drm/i915: Fix GEN8_MCR_SELECTOR programming

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: 1e40d4ae ("drm/i915/cnl: Implement WaProgramMgsrForCorrectSliceSpecificMmioReads")
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190717180624.20354-2-tvrtko.ursulin@linux.intel.com

drivers/gpu/drm/i915/gt/intel_engine_cs.c

@@ -962,9 +962,14 @@ 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;
+	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--;
 
 	if (IS_GEN(dev_priv, 10))
 		mcr_s_ss_select = GEN8_MCR_SLICE(slice) |