[Mesa-dev] [PATCH] st/dri: decrease input lag by syncing sooner in SwapBuffers

[Michel Dänzer]

On 2019-04-26 4:06 a.m., Marek Olšák wrote:
> From: Marek Olšák <marek.olsak at amd.com>
> 
> It's done by:
> - decrease the number of frames in flight by 1
> - flush before throttling in SwapBuffers
>   (instead of wait-then-flush, do flush-then-wait)
> 
> The improvement is apparent with Unigine Heaven.
> 
> Previously:
>     draw frame 2
>     wait frame 0
>     flush frame 2
>     present frame 2
> 
>     The input lag is 2 frames.
> 
> Now:
>     draw frame 2
>     flush frame 2
>     wait frame 1
>     present frame 2
> 
>     The input lag is 1 frame. Flushing is done before waiting, because
>     otherwise the device would be idle after waiting.

Nice idea. Not sure offhand about all ramifications, but certainly worth
a go.


> Nine is affected because it also uses the pipe cap.
> ---
>  src/gallium/auxiliary/util/u_screen.c         |  2 +-
>  src/gallium/state_trackers/dri/dri_drawable.c | 20 +++++++++----------
>  2 files changed, 11 insertions(+), 11 deletions(-)
> 
> diff --git a/src/gallium/auxiliary/util/u_screen.c b/src/gallium/auxiliary/util/u_screen.c
> index 27f51e0898e..410f17421e6 100644
> --- a/src/gallium/auxiliary/util/u_screen.c
> +++ b/src/gallium/auxiliary/util/u_screen.c
> @@ -349,21 +349,21 @@ u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
>     case PIPE_CAP_MAX_VARYINGS:
>        return 8;
>  
>     case PIPE_CAP_COMPUTE_GRID_INFO_LAST_BLOCK:
>        return 0;
>  
>     case PIPE_CAP_COMPUTE_SHADER_DERIVATIVES:
>        return 0;
>  
>     case PIPE_CAP_MAX_FRAMES_IN_FLIGHT:
> -      return 2;
> +      return 1;

This might be slightly misleading, as there can still be two frames in
flight (on the GPU) at the same time. Might be better to leave this at 2
(so Nine isn't affected) and adjust its treatment in
src/gallium/state_trackers/dri/dri_drawable.c .


-- 
Earthling Michel Dänzer               |              https://www.amd.com
Libre software enthusiast             |             Mesa and X developer