[Intel-gfx] [PATCH] drm/i915: Fix off-by-one in looking up icl sseu slice

[Chris Wilson]

Quoting Summers, Stuart (2019-05-28 21:45:05)
> On Tue, 2019-05-28 at 21:06 +0100, Chris Wilson wrote:
> > We want the index corresponding to the set bit but fls() returns the
> > 1-index value.
> > 
> > Otherwise, we trigger the sanitycheck
> >       intel_sseu_get_subslices:46 GEM_BUG_ON(slice >= sseu-
> > >max_slices)
> > when we look up the invalid slice.
> > 
> > The only remaining question then is just how reliable the rest of
> > intel_calculate_mcr_s_ss_select() is -- how many more of those fls()
> > are
> > also off-by-one.
> > 
> > Fixes: 1ac159e23c2c ("drm/i915: Expand subslice mask")
> > Fixes: 1e40d4aea57b ("drm/i915/cnl: Implement
> > WaProgramMgsrForCorrectSliceSpecificMmioReads")
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
> > Cc: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> > Cc: Stuart Summers <stuart.summers at intel.com>
> > Cc: Manasi Navare <manasi.d.navare at intel.com>
> > ---
> >  drivers/gpu/drm/i915/gt/intel_workarounds.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/intel_workarounds.c
> > b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> > index fbc853085809..485cd1c8ecc4 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_workarounds.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_workarounds.c
> > @@ -781,7 +781,7 @@ wa_init_mcr(struct drm_i915_private *i915, struct
> > i915_wa_list *wal)
> >                * read FUSE3 for enabled L3 Bank IDs, if L3 Bank
> > matches
> >                * enabled subslice, no need to redirect MCR packet
> >                */
> > -             u32 slice = fls(sseu->slice_mask);
> > +             u32 slice = __fls(sseu->slice_mask);
> 
> The condition around this (is_power_of_2) makes sure we meet the case
> where the slice_mask is uninitialized. This is going to work here, but
> might not work in all other places. If we propagate this change to the
> other places we call fls(slice_mask), which I'd recommend, we'll want
> to make sure we check for that.
> 
> Once we show results in CI:
> Reviewed-by: Stuart Summers <stuart.summers at intel.com>

This brought icl back from the dead. The other fls can be fixed up at
leisure! Ta,
-Chris