[RESEND,v2,2/2] drm/drm_vblank: set the dma-fence timestamp during send_vblank_event

The explicit out-fences in crtc are signaled as part of vblank event,
indicating all framebuffers present on the Atomic Commit request are
scanned out on the screen. Though the fence signal and the vblank event
notification happens at the same time, triggered by the same hardware
vsync event, the timestamp set in both are different. With drivers
supporting precise vblank timestamp the difference between the two
timestamps would be even higher. This might have an impact on use-mode
frameworks using these fence timestamps for purposes other than simple
buffer usage. For instance, the Android framework [1] uses the
retire-fences as an alternative to vblank when frame-updates are in
progress. Set the fence timestamp during send vblank event using a new
drm_send_event_timestamp_locked variant to avoid discrepancies.

[1] https://android.googlesource.com/platform/frameworks/native/+/master/
services/surfaceflinger/Scheduler/Scheduler.cpp#397

Changes in v2:
- Use drm_send_event_timestamp_locked to update fence timestamp
- add more information to commit text

Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>
---
 drivers/gpu/drm/drm_file.c   | 43 +++++++++++++++++++++++++++++++++++++++++++
 drivers/gpu/drm/drm_vblank.c |  9 ++++++++-
 include/drm/drm_file.h       |  3 +++
 3 files changed, 54 insertions(+), 1 deletion(-)

On Thu, Jan 7, 2021 at 12:54 AM Veera Sundaram Sankaran
<veeras@codeaurora.org> wrote:
>
> The explicit out-fences in crtc are signaled as part of vblank event,
> indicating all framebuffers present on the Atomic Commit request are
> scanned out on the screen. Though the fence signal and the vblank event
> notification happens at the same time, triggered by the same hardware
> vsync event, the timestamp set in both are different. With drivers
> supporting precise vblank timestamp the difference between the two
> timestamps would be even higher. This might have an impact on use-mode
> frameworks using these fence timestamps for purposes other than simple
> buffer usage. For instance, the Android framework [1] uses the
> retire-fences as an alternative to vblank when frame-updates are in
> progress. Set the fence timestamp during send vblank event using a new
> drm_send_event_timestamp_locked variant to avoid discrepancies.
>
> [1] https://android.googlesource.com/platform/frameworks/native/+/master/
> services/surfaceflinger/Scheduler/Scheduler.cpp#397
>
> Changes in v2:
> - Use drm_send_event_timestamp_locked to update fence timestamp
> - add more information to commit text

Thanks for sending this out! One small note:

> @@ -775,6 +775,49 @@ void drm_event_cancel_free(struct drm_device *dev,
>  EXPORT_SYMBOL(drm_event_cancel_free);
>
>  /**
> + * drm_send_event_timestamp_locked - send DRM event to file descriptor
> + * @dev: DRM device
> + * @e: DRM event to deliver
> + * @timestamp: timestamp to set for the fence event
> + *
> + * This function sends the event @e, initialized with drm_event_reserve_init(),
> + * to its associated userspace DRM file. Callers must already hold
> + * &drm_device.event_lock, see drm_send_event() for the unlocked version.
> + *
> + * Note that the core will take care of unlinking and disarming events when the
> + * corresponding DRM file is closed. Drivers need not worry about whether the
> + * DRM file for this event still exists and can call this function upon
> + * completion of the asynchronous work unconditionally.
> + */
> +void drm_send_event_timestamp_locked(struct drm_device *dev,
> +                       struct drm_pending_event *e, ktime_t timestamp)
> +{
> +       assert_spin_locked(&dev->event_lock);
> +
> +       if (e->completion) {
> +               complete_all(e->completion);
> +               e->completion_release(e->completion);
> +               e->completion = NULL;
> +       }
> +
> +       if (e->fence) {
> +               dma_fence_signal_timestamp(e->fence, timestamp);
> +               dma_fence_put(e->fence);
> +       }
> +
> +       if (!e->file_priv) {
> +               kfree(e);
> +               return;
> +       }
> +
> +       list_del(&e->pending_link);
> +       list_add_tail(&e->link,
> +                     &e->file_priv->event_list);
> +       wake_up_interruptible(&e->file_priv->event_wait);
> +}
> +EXPORT_SYMBOL(drm_send_event_timestamp_locked);

This seems to duplicate much of drm_send_event_locked().  Should a
common backend function be used between them?

thanks
-john