Re: [PATCH 1/4] drm/i915: Fix GEN8_MCR_SELECTOR programming

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 09/07/2019 22:09, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2019-07-09 22:06:17)
>> From: Tvrtko Ursulin <tvrtko.ursulin@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@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@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?

Regards,

Tvrtko


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