[Mesa-dev] [PATCH] loader/dri3: Overhaul dri3_update_num_back

[Michel Dänzer]

On 20/08/16 04:42 AM, Eric Anholt wrote:
> Michel Dänzer <michel at daenzer.net> writes:
> 
>> From: Michel Dänzer <michel.daenzer at amd.com>
>>
>> Always use 3 buffers when flipping. With only 2 buffers, we have to wait
>> for a flip to complete (which takes non-0 time even with asynchronous
>> flips) before we can start working on the next frame. We were previously
>> only using 2 buffers for flipping if the X server supports asynchronous
>> flips, even when we're not using asynchronous flips. This could result
>> in bad performance (the referenced bug report is an extreme case, where
>> the inter-frame stalls were preventing the GPU from reaching its maximum
>> clocks).
>>
>> I couldn't measure any performance boost using 4 buffers with flipping.
>> Performance actually seemed to go down slightly, but that might have
>> been just noise.
>>
>> Without flipping, a single back buffer is enough for swap interval 0,
>> but we need to use 2 back buffers when the swap interval is non-0,
>> otherwise we have to wait for the swap interval to pass before we can
>> start working on the next frame. This condition was previously reversed.
>>
>> Cc: "12.0 11.2" <mesa-stable at lists.freedesktop.org>
>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97260
>> Signed-off-by: Michel Dänzer <michel.daenzer at amd.com>
> 
> Reviewed-by: Eric Anholt <eric at anholt.net>

Thanks.

Note that four piglit EGL tests fail with this change, but AFAICT all of
those tests are broken. I submitted fixes for three of them:

https://patchwork.freedesktop.org/patch/106809/
https://patchwork.freedesktop.org/patch/106807/
https://patchwork.freedesktop.org/patch/106808/

The last one is egl_chromium_sync_control. It calls eglSwapBuffers twice
in a row and expects that the SBC is higher than before immediately
after that. I.e. it expects that there is only one back buffer, so that
at least the second eglSwapBuffers has to block until the first swap
finishes. Which is no longer true with this DRI3 change. Not sure what
to do about this one.


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

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