[Mesa-dev] [PATCH 2/3] anv: only expose up to 28 vertex attributes

[Iago Toral]

On Tue, 2017-07-25 at 00:46 +0100, Lionel Landwerlin wrote:
> On 21/07/17 09:26, Iago Toral Quiroga wrote:
> > We were exposing 31 because the hardware allows for 33 and we need
> > to reserve one vertex buffer to upload VertexIndex/InstanceIndex
> > and
> > another to upload DrawID when needed, however, there is another
> > restriction in the hardware regarding the size of the VUE entries
> > that limits this down to 30 (28 after VertexIndex and DrawID):
> 
> Something's weird here in the description from the PRMs (sorry, not
> your 
> fault).
> EU limit of 128 GRFs should allow 32 vertex elements of 4 GRFs.
> Looking at the PRMs, it seems the limits arises from the maximum
> value 
> allowed in "Vertex URB Entry Read Length" in SIMD8, which is 15.
> And 15 * 8 = 120 gives us a limit to 30 vertex elements.
> 
> Is that a fair description?

Yeah, I copied the description from the PRMs but you are right, the
limit comes from the vertex urb entry size. I'll update the commit log.
Thanks!

> >  Than
> >  From 3DSTATE_VS, Vertex URB entry read length:
> > 
> >    "For SIMD8 dispatch, each vertex element requires 4 GRFs of
> >     payload data, therefore the number of GRFs with vertex data
> > will
> >     be 8 times the value programmed in this field. The EU limit of
> > 128
> >     GRFs imposes a maximum limit of 30 elements per vertex pushed
> > into
> >     the payload, though the pactical limit may be lower."
> > 
> > There is an assertion in place to check that we don't exceed this
> > limit
> > and that is triggered when we expose more than 28 attributes and we
> > have
> > shaders that use more than 30 attributes in total (including
> > VertexIndex/InstanceIndex and DrawID).
> > ---
> >   src/intel/vulkan/anv_private.h | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/src/intel/vulkan/anv_private.h
> > b/src/intel/vulkan/anv_private.h
> > index 4dce360c76..f83f49b671 100644
> > --- a/src/intel/vulkan/anv_private.h
> > +++ b/src/intel/vulkan/anv_private.h
> > @@ -89,7 +89,7 @@ struct gen_l3_config;
> >    */
> >   #define ANV_HZ_FC_VAL 1.0f
> >   
> > -#define MAX_VBS         31
> > +#define MAX_VBS         28
> >   #define MAX_SETS         8
> >   #define MAX_RTS          8
> >   #define MAX_VIEWPORTS   16
> 
> 
>