[PATCH xserver] glamor: Limit outstanding drawing queue with glClientWaitSync

[Eric Anholt]

Keith Packard <keithp at keithp.com> writes:

> Michel Dänzer <michel at daenzer.net> writes:
>
>> Is this about x11perf by any chance? I was seeing long lag with that as
>> well, which turned out to be because we weren't correctly synchronizing
>> with the hardware for XGetImage.
>
> x11perf only does XGetImage once per run of the test; without some
> intra-test queue management, other applications are unusable while the
> test is running. This particular case isn't terribly interesting, but as
> we improve glamor to need fewer and fewer synchronizations between CPU
> and GPU, this should become relevant for interactions between regular
> applications.
>
>> In the amdgpu driver we now have a GPU scheduler managing the userspace
>> command submissions.
>
> That does seem like a fine place to manage this; right now, the intel
> driver appears to only do this in SwapBuffers.

vc4 just waits until you don't have too many (5, arbitrarily) execs
outstanding before submitting another one.  I think this is a problem of
the Intel driver, not glamor.  We ran into it with Intel on cairo-gl,
too, and I should have just fixed it in the driver back then.
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