[2/2] drm/bridge: Add support for a virtual display bridge

This adds a very small and simple driver to read a virtual
display characteristic from the device tree and reflect it
back into DRM so a display driver in a virtual environment
knows how to configure its output.

This was created for the ARM RTSM aemv8a emulator as a way
forward to convert all ARM reference designs to use the
PL11x DRM driver.

Cc: Liviu Dudau <Liviu.Dudau@arm.com>
Cc: Ryan Harkin <ryan.harkin@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 drivers/gpu/drm/bridge/Kconfig           |  10 ++
 drivers/gpu/drm/bridge/Makefile          |   1 +
 drivers/gpu/drm/bridge/virtual-display.c | 186 +++++++++++++++++++++++
 3 files changed, 197 insertions(+)
 create mode 100644 drivers/gpu/drm/bridge/virtual-display.c

Hi Linus,

On Fri, Aug 24, 2018 at 02:23:32PM +0200, Linus Walleij wrote:
> This adds a very small and simple driver to read a virtual

.... and simple bridge driver ....

> display characteristic from the device tree and reflect it
> back into DRM so a display driver in a virtual environment
> knows how to configure its output.

It is the job of the encoder to configure itself as well, and I
think that for RTSM you need a virtual encoder as well.

> 
> This was created for the ARM RTSM aemv8a emulator as a way
> forward to convert all ARM reference designs to use the
> PL11x DRM driver.
> 
> Cc: Liviu Dudau <Liviu.Dudau@arm.com>
> Cc: Ryan Harkin <ryan.harkin@linaro.org>
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
> ---
>  drivers/gpu/drm/bridge/Kconfig           |  10 ++
>  drivers/gpu/drm/bridge/Makefile          |   1 +
>  drivers/gpu/drm/bridge/virtual-display.c | 186 +++++++++++++++++++++++
>  3 files changed, 197 insertions(+)
>  create mode 100644 drivers/gpu/drm/bridge/virtual-display.c
> 
> diff --git a/drivers/gpu/drm/bridge/Kconfig b/drivers/gpu/drm/bridge/Kconfig
> index fa2c7997e2fd..cfb61305b3f6 100644
> --- a/drivers/gpu/drm/bridge/Kconfig
> +++ b/drivers/gpu/drm/bridge/Kconfig
> @@ -126,6 +126,16 @@ config DRM_TI_TFP410
>  	---help---
>  	  Texas Instruments TFP410 DVI/HDMI Transmitter driver
>  
> +config DRM_VIRTUAL_DISPLAY_BRIDGE
> +	tristate "Virtual Display Bridge support"
> +	depends on OF
> +	select DRM_KMS_HELPER
> +	select VIDEOMODE_HELPERS
> +	help
> +	  Support for virtualized environments where the avilable
> +	  resolution is controlled by software configuration in
> +	  the device tree.
> +
>  source "drivers/gpu/drm/bridge/analogix/Kconfig"
>  
>  source "drivers/gpu/drm/bridge/adv7511/Kconfig"
> diff --git a/drivers/gpu/drm/bridge/Makefile b/drivers/gpu/drm/bridge/Makefile
> index 35f88d48ec20..2bdf67d98972 100644
> --- a/drivers/gpu/drm/bridge/Makefile
> +++ b/drivers/gpu/drm/bridge/Makefile
> @@ -14,4 +14,5 @@ obj-$(CONFIG_DRM_TOSHIBA_TC358767) += tc358767.o
>  obj-$(CONFIG_DRM_ANALOGIX_DP) += analogix/
>  obj-$(CONFIG_DRM_I2C_ADV7511) += adv7511/
>  obj-$(CONFIG_DRM_TI_TFP410) += ti-tfp410.o
> +obj-$(CONFIG_DRM_VIRTUAL_DISPLAY_BRIDGE) += virtual-display.o
>  obj-y += synopsys/
> diff --git a/drivers/gpu/drm/bridge/virtual-display.c b/drivers/gpu/drm/bridge/virtual-display.c
> new file mode 100644
> index 000000000000..ab55b3d6be8a
> --- /dev/null
> +++ b/drivers/gpu/drm/bridge/virtual-display.c
> @@ -0,0 +1,186 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * (C) Copyright 2018 Linus Walleij <linus.walleij@linaro.org>
> + */
> +
> +#include <linux/module.h>
> +#include <drm/drmP.h>
> +#include <drm/drm_atomic_helper.h>
> +#include <drm/drm_crtc_helper.h>
> +#include <drm/drm_modes.h>
> +#include <video/of_display_timing.h>
> +
> +struct virtenc {

I think the name you picked is going to be confusing, as this is not an
encoder at all, but a virtual connector.

