drm/i915: Only apply a rmw mmio update if the value changes

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@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190917080029.27632-1-chris@chris-wilson.co.uk

diff --git a/drivers/gpu/drm/i915/intel_uncore.h b/drivers/gpu/drm/i915/intel_uncore.h
index 414fc2cb04597f09d8b891e381a3523081bc6836..dcfa243892c67e8c842f5f08c987b08a1dc5fcc1 100644 (file)
--- 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,