backend/drm: short-circuit no-op commits

Some output commits (changing e.g. the output scale or transform)
don't require any change in the KMS state. Instead of going through
a KMS commit, return early. Blocking KMS commits can be expensive.

backend/drm/drm.c

@@ -31,13 +31,16 @@
 #include "render/wlr_renderer.h"
 #include "util/signal.h"
 
-static const uint32_t SUPPORTED_OUTPUT_STATE =
-	WLR_OUTPUT_STATE_BACKEND_OPTIONAL |
+// Output state which needs a KMS commit to be applied
+static const uint32_t COMMIT_OUTPUT_STATE =
 	WLR_OUTPUT_STATE_BUFFER |
 	WLR_OUTPUT_STATE_MODE |
 	WLR_OUTPUT_STATE_ENABLED |
 	WLR_OUTPUT_STATE_GAMMA_LUT;
 
+static const uint32_t SUPPORTED_OUTPUT_STATE =
+	WLR_OUTPUT_STATE_BACKEND_OPTIONAL | COMMIT_OUTPUT_STATE;
+
 bool check_drm_features(struct wlr_drm_backend *drm) {
 	if (drmGetCap(drm->fd, DRM_CAP_CURSOR_WIDTH, &drm->cursor_width)) {
 		drm->cursor_width = 64;
@@ -474,6 +477,11 @@ static bool drm_connector_test(struct wlr_output *output,
 		return false;
 	}
 
+	if ((state->committed & ~COMMIT_OUTPUT_STATE) == 0) {
+		// This commit doesn't change the KMS state
+		return true;
+	}
+
 	if ((state->committed & WLR_OUTPUT_STATE_ENABLED) && state->enabled) {
 		if (output->current_mode == NULL &&
 				!(state->committed & WLR_OUTPUT_STATE_MODE)) {
@@ -560,6 +568,11 @@ bool drm_connector_commit_state(struct wlr_drm_connector *conn,
 		return false;
 	}
 
+	if ((base->committed & ~COMMIT_OUTPUT_STATE) == 0) {
+		// This commit doesn't change the KMS state
+		return true;
+	}
+
 	struct wlr_drm_connector_state pending = {0};
 	drm_connector_state_init(&pending, conn, base);