> +	struct device *dev;
> +	struct drm_device *drm;
> +	struct drm_bridge bridge;
> +	struct drm_connector connector;
> +	struct drm_display_mode mode;
> +	u32 bus_flags;
> +};
> +
> +static inline struct virtenc *bridge_to_virtenc(struct drm_bridge *bridge)
> +{
> +	return container_of(bridge, struct virtenc, bridge);
> +}
> +
> +static inline struct virtenc *connector_to_virtenc(struct drm_connector *con)
> +{
> +	return container_of(con, struct virtenc, connector);
> +}
> +
> +static enum drm_connector_status
> +virtenc_connector_detect(struct drm_connector *connector, bool force)
> +{
> +	return connector_status_connected;
> +}
> +
> +static const struct drm_connector_funcs virtenc_connector_funcs = {
> +	.detect = virtenc_connector_detect,
> +	.fill_modes = drm_helper_probe_single_connector_modes,
> +	.destroy = drm_connector_cleanup,
> +	.reset = drm_atomic_helper_connector_reset,
> +	.atomic_duplicate_state = drm_atomic_helper_connector_duplicate_state,
> +	.atomic_destroy_state = drm_atomic_helper_connector_destroy_state,
> +};
> +
> +static int virtenc_get_modes(struct drm_connector *connector)
> +{
> +	struct virtenc *virtenc = connector_to_virtenc(connector);
> +	struct drm_display_mode *mode = drm_mode_create(virtenc->drm);
> +	u32 bus_format = MEDIA_BUS_FMT_RGB888_1X24;
> +	int ret;
> +
> +	drm_mode_copy(mode, &virtenc->mode);
> +	mode->type = DRM_MODE_TYPE_DRIVER | DRM_MODE_TYPE_PREFERRED;
> +	mode->width_mm = 80;
> +	mode->height_mm = 60;
> +	drm_mode_set_name(mode);
> +
> +	drm_mode_probed_add(connector, mode);
> +	ret = drm_display_info_set_bus_formats(&connector->display_info,
> +					       &bus_format, 1);
> +	if (ret)
> +		return ret;
> +
> +	return 1;
> +}
> +
> +static enum drm_mode_status virtenc_mode_valid(struct drm_connector *connector,
> +					       struct drm_display_mode *mode)
> +{
> +	return MODE_OK;
> +}
> +
> +static const struct drm_connector_helper_funcs
> +virtenc_connector_helper_funcs = {
> +	.get_modes = virtenc_get_modes,
> +	.mode_valid = virtenc_mode_valid,
> +};
> +
> +static void virtenc_bridge_disable(struct drm_bridge *bridge)
> +{
> +}
> +
> +static void virtenc_bridge_enable(struct drm_bridge *bridge)
> +{
> +}

I don't think you need these enable/disable hooks, the drm_bridge.c code
seems to be checking if they are present and skips them if they're not.

> +
> +static void virtenc_bridge_mode_set(struct drm_bridge *bridge,
> +				    struct drm_display_mode *mode,
> +				    struct drm_display_mode *adj)
> +{
> +}

Same for this one, AFAICT.

> +
> +static int virtenc_bridge_attach(struct drm_bridge *bridge)
> +{
> +	struct virtenc *virtenc = bridge_to_virtenc(bridge);
> +	struct drm_device *drm = bridge->dev;
> +	int ret;
> +
> +	virtenc->drm = drm;
> +	drm_connector_helper_add(&virtenc->connector,
> +				 &virtenc_connector_helper_funcs);
> +
> +	if (!drm_core_check_feature(drm, DRIVER_ATOMIC)) {
> +		dev_err(virtenc->dev,
> +			"Virtual Display bridge driver is only compatible with DRM devices supporting atomic updates\n");
> +		return -ENOTSUPP;
> +	}
> +
> +	ret = drm_connector_init(drm, &virtenc->connector,
> +				 &virtenc_connector_funcs,
> +				 DRM_MODE_CONNECTOR_VIRTUAL);
> +	if (ret)
> +		return ret;
> +
> +	virtenc->connector.polled = DRM_CONNECTOR_POLL_CONNECT;
> +
> +	drm_mode_connector_attach_encoder(&virtenc->connector, bridge->encoder);
> +
> +	return 0;
> +}
> +
> +static const struct drm_bridge_funcs virtenc_bridge_funcs = {
> +	.attach = virtenc_bridge_attach,
> +	.mode_set = virtenc_bridge_mode_set,
> +	.disable = virtenc_bridge_disable,
> +	.enable = virtenc_bridge_enable,
> +};
> +
> +static int virtenc_probe(struct platform_device *pdev)
> +{
> +	struct device *dev = &pdev->dev;
> +	struct device_node *np = dev->of_node;
> +	struct virtenc *virtenc;
> +	int ret;
> +
> +	virtenc = devm_kzalloc(dev, sizeof(*virtenc), GFP_KERNEL);
> +	if (!virtenc)
> +		return -ENOMEM;
> +
> +	ret = of_get_drm_display_mode(np, &virtenc->mode,
> +				      &virtenc->bus_flags,
> +				      0);
> +	if (ret)
> +		return ret;
> +
> +	virtenc->dev = dev;
> +	virtenc->bridge.funcs = &virtenc_bridge_funcs;
> +	virtenc->bridge.of_node = dev->of_node;
> +	drm_bridge_add(&virtenc->bridge);
> +	platform_set_drvdata(pdev, virtenc);
> +	dev_info(dev, "added virtual display bridge\n");
> +
> +	return 0;
> +}
> +
> +static int virtenc_remove(struct platform_device *pdev)
> +
> +{
> +	struct virtenc *virtenc = platform_get_drvdata(pdev);
> +
> +	drm_bridge_remove(&virtenc->bridge);
> +
> +	return 0;
> +}
> +
> +static const struct of_device_id virtenc_dt_ids[] = {
> +	{ .compatible = "virtual-display-bridge", },
> +	{ }
> +};
> +MODULE_DEVICE_TABLE(of, virtenc_dt_ids);
> +
> +static struct platform_driver virtenc_driver = {
> +	.driver = {
> +		.name = "virtenc",
> +		.of_match_table = virtenc_dt_ids,
> +	},
> +	.probe = virtenc_probe,
> +	.remove = virtenc_remove,
> +};
> +module_platform_driver(virtenc_driver);
> +
> +MODULE_AUTHOR("Linus Walleij <linus.walleij@linaro.org>");
> +MODULE_DESCRIPTION("Virtual Display Bridge");
> +MODULE_LICENSE("GPL");
> -- 
> 2.17.1
> 

I need to check how your driver behaves compared with my old virtual
encoder driver but at a glance things look OK to me.

Best regards,
Liviu