drm/amdgpu: Bring bo creation in line with radeon driver (v2)

Add the bo creation changes that have been done to the radeon driver in
recent times, e.g. disable GTT WC on 32 bit because it is broken there,
and also disable it generally (and print a warning message) when
CONFIG_X86_PAT is not set.

v2: agd: fix warning in defined(CONFIG_X86) && !defined(CONFIG_X86_PAT)
    case
Reviewed-by: NMichel Dänzer <michel.daenzer@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NNils Holland <nholland@tisys.org>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_object.c

@@ -363,11 +363,31 @@ int amdgpu_bo_create_restricted(struct amdgpu_device *adev,
 
 	bo->flags = flags;
 
+#ifdef CONFIG_X86_32
+	/* XXX: Write-combined CPU mappings of GTT seem broken on 32-bit
+	 * See https://bugs.freedesktop.org/show_bug.cgi?id=84627
+	 */
+	bo->flags &= ~AMDGPU_GEM_CREATE_CPU_GTT_USWC;
+#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
+
+	if (bo->flags & AMDGPU_GEM_CREATE_CPU_GTT_USWC)
+		DRM_INFO_ONCE("Please enable CONFIG_MTRR and CONFIG_X86_PAT for "
+			      "better performance thanks to write-combining\n");
+	bo->flags &= ~AMDGPU_GEM_CREATE_CPU_GTT_USWC;
+#else
 	/* For architectures that don't support WC memory,
 	 * mask out the WC flag from the BO
 	 */
 	if (!drm_arch_can_wc_memory())
 		bo->flags &= ~AMDGPU_GEM_CREATE_CPU_GTT_USWC;
+#endif
 
 	amdgpu_fill_placement_to_bo(bo, placement);
 	/* Kernel allocation are uninterruptible */