[Mesa-dev] [PATCH 1/7] gallium: add pipe_blend_state::srgb_enable and the CAP

[Marek Olšák]

On Tue, Jun 13, 2017 at 3:43 PM, Marek Olšák <maraeo at gmail.com> wrote:
> On Tue, Jun 13, 2017 at 1:40 PM, Jose Fonseca <jfonseca at vmware.com> wrote:
>> On 12/06/17 22:56, Marek Olšák wrote:
>>>
>>> On Mon, Jun 12, 2017 at 10:43 PM, Jose Fonseca <jfonseca at vmware.com>
>>> wrote:
>>>>
>>>> On 12/06/17 21:25, Marek Olšák wrote:
>>>>>
>>>>>
>>>>> On Mon, Jun 12, 2017 at 9:51 PM, Jose Fonseca <jfonseca at vmware.com>
>>>>> wrote:
>>>>>>
>>>>>>
>>>>>> How does this help exactly?
>>>>>>
>>>>>> Are applications actually rendering to the same FBO w/ and w/o SRGB
>>>>>> decoding?
>>>>>>
>>>>>> Or is the problem here GL_SRGB_WRITE state getting spuriously dirtied
>>>>>> by
>>>>>> the
>>>>>> application?
>>>>>>
>>>>>> And even if they do, why is toggling surface views in framebuffer state
>>>>>> so
>>>>>> expensive?
>>>>>>
>>>>>> I don't object per se, but it looks like an unusual thing to optimize
>>>>>> for.
>>>>>>
>>>>>
>>>>> set_framebuffer_state is basically a memory barrier. We have different
>>>>> caches between FB and textures and we have to flush them when a
>>>>> texture is unbound from the framebuffer and set as a sampler view. To
>>>>> keep thing simple, set_framebuffer_state is the barrier. When we
>>>>> change the blend state, the barrier is avoided. Note that the barrier
>>>>> makes set_framebuffer_state a function that is always GPU-bound.
>>>>
>>>>
>>>>
>>>> I see.
>>>>
>>>> And you're sure that the incoming set_framebuffer_state are not spurious?
>>>>
>>>> I know cso_context always eliminates redundant
>>>> pipe_context::set_framebuffer_state calls, but it is perhaps possible
>>>> that
>>>> Mesa state tracker is reseting the framebuffer state with different
>>>> surface
>>>> views, but that in practice are exactly the same as the previous one?
>>>>
>>>> Like I said, it seems odd apps are doing this: it doesn't make much sense
>>>> to
>>>> me to change colorspace of the fragments between draws. (Unless some of
>>>> the
>>>> assets are already in SRGB and the app is trying to be too smart for its
>>>> own
>>>> good to avoid the sRGB->RGB->sRGB.)  It seems much more likely that these
>>>> framebuffer state changes are self-inflicted some where in our stack,
>>>> than
>>>> something truly demanded by the app.
>>>>
>>>> And if that's the case and we can fix it, then it would be a better
>>>> solution
>>>> all around.
>>>
>>>
>>> Yeah the funny part and the reason is that we have a microbenchmark in
>>> piglit (drawoverhead) changing this state between draw calls. :)
>>>
>>> Marek
>>>
>>
>> I couldn't find that piglit microbenchmark.  mesademos has
>> src/perf/drawoverhead.c but it doesn't set GL_SRGB_WRITE.  So if fbo is
>> changing internally, then it's a perf bug in Mesa state tracker.
>>
>> Unless it's mimicking something that real apps do, then it's probably better
>> to fix the microbenchmark to use a more realistic tests.
>
> If you build piglit, it's in bin/drawoverhead.
>
> You're right that this subtest (switching GL_FRAMEBUFFER_SRGB) is
> rather artificial and fairly unlikely to occur with real apps.

FYI, I'm dropping this series and I don't have it in my repo anymore.
piglit/drawoverhead will be updated not to test this state change.

Marek