drm/i915: Make contexts non-snooped on non-LLC platforms

We don't do CPU access to GPU contexts so making the GPU access snoop
the CPU caches seems silly, and potentially expensive.

v2: Use !IS_VALLEYVIEW instead of HAS_LLC as this is really
    about what the PTEs can represent.
    Add a comment clarifying the situation.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_context.c

@@ -240,7 +240,15 @@ __create_hw_context(struct drm_device *dev,
 		return ERR_PTR(-ENOMEM);
 	}
 
-	if (INTEL_INFO(dev)->gen >= 7) {
+	/*
+	 * Try to make the context utilize L3 as well as LLC.
+	 *
+	 * On VLV we don't have L3 controls in the PTEs so we
+	 * shouldn't touch the cache level, especially as that
+	 * would make the object snooped which might have a
+	 * negative performance impact.
+	 */
+	if (INTEL_INFO(dev)->gen >= 7 && !IS_VALLEYVIEW(dev)) {
 		ret = i915_gem_object_set_cache_level(ctx->obj,
 						      I915_CACHE_L3_LLC);
 		/* Failure shouldn't ever happen this early */