[Intel-gfx] [PATCH] drm/i915: Only apply a rmw mmio update if the value changes

[Chris Wilson]

If we try to clear, or even set, a bit in the register that doesn't
change the register state; skip the write. There's a slight danger in
that the register acts as a latch-on-write, but I do not think we use a
rmw cycle with any such latch registers.

Suggested-by: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio at intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
---
 drivers/gpu/drm/i915/intel_uncore.h | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_uncore.h b/drivers/gpu/drm/i915/intel_uncore.h
index 414fc2cb0459..dcfa243892c6 100644
--- a/drivers/gpu/drm/i915/intel_uncore.h
+++ b/drivers/gpu/drm/i915/intel_uncore.h
@@ -378,23 +378,23 @@ intel_uncore_read64_2x32(struct intel_uncore *uncore,
 static inline void intel_uncore_rmw(struct intel_uncore *uncore,
 				    i915_reg_t reg, u32 clear, u32 set)
 {
-	u32 val;
+	u32 old, val;
 
-	val = intel_uncore_read(uncore, reg);
-	val &= ~clear;
-	val |= set;
-	intel_uncore_write(uncore, reg, val);
+	old = intel_uncore_read(uncore, reg);
+	val = (old & ~clear) | set;
+	if (val != old)
+		intel_uncore_write(uncore, reg, val);
 }
 
 static inline void intel_uncore_rmw_fw(struct intel_uncore *uncore,
 				       i915_reg_t reg, u32 clear, u32 set)
 {
-	u32 val;
+	u32 old, val;
 
-	val = intel_uncore_read_fw(uncore, reg);
-	val &= ~clear;
-	val |= set;
-	intel_uncore_write_fw(uncore, reg, val);
+	old = intel_uncore_read_fw(uncore, reg);
+	val = (old & ~clear) | set;
+	if (val != old)
+		intel_uncore_write_fw(uncore, reg, val);
 }
 
 static inline int intel_uncore_write_and_verify(struct intel_uncore *uncore,
-- 
2.23.0