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

["Saarinen, Jani" <jani.saarinen@intel.com>]

HI, 

> -----Original Message-----
> From: Intel-gfx [mailto:intel-gfx-bounces@lists.freedesktop.org] On Behalf Of Jani
> Nikula
> Sent: keskiviikko 29. toukokuuta 2019 17.23
> To: Chris Wilson <chris@chris-wilson.co.uk>; intel-gfx@lists.freedesktop.org
> Subject: Re: [Intel-gfx] [PATCH] drm/i915: Fix off-by-one in looking up icl sseu slice
> 
> On Wed, 29 May 2019, Jani Nikula <jani.nikula@linux.intel.com> wrote:
> > On Tue, 28 May 2019, Chris Wilson <chris@chris-wilson.co.uk> 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")
> >
> > I sent a revert of this commit [1] for reasons explained in the commit
> > message.
> 
> I've gone ahead and pushed the revert.
> 
> We can now calmly get back to the drawing board, and get this sorted out.
Ack ;)

> 
> BR,
> Jani.
> 
> 
> >
> > BR,
> > Jani.
> >
> >
> > [1]
> > http://patchwork.freedesktop.org/patch/msgid/20190529082150.31526-1-ja
> > ni.nikula@intel.com
> >
> >
> >> Fixes: 1e40d4aea57b ("drm/i915/cnl: Implement
> >> WaProgramMgsrForCorrectSliceSpecificMmioReads")
> >> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> >> Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
> >> Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
> >> Cc: Stuart Summers <stuart.summers@intel.com>
> >> Cc: Manasi Navare <manasi.d.navare@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);
> >>  		u32 fuse3 =
> >>  			intel_uncore_read(&i915->uncore,
> GEN10_MIRROR_FUSE3);
> >>  		u32 ss_mask = intel_sseu_get_subslices(sseu, slice);
> 
> --
> Jani Nikula, Intel Open Source Graphics Center
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