backend-drm: allow to disable GBM modifiers

Allow to disable GBM modifiers at runtime using the environment variable
WESTON_DISABLE_GBM_MODIFIERS.

This can be useful for debugging or when modifiers cause issues, e.g. in
case modifiers use higher memory bandwidth and hence impose a lower
resolution limit as it is the case with Intel Kaby Lake graphics.

Related to: https://gitlab.freedesktop.org/wayland/weston/-/issues/404
Signed-off-by: Stefan Agner <stefan@agner.ch>

libweston/backend-drm/drm-internal.h

@@ -634,7 +634,8 @@ drm_property_get_range_values(struct drm_property_info *info,
 			      const drmModeObjectProperties *props);
 int
 drm_plane_populate_formats(struct drm_plane *plane, const drmModePlane *kplane,
-			   const drmModeObjectProperties *props);
+			   const drmModeObjectProperties *props,
+			   const bool use_modifiers);
 void
 drm_property_info_free(struct drm_property_info *info, int num_props);
 

libweston/backend-drm/drm.c

@@ -808,7 +808,8 @@ drm_plane_create(struct drm_backend *b, const drmModePlane *kplane,
 			plane->zpos_max = DRM_PLANE_ZPOS_INVALID_PLANE;
 		}
 
-		if (drm_plane_populate_formats(plane, kplane, props) < 0) {
+		if (drm_plane_populate_formats(plane, kplane, props,
+					       b->fb_modifiers) < 0) {
 			drmModeFreeObjectProperties(props);
 			goto err;
 		}

libweston/backend-drm/kms.c

@@ -434,7 +434,8 @@ modifiers_ptr(struct drm_format_modifier_blob *blob)
  */
 int
 drm_plane_populate_formats(struct drm_plane *plane, const drmModePlane *kplane,
-			   const drmModeObjectProperties *props)
+			   const drmModeObjectProperties *props,
+			   const bool use_modifiers)
 {
 	unsigned i;
 	drmModePropertyBlobRes *blob;
@@ -443,6 +444,9 @@ drm_plane_populate_formats(struct drm_plane *plane, const drmModePlane *kplane,
 	uint32_t *blob_formats;
 	uint32_t blob_id;
 
+	if (!use_modifiers)
+		goto fallback;
+
 	blob_id = drm_property_get_value(&plane->props[WDRM_PLANE_IN_FORMATS],
 				         props,
 				         0);
@@ -1474,11 +1478,13 @@ init_kms_caps(struct drm_backend *b)
 	weston_log("DRM: %s atomic modesetting\n",
 		   b->atomic_modeset ? "supports" : "does not support");
 
+	if (!getenv("WESTON_DISABLE_GBM_MODIFIERS")) {
 		ret = drmGetCap(b->drm.fd, DRM_CAP_ADDFB2_MODIFIERS, &cap);
 		if (ret == 0)
 			b->fb_modifiers = cap;
-	else
+	}
-		b->fb_modifiers = 0;
+	weston_log("DRM: %s GBM modifiers\n",
+		   b->fb_modifiers ? "supports" : "does not support");
 
 	/*
 	 * KMS support for hardware planes cannot properly synchronize