[Mesa-dev] Question: st/mesa and context-shareable shaders

[Marek Olšák]

On Sun, Sep 27, 2015 at 10:41 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> On Sun, Sep 27, 2015 at 2:25 PM, Marek Olšák <maraeo at gmail.com> wrote:
>> On Sun, Sep 27, 2015 at 8:21 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
>>> On Sun, Sep 27, 2015 at 2:14 PM, Marek Olšák <maraeo at gmail.com> wrote:
>>>> Hi,
>>>>
>>>> For some reason, st/mesa assumes that shaders can't be shared by
>>>> multiple contexts and therefore has a context pointer in shader keys.
>>>> I'd like to get rid of this dependency, because there is no reason why
>>>> shaders would need to be tied to a context. In fact, shaders don't
>>>> even need a device, they just need a compiler.
>>>>
>>>> This is becoming a bigger issue with latest games that might prefer
>>>> compiling shaders in another thread using a different GL context.
>>>>
>>>> As far as I know, shaders should be context-shareable on all hardware
>>>> drivers. I see only 2 options out of this:
>>>>
>>>> 1) Removing the context pointer from the shader keys. If drivers need
>>>> this, they should handle it by themselves. This will simplify st/mesa,
>>>> because TCS, TES, GS won't need the variant-tracking machinery.
>>>>
>>>> 2) Adding PIPE_CAP_SHAREABLE_SHADERS and if the cap is 1, set the
>>>> context pointer to NULL in all keys.
>>>>
>>>> What do you think?
>>>
>>> FWIW I believe this should work for at least nv50 and nvc0, as they
>>> have a screen-shared code segment that all shaders get uploaded to.
>>> However I believe that it would be crucial to have a piglit test that
>>> exercises this functionality, as debugging this stuff esp in an actual
>>> game is extremely difficult.
>>
>> Who needs to debug it. If there's a regression, it's bisectable. After
>> that, you know your shaders aren't shareable and you need to take a
>> closer look at your code.
>
> OK, well, in that case I'm just going to set such a cap to 0 until a
> test appears -- I'm not sufficiently confident in how it all works to
> say that it's definitely fine on nouveau. There are enough bugs that
> failures from such an issue may not be easily bisected, or it may not
> even be clear that old versions worked.

Yeah, I'll probably add some test for simple sharing, i.e. one context
compiles it and another one uses it.

Marek