[Mesa-stable] [PATCH 2/3] i965: Set subslice_total on Haswell.

[Kenneth Graunke]

On Thursday, June 9, 2016 1:34:15 PM PDT Francisco Jerez wrote:
> Kenneth Graunke <kenneth at whitecape.org> writes:
> 
> > We'll use this for compute shader thread counts shortly.
> >
> > Cc: "12.0" <mesa-stable at lists.freedesktop.org>
> > Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
> > ---
> >  src/mesa/drivers/dri/i965/intel_screen.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > I'm not sure whether I want to commit this or not...there still seem to
> > be some issues on Haswell.  I think this is right, but maybe there are
> > just other bugs.
> >
> Yeah, I believe the formula below should work for the time being until
> the kernel is fixed to support the right get-params on Gen7.  I wonder
> though what should we do on IVB?  AFAIK IVB GT2 had two subslices rather
> than one, but if you simply multiply the current max_cs_threads value by
> the number of subslices you'll go over the total thread count of the
> GPU.  The current max_cs_threads value for IVB GT2 seems bogus AFAICT,
> it's higher than the thread count per subslice (48?) but lower than the
> total thread count (96).  I wonder if barriers are broken on IVB right
> now for large enough workgroup size.

I think the Configurations[IVB] > Device Attributes[IVB] page has
incorrect information about Ivybridge GT2.  It claims there are 12 EUs
and 8 Threads/EU.  However, check the "Configurations Overview" page.
It claims that IVB GT2 has 16 total EUs.  The simulator also indicates
that IVB GT2 has 2 subslices (half slices), 8 EUs per half slice, and
8 Threads/EU.  This gives us 8 * 8 = 64, which is the value we use now.

Ugh.  This wouldn't be the first time the documentation's been wrong
in this area.  I've come to trust the simulator more.
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