[Intel-gfx] [PATCH 2/4] drm/i915: Fix WaProgramMgsrForL3BankSpecificMmioReads

[Tvrtko Ursulin]

From: Tvrtko Ursulin <tvrtko.ursulin at intel.com>

Two issues in this code:

1.
fls() usage is incorrect causing off by one in subslice mask lookup,
which in other words means subslice mask of all zeroes is always used
(subslice mask of a slice which is not present, or even out of bounds
array access), rendering the checks in wa_init_mcr either futile or
random.

2.
Condition in WARN_ON is not correct. It is doing a bitwise and operation
between a positive (present subslices) and negative mask (disabled L3
banks).

This means that with corrected fls() usage the assert would always
incorrectly fail.

We can fix this by invereting the fuse bits in the check.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
Fixes: fe864b76c2ab ("drm/i915: Implement WaProgramMgsrForL3BankSpecificMmioReads")
---
 drivers/gpu/drm/i915/gt/intel_workarounds.c | 26 ++++++++++-----------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/i915/gt/intel_workarounds.c b/drivers/gpu/drm/i915/gt/intel_workarounds.c
index 9e069286d3ce..b5f19ad48d22 100644
--- a/drivers/gpu/drm/i915/gt/intel_workarounds.c
+++ b/drivers/gpu/drm/i915/gt/intel_workarounds.c
@@ -776,26 +776,26 @@ wa_init_mcr(struct drm_i915_private *i915, struct i915_wa_list *wal)
 	 * something more complex that requires checking the range of every
 	 * MMIO read).
 	 */
-	if (INTEL_GEN(i915) >= 10 &&
-	    is_power_of_2(sseu->slice_mask)) {
+	if (INTEL_GEN(i915) >= 10 && is_power_of_2(sseu->slice_mask)) {
 		/*
-		 * read FUSE3 for enabled L3 Bank IDs, if L3 Bank matches
-		 * enabled subslice, no need to redirect MCR packet
+		 * 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 fuse3 =
-			intel_uncore_read(&i915->uncore, GEN10_MIRROR_FUSE3);
-		u8 ss_mask = sseu->subslice_mask[slice];
+		unsigned int slice = fls(sseu->slice_mask) - 1;
+		u8 ss, en, dis;
 
-		u8 enabled_mask = (ss_mask | ss_mask >>
-				   GEN10_L3BANK_PAIR_COUNT) & GEN10_L3BANK_MASK;
-		u8 disabled_mask = fuse3 & GEN10_L3BANK_MASK;
+		GEM_BUG_ON(slice >= ARRAY_SIZE(sseu->subslice_mask));
+		ss = sseu->subslice_mask[slice];
+
+		en = (ss | ss >> GEN10_L3BANK_PAIR_COUNT) & GEN10_L3BANK_MASK;
+		dis = intel_uncore_read(&i915->uncore, GEN10_MIRROR_FUSE3) &
+		      GEN10_L3BANK_MASK;
 
 		/*
 		 * Production silicon should have matched L3Bank and
-		 * subslice enabled
+		 * subslice enabled.
 		 */
-		WARN_ON((enabled_mask & disabled_mask) != enabled_mask);
+		WARN_ON((en & ~dis) != en);
 	}
 
 	if (INTEL_GEN(i915) >= 11)
-- 
2.20.1