[Mesa-dev] [RFC] [PATCH 0/4] Add frequency declaration for vertex elements

[Alex Deucher]

On Mon, Aug 16, 2010 at 12:01 PM, Luca Barbieri <luca at luca-barbieri.com> wrote:
>> It seems like there is an alternate fix possible -- modify the mesa
>> fixed-function vertex program generator to put these constant values in
>> the constant buffer, rather than passing them as vertex data.  That
>> would remove the need for us to have this unique capability at the
>> gallium level that none of the other graphics abstractions seem to
>> consider necessary.
>>
>> I think we're putting more effort into accommodating the behaviour of
>> that code than really makes sense...
>
> I'm not totally sure about this.
> Right now drivers are supposed to support stride == 0, so this already
> works, but is not optimal for hardware where this causes repeated
> memory access.
>
> The change I'm proposing makes it easier to implement this optimally
> on hardware that has some "set constant vertex attribute" command
> (e.g. all nVidia cards), since the path taken by each element becomes
> known at CSO creation time.
>
> A shader/constant-based solution would be necessary for optimal
> performance on hardware that does repeated fetches on stride == 0 and
> has no such command, but might be detrimental on hardware that either
> has a "set constant" command, or internally optimizes for stride == 0.
>
> Note that it is possible to have both solutions: if a cap bit is set,
> then constant elements would be used, and otherwise the state tracker
> will modify the shader.
>
> It would be interesting to know what would be optimal for r300 and r600.

IIRC, the vertex fetch constants for r6xx+ accept 0 stride.

Alex