[Mesa-dev] [PATCH 0/3] verify max vertex attrib stride

[Roland Scheidegger]

Am 06.07.2018 um 12:03 schrieb Erik Faye-Lund:
> OpenGL 4.4 and OpenGL ES 3.1 both require the maximum
> vertex attrib stride to be at least 2048. If this isn't
> the case, we shouldn't expose these API versions.
> 
> Unfortunately, the r600 driver only supports 2047. To
> avoid regressions in the supported GL version, the
> first patch modifies the returned value.
> 
> I'm not sure if that last one is a good idea or not, as
> it's strictly speaking non-conformant. But applications
> won't expect GL errors generated when using strides of
> 2048 either, which is what currently happens.
> 
> The initial motivation for this patch-series is to avoid
> exposing a too high spec version in virgl and then get
> dEQP failures when running on old hardware. The virgl
> specific bits are being sent separately, because they
> depend on some other not-yet-upstream things ATM.
> 
> Thoughts?
> 
> Erik Faye-Lund (3):
>   r600: report incorrect max-vertex-attrib for GL 4.4
>   mesa: verify MaxVertexAttribStride for GL 4.4
>   mesa: verify MaxVertexAttribStride for GLES 3.1
> 
>  src/gallium/drivers/r600/r600_pipe.c | 3 ++-
>  src/mesa/main/version.c              | 2 ++
>  2 files changed, 4 insertions(+), 1 deletion(-)
> 

Personally I think it's _much_ better to lie about the supported GL
version rather than the maximum vertex attrib stride (I don't know if
dEQP would actually have a test which tests for the max stride also
working than just being advertized, which would be way more relevant and
fail in any case).
(Because if you're going to use stride 2048, it is hugely unlikely you'd
just rely on GL 4.4 for that, hence it wouldn't work with older versions
of GL anyway neither, where ALL strides are just supposed to work.)

Roland

FWIW, I think it should be possible to work around that limitation on
r600. Since vertex fetch just uses a fetch shader, you could have a
shader key bit indicating strides of 2048, program a stride of 1024
instead and multiply the index to fetch by 2 in the shader. Of course
that could cause shader recompiles (never in practice...) and there's
some overhead associated with it, so might not be worth it...

Roland