[Mesa-dev] [PATCH] loader/dri3: Always use at least two back buffers

[Michel Dänzer]

On 06/09/16 12:08 PM, Jason Ekstrand wrote:
> On Mon, Sep 5, 2016 at 7:39 PM, Michel Dänzer <michel at daenzer.net
> <mailto:michel at daenzer.net>> wrote:
> 
>     From: Michel Dänzer <michel.daenzer at amd.com
>     <mailto:michel.daenzer at amd.com>>
> 
>     This can make a significant difference for performance with some extreme
>     test cases such as vblank_mode=0 glxgears.
> 
>     Fixes: 1e3218bc5ba2 ("loader/dri3: Overhaul dri3_update_num_back")
>     Cc: "12.0 11.2" <mesa-stable at lists.freedesktop.org
>     <mailto:mesa-stable at lists.freedesktop.org>>
>     Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97549
>     <https://bugs.freedesktop.org/show_bug.cgi?id=97549>
>     Signed-off-by: Michel Dänzer <michel.daenzer at amd.com
>     <mailto:michel.daenzer at amd.com>>
>     ---
> 
>     I could swear I tested vblank_mode=0 glxgears with my previous change
>     and couldn't measure any difference, but I can now, so I must have
>     messed up my previous testing somehow... Apologies for any inconvenience
>     this caused.
> 
>      src/loader/loader_dri3_helper.c | 4 +---
>      1 file changed, 1 insertion(+), 3 deletions(-)
> 
>     diff --git a/src/loader/loader_dri3_helper.c
>     b/src/loader/loader_dri3_helper.c
>     index 86ae5ae..3ce0352 100644
>     --- a/src/loader/loader_dri3_helper.c
>     +++ b/src/loader/loader_dri3_helper.c
>     @@ -70,10 +70,8 @@ dri3_update_num_back(struct loader_dri3_drawable
>     *draw)
>      {
>         if (draw->flipping)
>            draw->num_back = 3;
>     -   else if (draw->vtable->get_swap_interval(draw) != 0)
>     -      draw->num_back = 2;
>         else
>     -      draw->num_back = 1;
>     +      draw->num_back = 2;
> 
> 
> With this change, the function is logically identical to the old
> function with the async flipping case removed.

Not sure what you mean exactly, but I hope we can agree that the code is
easier to understand and reason about now. :)


> I'm still not sure why we aren't getting a stall with 3 butters in the
> swapinterval=0 flipping case, but if it doesn't have a perf impact, it must
> not be a problem.

Right, there is a stall, but it doesn't seem significant, at least on my
setup.


> Reviewed-by: Jason Ekstrand <jason at jlekstrand.net
> <mailto:jason at jlekstrand.net>>

Thanks!


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