drm/i915: Access to snooped system memory through the GTT is incoherent

We ignore all the user requests to handle flushing to the GTT domain if
the user requests such on a snoopable bo, and as such access through the
GTT to such pages remains incoherent. The specs even warn that such
behaviour is undefined - a strong reason never to do so.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

1 files changed, 6 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 6380c6083cb2..d15c86279d02 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1341,6 +1341,12 @@ int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 
         trace_i915_gem_object_fault(obj, page_offset, true, write);
 
+        /* Access to snoopable pages through the GTT is incoherent. */
+        if (obj->cache_level != I915_CACHE_NONE && !HAS_LLC(dev)) {
+                ret = -EINVAL;
+                goto unlock;
+        }
+
         /* Now bind it into the GTT if needed */
         ret = i915_gem_object_pin(obj, 0, true, false);
         if (ret)