drm/i915: Do not clflush snooped objects

Rely on the GPU snooping into the CPU cache for appropriately bound
objects on MI_FLUSH. Or perhaps one day we will have a cache-coherent
CPU/GPU package...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Keith Packard <keithp@keithp.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 264bec8f1f1d..bf32527fc928 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2878,6 +2878,17 @@ i915_gem_clflush_object(struct drm_i915_gem_object *obj)
         if (obj->pages == NULL)
                 return;
 
+        /* If the GPU is snooping the contents of the CPU cache,
+         * we do not need to manually clear the CPU cache lines.  However,
+         * the caches are only snooped when the render cache is
+         * flushed/invalidated.  As we always have to emit invalidations
+         * and flushes when moving into and out of the RENDER domain, correct
+         * snooping behaviour occurs naturally as the result of our domain
+         * tracking.
+         */
+        if (obj->cache_level != I915_CACHE_NONE)
+                return;
+
         trace_i915_gem_object_clflush(obj);
 
         drm_clflush_pages(obj->pages, obj->base.size / PAGE_SIZE);