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

Gert Wollny gert.wollny at collabora.com
Mon Jul 9 09:51:44 UTC 2018

Am Montag, den 09.07.2018, 09:54 +0200 schrieb Erik Faye-Lund:
> On 06. juli 2018 18:43, Roland Scheidegger wrote:
> > 
> > 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).
> I don't think I agree in this case; OpenGL 4.4 applications that use
> *exactly* strides of 2048probably won't even check the max-stride, as
> it's guaranteed to work by the standard anyway. Applications that use
> *more* might check for this value, but as the query was introduced in
> 4.4 it's likely that applications will just get a gl-error down an
> unexpected code-path, which can lead to the wrong attribute bound
> (possibly pointing to client-memory, which can lead to segfaults).
> Lying about the max attribute stride will "just" lead to rendering
> glitches.

FWIW: The value probably comes from the definition of
SQ_VTX_CONSTANT_WORD2_0 where a stride is defined as a value of 11 bits
 that is 0-2047, and when the value is send to the graphics card it
will be masked to use only these 11 bits, which means when an
applciation uses a stride of 2048 then the value zero will be uploaded.

Also I seem to remember that the blob advertised OpenGL 4.4 - now
whether they lied about it, had some special code path, or a value of
zero is actually interpreted as 2048 (which would actually make sense
to me), I don't know. - Well, when I get around to it, I'll write a
piglit that sees what happens in that case ... 


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