Quoting Tvrtko Ursulin (2019-07-10 07:21:19)
>
> On 09/07/2019 22:09, Chris Wilson wrote:
> > Quoting Tvrtko Ursulin (2019-07-09 22:06:17)
> >> From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> >>
> >> fls returns bit positions starting from one for the lsb and the MCR
> >> register expects zero based (sub)slice addressing.
> >>
> >> Incorrent MCR programming can have the effect of directing MMIO reads of
> >> registers in the 0xb100-0xb3ff range to invalid subslice returning zeroes
> >> instead of actual content.
> >>
> >> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> >> Fixes: 1e40d4aea57b ("drm/i915/cnl: Implement WaProgramMgsrForCorrectSliceSpecificMmioReads")
> >
> > Makes sense to me, just from my meagre understanding of arrays
> > Reviewed-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> >
> >> ---
> >> drivers/gpu/drm/i915/gt/intel_engine_cs.c | 9 +++++++--
> >> 1 file changed, 7 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/i915/gt/intel_engine_cs.c b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
> >> index bdf279fa3b2e..ee15d1934486 100644
> >> --- a/drivers/gpu/drm/i915/gt/intel_engine_cs.c
> >> +++ b/drivers/gpu/drm/i915/gt/intel_engine_cs.c
> >> @@ -975,9 +975,14 @@ const char *i915_cache_level_str(struct drm_i915_private *i915, int type)
> >> u32 intel_calculate_mcr_s_ss_select(struct drm_i915_private *dev_priv)
> >> {
> >> const struct sseu_dev_info *sseu = &RUNTIME_INFO(dev_priv)->sseu;
> >> + unsigned int slice = fls(sseu->slice_mask) - 1;
> >
> > I'd vote for __fls() here instead of fls() - 1.
>
> With fls() I get zero slice mask check for free, in the array out of
> bounds check below.
>
> >
> >> + unsigned int subslice;
> >> u32 mcr_s_ss_select;
> >> - u32 slice = fls(sseu->slice_mask);
> >> - u32 subslice = fls(sseu->subslice_mask[slice]);
> >> +
> >> + GEM_BUG_ON(slice >= ARRAY_SIZE(sseu->subslice_mask));
> >> + subslice = fls(sseu->subslice_mask[slice]);
> >> + GEM_BUG_ON(!subslice);
> >> + subslice--;
> >
> > And I think we're a bit late on the BUG_ON here (it's shouldn't change
> > after probing) so could be happily reduced to __fls().
>
> Why late? This one is not checking the array for out of bounds, just if
> zero subslice mask happens to be in a valid slot. Too paranoid?
Just saying this won't change from setup where we should have validated
our HW discovery.
-Chris
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