[Intel-gfx] [PATCH 1/4] drm/i915: Fix GEN8_MCR_SELECTOR programming

[Chris Wilson]

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 at intel.com>
> >>
> >> 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 at intel.com>
> >> Fixes: 1e40d4aea57b ("drm/i915/cnl: Implement WaProgramMgsrForCorrectSliceSpecificMmioReads")
> > 
> > Makes sense to me, just from my meagre understanding of arrays
> > Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
> > 
> >> ---
> >>   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