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

[Luca Barbieri]

> 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.