backend/drm: allow to pass empty gamma ramp to reset it

backend/drm/drm.c

@@ -228,19 +228,12 @@ static bool drm_connector_swap_buffers(struct wlr_output *output,
 	return true;
 }
 
-static bool drm_connector_set_gamma(struct wlr_output *output,
-		uint32_t size, uint16_t *r, uint16_t *g, uint16_t *b) {
-	struct wlr_drm_connector *conn = (struct wlr_drm_connector *)output;
-	struct wlr_drm_backend *drm = (struct wlr_drm_backend *)output->backend;
-
-	bool ok = false;
-	if (conn->crtc) {
-		ok = drm->iface->crtc_set_gamma(drm, conn->crtc, r, g, b, size);
-		if (ok) {
-			wlr_output_update_needs_swap(output);
-		}
+static void fill_empty_gamma_table(uint32_t size,
+		uint16_t *r, uint16_t *g, uint16_t *b) {
+	for (uint32_t i = 0; i < size; ++i) {
+		uint16_t val = (uint32_t)0xffff * (uint32_t)i / (uint32_t)(size - 1);
+		r[i] = g[i] = b[i] = val;
 	}
-	return ok;
 }
 
 static uint32_t drm_connector_get_gamma_size(struct wlr_output *output) {
@@ -254,6 +247,37 @@ static uint32_t drm_connector_get_gamma_size(struct wlr_output *output) {
 	return 0;
 }
 
+static bool drm_connector_set_gamma(struct wlr_output *output,
+		uint32_t size, uint16_t *r, uint16_t *g, uint16_t *b) {
+	struct wlr_drm_connector *conn = (struct wlr_drm_connector *)output;
+	struct wlr_drm_backend *drm = (struct wlr_drm_backend *)output->backend;
+
+	if (!conn->crtc) {
+		return false;
+	}
+
+	uint16_t *reset_table = NULL;
+	if (size == 0) {
+		size = drm_connector_get_gamma_size(output);
+		reset_table = malloc(3 * size * sizeof(uint16_t));
+		if (reset_table == NULL) {
+			wlr_log(WLR_ERROR, "Failed to allocate gamma table");
+			return false;
+		}
+		r = reset_table;
+		g = reset_table + size;
+		b = reset_table + 2 * size;
+		fill_empty_gamma_table(size, r, g, b);
+	}
+
+	bool ok = drm->iface->crtc_set_gamma(drm, conn->crtc, r, g, b, size);
+	if (ok) {
+		wlr_output_update_needs_swap(output);
+	}
+	free(reset_table);
+	return ok;
+}
+
 static bool drm_connector_export_dmabuf(struct wlr_output *output,
 		struct wlr_dmabuf_attributes *attribs) {
 	struct wlr_drm_connector *conn = (struct wlr_drm_connector *)output;

include/wlr/types/wlr_output.h

@@ -182,6 +182,8 @@ uint32_t wlr_output_get_gamma_size(struct wlr_output *output);
  * Sets the gamma table for this output. `r`, `g` and `b` are gamma ramps for
  * red, green and blue. `size` is the length of the ramps and must not exceed
  * the value returned by `wlr_output_get_gamma_size`.
+ *
+ * Providing zero-sized ramps resets the gamma table.
  */
 bool wlr_output_set_gamma(struct wlr_output *output,
 	uint32_t size, uint16_t *r, uint16_t *g, uint16_t *b);