[Mesa-dev] [PATCH 2/5] st/dri: flush before throttling in SwapBuffers

[Marek Olšák]

On Tue, Apr 23, 2019 at 7:30 PM Marek Olšák <maraeo at gmail.com> wrote:

> On Tue, Apr 23, 2019 at 4:49 PM Kenneth Graunke <kenneth at whitecape.org>
> wrote:
>
>> On Monday, April 22, 2019 6:29:43 PM PDT Marek Olšák wrote:
>> > From: Marek Olšák <marek.olsak at amd.com>
>> >
>> > for better CPU-GPU parallelism
>> > ---
>> >  src/gallium/state_trackers/dri/dri_drawable.c | 20 +++++++++----------
>> >  1 file changed, 10 insertions(+), 10 deletions(-)
>> >
>> > diff --git a/src/gallium/state_trackers/dri/dri_drawable.c
>> b/src/gallium/state_trackers/dri/dri_drawable.c
>> > index 26bfdbecc53..c1de3bed9dd 100644
>> > --- a/src/gallium/state_trackers/dri/dri_drawable.c
>> > +++ b/src/gallium/state_trackers/dri/dri_drawable.c
>> > @@ -555,33 +555,33 @@ dri_flush(__DRIcontext *cPriv,
>> >         *
>> >         * This pulls a fence off the throttling queue and waits for it
>> if the
>> >         * number of fences on the throttling queue has reached the
>> desired
>> >         * number.
>> >         *
>> >         * Then flushes to insert a fence at the current rendering
>> position, and
>> >         * pushes that fence on the queue. This requires that the
>> st_context_iface
>> >         * flush method returns a fence even if there are no commands to
>> flush.
>> >         */
>> >        struct pipe_screen *screen = drawable->screen->base.screen;
>> > -      struct pipe_fence_handle *fence;
>> > +      struct pipe_fence_handle *oldest_fence, *new_fence = NULL;
>> >
>> > -      fence = swap_fences_pop_front(drawable);
>> > -      if (fence) {
>> > -         (void) screen->fence_finish(screen, NULL, fence,
>> PIPE_TIMEOUT_INFINITE);
>> > -         screen->fence_reference(screen, &fence, NULL);
>> > -      }
>> > +      st->flush(st, flush_flags, &new_fence);
>> >
>> > -      st->flush(st, flush_flags, &fence);
>> > +      oldest_fence = swap_fences_pop_front(drawable);
>> > +      if (oldest_fence) {
>> > +         screen->fence_finish(screen, NULL, oldest_fence,
>> PIPE_TIMEOUT_INFINITE);
>> > +         screen->fence_reference(screen, &oldest_fence, NULL);
>> > +      }
>> >
>> > -      if (fence) {
>> > -         swap_fences_push_back(drawable, fence);
>> > -         screen->fence_reference(screen, &fence, NULL);
>> > +      if (new_fence) {
>> > +         swap_fences_push_back(drawable, new_fence);
>> > +         screen->fence_reference(screen, &new_fence, NULL);
>> >        }
>> >     }
>> >     else if (flags & (__DRI2_FLUSH_DRAWABLE | __DRI2_FLUSH_CONTEXT)) {
>> >        st->flush(st, flush_flags, NULL);
>> >     }
>> >
>> >     if (drawable) {
>> >        drawable->flushing = FALSE;
>> >     }
>>
>> It seems like this will let us submit one more batch before throttling,
>> which is a little like increasing the throttle value from 2 to 3...but
>> not exactly.  I'm not sure I have an opinion on which is better...
>>
>
> "Wait then flush" adds latency (input lag) to the unflushed frame.
> "Flush then wait" doesn't add latency.
>

Actually, if it has to wait, there is already large input lag, so I guess
this patch doesn't change anything other than increasing the number. I'll
drop it.

Marek
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