[Intel-gfx] [PATCH 4/4] drm/i915: Review the memory barriers around CPU access to buffers

[Chris Wilson]

We need to treat the GPU core as a distinct processor and so apply the
same SMP memory barriers. In this case, in addition to flushing the
chipset cache, which is a no-op on LLC platforms, apply a write barrier
beforehand. And then when we invalidate the CPU cache, make sure the
memory is coherent (again this was a no-op on LLC platforms).

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/char/agp/intel-gtt.c    |    1 +
 drivers/gpu/drm/i915/i915_gem.c |    1 +
 2 files changed, 2 insertions(+)

diff --git a/drivers/char/agp/intel-gtt.c b/drivers/char/agp/intel-gtt.c
index 8b0f6d19..1223128 100644
--- a/drivers/char/agp/intel-gtt.c
+++ b/drivers/char/agp/intel-gtt.c
@@ -1706,6 +1706,7 @@ EXPORT_SYMBOL(intel_gtt_get);
 
 void intel_gtt_chipset_flush(void)
 {
+	wmb();
 	if (intel_private.driver->chipset_flush)
 		intel_private.driver->chipset_flush();
 }
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index ed8d21a..b1ebb88 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -3528,6 +3528,7 @@ i915_gem_object_set_to_cpu_domain(struct drm_i915_gem_object *obj, bool write)
 	/* Flush the CPU cache if it's still invalid. */
 	if ((obj->base.read_domains & I915_GEM_DOMAIN_CPU) == 0) {
 		i915_gem_clflush_object(obj);
+		mb(); /* in case the clflush above is optimised away */
 
 		obj->base.read_domains |= I915_GEM_DOMAIN_CPU;
 	}
-- 
1.7.10.4