drm/connector: demote connector force-probes for non-master clients

Force-probing a connector can be slow and cause flickering. As this
affects the global KMS state, let's make it so only the DRM master
can force-probe a connector.

Non-master DRM clients won't be able to force-probe a connector
anymore. Instead, KMS will perform a regular read-only connector
query.
Signed-off-by: NSimon Ser <contact@emersion.fr>
Acked-by: NPekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: NDaniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210402112212.5625-1-contact@emersion.fr

drivers/gpu/drm/drm_connector.c

@@ -20,6 +20,7 @@
  * OF THIS SOFTWARE.
  */
 
+#include <drm/drm_auth.h>
 #include <drm/drm_connector.h>
 #include <drm/drm_edid.h>
 #include <drm/drm_encoder.h>
@@ -2376,9 +2377,13 @@ int drm_mode_getconnector(struct drm_device *dev, void *data,
 
 	mutex_lock(&dev->mode_config.mutex);
 	if (out_resp->count_modes == 0) {
-		connector->funcs->fill_modes(connector,
-					     dev->mode_config.max_width,
-					     dev->mode_config.max_height);
+		if (drm_is_current_master(file_priv))
+			connector->funcs->fill_modes(connector,
+						     dev->mode_config.max_width,
+						     dev->mode_config.max_height);
+		else
+			drm_dbg_kms(dev, "User-space requested a forced probe on [CONNECTOR:%d:%s] but is not the DRM master, demoting to read-only probe",
+				    connector->base.id, connector->name);
 	}
 
 	out_resp->mm_width = connector->display_info.width_mm;

include/uapi/drm/drm_mode.h

@@ -413,9 +413,10 @@ enum drm_mode_subconnector {
  *
  * **Force-probing a connector**
  *
- * If the @count_modes field is set to zero, the kernel will perform a forced
- * probe on the connector to refresh the connector status, modes and EDID.
- * A forced-probe can be slow, might cause flickering and the ioctl will block.
+ * If the @count_modes field is set to zero and the DRM client is the current
+ * DRM master, the kernel will perform a forced probe on the connector to
+ * refresh the connector status, modes and EDID. A forced-probe can be slow,
+ * might cause flickering and the ioctl will block.
  *
  * User-space needs to force-probe connectors to ensure their metadata is
  * up-to-date at startup and after receiving a hot-plug event. User-space