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

[Daniel Vetter <daniel@ffwll.ch>]

On Thu, Nov 12, 2020 at 10:27:23AM -0800, Veera Sundaram Sankaran 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 uses the retire-fences
> as an alternative to vblank when frame-updates are in progress Set the
> fence timestamp during send vblank event to avoid discrepancies.

I think a reference to the exact source code in android that does this
would be really useful. Something in drm_hwcomposer or whatever is doing
this.

Aside from documenting why we want to do this I think this all looks
reasonable.
-Daniel

> 
> Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>
> ---
>  drivers/gpu/drm/drm_vblank.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index b18e1ef..b38e50c 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -24,6 +24,7 @@
>   * OTHER DEALINGS IN THE SOFTWARE.
>   */
>  
> +#include <linux/dma-fence.h>
>  #include <linux/export.h>
>  #include <linux/kthread.h>
>  #include <linux/moduleparam.h>
> @@ -999,6 +1000,14 @@ static void send_vblank_event(struct drm_device *dev,
>  		e->event.seq.time_ns = ktime_to_ns(now);
>  		break;
>  	}
> +
> +	/*
> +	 * update fence timestamp with the same vblank timestamp as both
> +	 * are signaled by the same event
> +	 */
> +	if (e->base.fence)
> +		e->base.fence->timestamp = now;
> +
>  	trace_drm_vblank_event_delivered(e->base.file_priv, e->pipe, seq);
>  	drm_send_event_locked(dev, &e->base);
>  }
> -- 
> 2.7.4
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch