drm/radeon: Don't try to enable write-combining without PAT

Doing so can cause things to become slow.

Print a warning at compile time and an informative message at runtime in
that case.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=88758
Cc: stable@vger.kernel.org
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_object.c

@@ -238,6 +238,18 @@ int radeon_bo_create(struct radeon_device *rdev,
 	 * See https://bugs.freedesktop.org/show_bug.cgi?id=84627
 	 */
 	bo->flags &= ~RADEON_GEM_GTT_WC;
+#elif defined(CONFIG_X86) && !defined(CONFIG_X86_PAT)
+	/* Don't try to enable write-combining when it can't work, or things
+	 * may be slow
+	 * See https://bugs.freedesktop.org/show_bug.cgi?id=88758
+	 */
+
+#warning Please enable CONFIG_MTRR and CONFIG_X86_PAT for better performance \
+	 thanks to write-combining
+
+	DRM_INFO_ONCE("Please enable CONFIG_MTRR and CONFIG_X86_PAT for "
+		      "better performance thanks to write-combining\n");
+	bo->flags &= ~RADEON_GEM_GTT_WC;
 #endif
 
 	radeon_ttm_placement_from_domain(bo, domain);