[Piglit] [PATCH 4/5] Test interaction between fast color clears and glXSwapBuffers.

[Eric Anholt]

Paul Berry <stereotype441 at gmail.com> writes:

> On 22 May 2013 16:20, Eric Anholt <eric at anholt.net> wrote:
>
>> Paul Berry <stereotype441 at gmail.com> writes:
>> > +/**
>> > + * \file glx-fast-color-clear.c
>> > + *
>> > + * On implementations that support fast color clear (e.g. i965/gen7+),
>> > + * verify that memory writes that were deferred as the result of a
>> > + * fast clear are properly resolved at the time of glXSwapBuffers.
>> > + *
>> > + * We can't use glReadPixels() to do this, since glReadPixels() also
>> > + * does a resolve, so instead we read the contents of the front buffer
>> > + * directly from X after the swap.
>> > + *
>> > + * Since glXSwapBuffers doesn't actually perform the swap (it simply
>> > + * queues it to happen later), the test needs to wait for the swap to
>> > + * occur before reading the front buffer.  If the implementation
>> > + * supports it, we do so using the GLX_INTEL_swap_event extension.
>> > + * Otherwise we sleep for 1 second to give time for the swap to occur.
>> > + */
>>
>> It seems to me like the test shouldn't have to do this. Particularly
>> this text:
>>
>> "glXSwapBuffers is in the OpenGL stream if and only if the display and
>> drawable are those belonging to the calling thread’s current context;
>> otherwise it is in the X stream."
>>
>> and
>>
>> "Subsequent OpenGL commands can be issued immediately, but will not be
>> executed until the buffer swapping has completed, typically during
>> vertical retrace of the display monitor."
>>
>> Our display and drawable are the calling thread's current context, so
>> the subsequent ReadPixels should be serialized against the result of the
>> swap.  I think this is a bug in the X Server, and maybe this is what's
>> affecting those other frontbuffer-reading tests.
>>
>
> But this test doesn't use ReadPixels (see the documentation above); it uses
> xcb_image_get().  Does that change your opinion about whether this is an X
> server bug?

I read the "can't just ReadPixels" as "can't just ReadPixels the back
buffer".

You should be able to serialize with XGetImage() by just doing
glXWaitGL(), but I think that's going to be broken in the way I was
seeing here.  Even if it's broken, you can wait for your swap to be done
much easier using GLX_OML_sync_control.

But even easier I think, you just want to glReadPixels on the front
buffer instead of the back, avoiding the need for using X to get the
front buffer pixels entirely.
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