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

[Marek Olšák]

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.

Marek