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

[Iago Toral]

On Tue, 2017-07-25 at 11:10 +0100, Lionel Landwerlin wrote:
> Cool, got it :)
> 
> Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>

Thanks!

> I think we might want this in stable right?

Yes, this and the first patch in the series too. I'll make sure to add
the CC when I push them.

> On 25/07/17 09:30, Iago Toral wrote:
> > 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
> > > 
> > > 
> 
>