+ * See also: drm_writeback_signal_completion()
+ */
+void drm_writeback_queue_job(struct drm_writeback_connector *wb_connector,
+			     struct drm_writeback_job *job)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&wb_connector->job_lock, flags);
+	list_add_tail(&job->list_entry, &wb_connector->job_queue);
+	spin_unlock_irqrestore(&wb_connector->job_lock, flags);
+}
+EXPORT_SYMBOL(drm_writeback_queue_job);
+
+/**
+ * drm_writeback_cleanup_job - Cleanup and free a writeback job
+ * @job: The writeback job to free
+ *
+ * Drops any references held by the writeback job, and frees the structure.
+ */
+void drm_writeback_cleanup_job(struct drm_writeback_job *job)
+{
+	if (job->fb)
+		drm_framebuffer_unreference(job->fb);
+}
+EXPORT_SYMBOL(drm_writeback_cleanup_job);
+
+/*
+ * @cleanup_work: deferred cleanup of a writeback job
+ *
+ * The job cannot be cleaned up directly in drm_writeback_signal_completion,
+ * because it may be called in interrupt context. Dropping the framebuffer
+ * reference can sleep, and so the cleanup is deferred to a workqueue.
+ */
+static void cleanup_work(struct work_struct *work)
+{
+	struct drm_writeback_job *job = container_of(work,
+						     struct drm_writeback_job,
+						     cleanup_work);
+	drm_writeback_cleanup_job(job);
+}
+
+/**
+ * drm_writeback_signal_completion - Signal the completion of a writeback job
+ * @wb_connector: The writeback connector whose job is complete
+ *
+ * Drivers should call this to signal the completion of a previously queued
+ * writeback job. It should be called as soon as possible after the hardware
+ * has finished writing, and may be called from interrupt context.
+ * It is the driver's responsibility to ensure that for a given connector, the
+ * hardware completes writeback jobs in the same order as they are queued.
+ *
+ * Unless the driver is holding its own reference to the framebuffer, it must
+ * not be accessed after calling this function.
+ *
+ * See also: drm_writeback_queue_job()
+ */
+void
+drm_writeback_signal_completion(struct drm_writeback_connector *wb_connector)
+{
+	unsigned long flags;
+	struct drm_writeback_job *job;
+
+	spin_lock_irqsave(&wb_connector->job_lock, flags);
+	job = list_first_entry_or_null(&wb_connector->job_queue,
+				       struct drm_writeback_job,
+				       list_entry);
+	if (job)
+		list_del(&job->list_entry);
+	spin_unlock_irqrestore(&wb_connector->job_lock, flags);
+
+	if (WARN_ON(!job))
+		return;
+
+	INIT_WORK(&job->cleanup_work, cleanup_work);
+	queue_work(system_long_wq, &job->cleanup_work);
+}
+EXPORT_SYMBOL(drm_writeback_signal_completion);
diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
index a57a8aa90ffb7..a5e904105db3a 100644
--- a/include/drm/drm_atomic.h
+++ b/include/drm/drm_atomic.h
@@ -594,6 +594,9 @@  void drm_atomic_set_fence_for_plane(struct drm_plane_state *plane_state,
 int __must_check
 drm_atomic_set_crtc_for_connector(struct drm_connector_state *conn_state,
 				  struct drm_crtc *crtc);
+int drm_atomic_set_writeback_fb_for_connector(
+		struct drm_connector_state *conn_state,
+		struct drm_framebuffer *fb);
 int __must_check
 drm_atomic_add_affected_connectors(struct drm_atomic_state *state,
 				   struct drm_crtc *crtc);
diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
index 675cc3f8cf852..60c0ffb947d5e 100644
--- a/include/drm/drm_connector.h
+++ b/include/drm/drm_connector.h
@@ -429,6 +429,19 @@  struct drm_connector_state {
 	 * protection. This is most commonly used for HDCP.
 	 */
 	unsigned int content_protection;
+
+	/**
+	 * @writeback_job: Writeback job for writeback connectors
+	 *
+	 * Holds the framebuffer for a writeback connector. As the writeback
+	 * completion may be asynchronous to the normal commit cycle, the
+	 * writeback job lifetime is managed separately from the normal atomic
+	 * state by this object.
+	 *
+	 * See also: drm_writeback_queue_job() and
+	 * drm_writeback_signal_completion()
+	 */
+	struct drm_writeback_job *writeback_job;
 };
 
 /**
diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
index 33b3a96d66d0a..9ca32bf7bf183 100644
--- a/include/drm/drm_mode_config.h
+++ b/include/drm/drm_mode_config.h
@@ -779,6 +779,21 @@  struct drm_mode_config {
 	 */
 	struct drm_property *panel_orientation_property;
 
