Re: [PATCH 07/11] drm/i915: Use subslice stride to set subslices for a given slice

["Summers, Stuart" <stuart.summers@intel.com>]

On Wed, 2019-08-21 at 23:36 +0100, Chris Wilson wrote:
> Quoting Stuart Summers (2019-08-21 00:05:40)
> > Add a subslice stride calculation when setting subslices. This
> > aligns more closely with the userspace expectation of the subslice
> > mask structure.
> > 
> > v2: Use local variable for subslice_mask on HSW and
> >     clean up a few other subslice_mask local variable
> >     changes
> > v3: Add GEM_BUG_ON for ss_stride to prevent array overflow (Chris)
> >     Split main set function and refactors in intel_device_info.c
> >     into separate patches (Chris)
> > 
> > Signed-off-by: Stuart Summers <stuart.summers@intel.com>
> > ---
> >  drivers/gpu/drm/i915/gt/intel_sseu.c | 12 ++++++++++--
> >  drivers/gpu/drm/i915/gt/intel_sseu.h |  2 +-
> >  2 files changed, 11 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/intel_sseu.c
> > b/drivers/gpu/drm/i915/gt/intel_sseu.c
> > index 79a9b5f186f9..5d537ec97fcc 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_sseu.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_sseu.c
> > @@ -31,9 +31,17 @@ intel_sseu_subslice_total(const struct
> > sseu_dev_info *sseu)
> >  }
> >  
> >  void intel_sseu_set_subslices(struct sseu_dev_info *sseu, int
> > slice,
> > -                             u8 ss_mask)
> > +                             u32 ss_mask)
> >  {
> > -       sseu->subslice_mask[i] = ss_mask & 0xff;
> > +       int i, offset;
> > +
> > +       GEM_BUG_ON(sseu->ss_stride > 32);
> > +
> > +       offset = slice * sseu->ss_stride;
> > +
> > +       for (i = 0; i < sseu->ss_stride; i++)
> 
> You are saying that i can be a maximum of 32, and
> u32 >> (8 * 32) is legal?

Yeah the stride is calculated as a div-by-8, so really this should be
max of 4, not 32. I'll fix this and repost.

> 
> > +               sseu->subslice_mask[offset + i] =
> > +                       (ss_mask >> (BITS_PER_BYTE * i)) & 0xff;
> 
> The 0xff is, and was, superfluous. Shrug.

I agree in the previous patch this wasn't really interesting. I can
remove that. But here, IMO, having the explicit AND makes this clearer.

Thanks,
Stuart

> -Chris
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