[Mesa-dev] [PATCH v2] gbm/drm: Pick the oldest available buffer in get_back_bo

[Pekka Paalanen]

On Wed, 23 Nov 2016 16:40:42 -0600
Derek Foreman <derekf at osg.samsung.com> wrote:

> Applications may query the back buffer age to efficiently perform
> partial updates. Generally the application will keep a fixed length
> damage history, and use this to calculate what needs to be redrawn
> based on the age of the back buffer it's about to render to.
> 
> If presented with a buffer that has an age greater than the
> length of the damage history, the application will likely have
> to completely repaint the buffer.
> 
> Our current buffer selection strategy is to pick the first available
> buffer without considering its age.  If an application frequently
> manages to fit within two buffers but occasionally requires a third,
> this extra buffer will almost always be old enough to fall outside
> of a reasonably long damage history, and require a full repaint.
> 
> This patch changes the buffer selection behaviour to prefer the oldest
> available buffer.
> 
> By selecting the oldest available buffer, the application will likely
> always be able to use its damage history, at a cost of having to
> perform slightly more work every frame.  This is an improvement if
> the cost of a full repaint is heavy, and the surface damage between
> frames is relatively small.
> 
> It should be noted that since we don't currently trim our queue in
> any way, an application that briefly needs a large number of buffers
> will continue to receive older buffers than it would if it only ever
> needed two buffers.
> 
> Reviewed-by: Daniel Stone <daniels at collabora.com>
> Signed-off-by: Derek Foreman <derekf at osg.samsung.com>
> ---
> 
> The only changes are in the commit log, which hopefully better
> expresses the rationale for the change.

Hi,

yes, I'm happy with the caveats being documented now. Therefore:

Reviewed-by: Pekka Paalanen <pekka.paalanen at collabora.co.uk>


Thanks,
pq

>  src/egl/drivers/dri2/platform_drm.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/src/egl/drivers/dri2/platform_drm.c b/src/egl/drivers/dri2/platform_drm.c
> index 2099314..f812ab5 100644
> --- a/src/egl/drivers/dri2/platform_drm.c
> +++ b/src/egl/drivers/dri2/platform_drm.c
> @@ -215,13 +215,15 @@ get_back_bo(struct dri2_egl_surface *dri2_surf)
>     struct dri2_egl_display *dri2_dpy =
>        dri2_egl_display(dri2_surf->base.Resource.Display);
>     struct gbm_dri_surface *surf = dri2_surf->gbm_surf;
> +   int age = 0;
>     unsigned i;
>  
>     if (dri2_surf->back == NULL) {
>        for (i = 0; i < ARRAY_SIZE(dri2_surf->color_buffers); i++) {
> -	 if (!dri2_surf->color_buffers[i].locked) {
> +	 if (!dri2_surf->color_buffers[i].locked &&
> +	      dri2_surf->color_buffers[i].age >= age) {
>  	    dri2_surf->back = &dri2_surf->color_buffers[i];
> -	    break;
> +	    age = dri2_surf->color_buffers[i].age;
>  	 }
>        }
>     }

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