+	/**
+	 * @writeback_fb_id_property: Property for writeback connectors, storing
+	 * the ID of the output framebuffer.
+	 * See also: drm_writeback_connector_init()
+	 */
+	struct drm_property *writeback_fb_id_property;
+
+	/**
+	 * @writeback_pixel_formats_property: Property for writeback connectors,
+	 * storing an array of the supported pixel formats for the writeback
+	 * engine (read-only).
+	 * See also: drm_writeback_connector_init()
+	 */
+	struct drm_property *writeback_pixel_formats_property;
+
 	/* dumb ioctl parameters */
 	uint32_t preferred_depth, prefer_shadow;
 
diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h
index 35e2a3a79fc56..3b289773297c2 100644
--- a/include/drm/drm_modeset_helper_vtables.h
+++ b/include/drm/drm_modeset_helper_vtables.h
@@ -974,6 +974,17 @@  struct drm_connector_helper_funcs {
 	 */
 	int (*atomic_check)(struct drm_connector *connector,
 			    struct drm_connector_state *state);
+
+	/**
+	 * @atomic_commit:
+	 *
+	 * This hook is to be used by drivers implementing writeback connectors
+	 * that need a point when to commit the writeback job to the hardware.
+	 *
+	 * This callback is used by the atomic modeset helpers.
+	 */
+	void (*atomic_commit)(struct drm_connector *connector,
+			      struct drm_writeback_job *writeback_job);
 };
 
 /**
diff --git a/include/drm/drm_writeback.h b/include/drm/drm_writeback.h
new file mode 100644
index 0000000000000..499c5c7b0a54e
--- /dev/null
+++ b/include/drm/drm_writeback.h
@@ -0,0 +1,90 @@ 
+/*
+ * (C) COPYRIGHT 2016 ARM Limited. All rights reserved.
+ * Author: Brian Starkey <brian.starkey@arm.com>
+ *
+ * This program is free software and is provided to you under the terms of the
+ * GNU General Public License version 2 as published by the Free Software
+ * Foundation, and any use by you of this program is subject to the terms
+ * of such GNU licence.
+ */
+
+#ifndef __DRM_WRITEBACK_H__
+#define __DRM_WRITEBACK_H__
+#include <drm/drm_connector.h>
+#include <drm/drm_encoder.h>
+#include <linux/workqueue.h>
+
+struct drm_writeback_connector {
+	struct drm_connector base;
+
+	/**
+	 * @encoder: Internal encoder used by the connector to fulfill
+	 * the DRM framework requirements. The users of the
+	 * @drm_writeback_connector control the behaviour of the @encoder
+	 * by passing the @enc_funcs parameter to drm_writeback_connector_init()
+	 * function.
+	 */
+	struct drm_encoder encoder;
+
+	/**
+	 * @pixel_formats_blob_ptr:
+	 *
+	 * DRM blob property data for the pixel formats list on writeback
+	 * connectors
+	 * See also drm_writeback_connector_init()
+	 */
+	struct drm_property_blob *pixel_formats_blob_ptr;
+
+	/** @job_lock: Protects job_queue */
+	spinlock_t job_lock;
+
+	/**
+	 * @job_queue:
+	 *
+	 * Holds a list of a connector's writeback jobs; the last item is the
+	 * most recent. The first item may be either waiting for the hardware
+	 * to begin writing, or currently being written.
+	 *
+	 * See also: drm_writeback_queue_job() and
+	 * drm_writeback_signal_completion()
+	 */
+	struct list_head job_queue;
+};
+
+struct drm_writeback_job {
+	/**
+	 * @cleanup_work:
+	 *
+	 * Used to allow drm_writeback_signal_completion to defer dropping the
+	 * framebuffer reference to a workqueue.
+	 */
+	struct work_struct cleanup_work;
+
+	/**
+	 * @list_entry:
+	 *
+	 * List item for the connector's @job_queue
+	 */
+	struct list_head list_entry;
+
+	/**
+	 * @fb:
+	 *
+	 * Framebuffer to be written to by the writeback connector. Do not set
+	 * directly, use drm_atomic_set_writeback_fb_for_connector()
+	 */
+	struct drm_framebuffer *fb;
+};
+
+int drm_writeback_connector_init(struct drm_device *dev,
+				 struct drm_writeback_connector *wb_connector,
+				 const struct drm_connector_funcs *con_funcs,
+				 const struct drm_encoder_helper_funcs *enc_helper_funcs,
+				 const u32 *formats, int n_formats);
+
+void drm_writeback_queue_job(struct drm_writeback_connector *wb_connector,
+			     struct drm_writeback_job *job);
+
+void drm_writeback_cleanup_job(struct drm_writeback_job *job);
+void drm_writeback_signal_completion(struct drm_writeback_connector *wb_connector);
+#endif
diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
index 4b3a1bb58e680..3e6541f0400b8 100644
--- a/include/uapi/drm/drm_mode.h
+++ b/include/uapi/drm/drm_mode.h
@@ -344,6 +344,7 @@  enum drm_mode_subconnector {
 #define DRM_MODE_CONNECTOR_VIRTUAL      15
 #define DRM_MODE_CONNECTOR_DSI		16
 #define DRM_MODE_CONNECTOR_DPI		17
+#define DRM_MODE_CONNECTOR_WRITEBACK	18
 
 struct drm_mode_get_connector {