[Mesa-dev] [RFC PATCH] dri3: Sync after buffer swaps with glXSwapBuffers

[Michel Dänzer]

On 20/06/17 03:00 AM, Thomas Hellstrom wrote:
> On 06/19/2017 07:44 PM, Thomas Hellstrom wrote:
>> On 06/19/2017 07:26 PM, Eric Anholt wrote:
>>> Thomas Hellstrom <thellstrom at vmware.com> writes:
>>>
>>>> Applications calling glXSwapBuffers should be able to expect that any X
>>>> rendering submitted after the call to glXSwapBuffers returns should
>>>> be ordered
>>>> with respect to the glXSwapBuffers call. (For example piglit reading
>>>> out from
>>>> a window). This appears not to be the case at least with the current
>>>> server
>>>> side present extension implementation.
>>>>
>>>> Fixes piglit glx-multithread-texture on svga/vmwgfx.
>>> I don't believe that's true.  From the 1.4 spec:
>>>
>>> "glXSwapBuffers is in the X stream if and only if the display and
>>> drawable are not those belonging to the calling thread’s current
>>> context;"
>>>
>>> So if you want to sync X rendering after swap, you need glXWaitForGL().
>>
>> Hmm. You're right.
>>
>> Then I guess we need to fix both dri3's glXWaitGL to include the wait
>> for sbc and in addition fix piglit to call glXWaitGL.
>> Another complication is in the glXWaitGL man page, which states that
>> glFinish can be used instead of glXWaitGL and I guess that might be a
>> bit problematic.
>
> Actually, looking into the piglit code it uses glReadPixels. It doesn't
> read from X, which means the problem is probably that GL is occasionally
> reading from the front before the swap has been posted to the device. So
> the patch might be correct after all but the fix is in the fact it
> introduces a GL stream read barrier rather than an X barrier. But since
> it also introduces an X barrier, it corrects the glXWaitGL behavior as
> well.

While your patch fixes the problem, I think it's not quite correct (the
same problem could happen with target_msc != 0) and overkill at the same
time. Instead, we should wait for pending presentations to complete
before using the front buffer.


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