backend/drm: set "max bpc" to the max

"max bpc" is a maximum value, the driver is free to choose a
smaller value depending on the bandwidth available.

Some faulty monitors misbehave with higher bpc values. We'll add
a workaround if users get hit by these in practice.

References: https://gitlab.freedesktop.org/wayland/weston/-/issues/612

backend/drm/atomic.c

@@ -284,6 +284,9 @@ static bool atomic_crtc_commit(struct wlr_drm_connector *conn,
 		atomic_add(&atom, conn->id, conn->props.content_type,
 			DRM_MODE_CONTENT_TYPE_GRAPHICS);
 	}
+	if (active && conn->props.max_bpc != 0 && conn->max_bpc > 0) {
+		atomic_add(&atom, conn->id, conn->props.max_bpc, conn->max_bpc);
+	}
 	atomic_add(&atom, crtc->id, crtc->props.mode_id, mode_id);
 	atomic_add(&atom, crtc->id, crtc->props.active, active);
 	if (active) {

backend/drm/drm.c

@@ -1317,6 +1317,14 @@ void scan_drm_connectors(struct wlr_drm_backend *drm,
 				wlr_conn->output.non_desktop = non_desktop;
 			}
 
+			wlr_conn->max_bpc = 0;
+			if (wlr_conn->props.max_bpc != 0) {
+				if (!introspect_drm_prop_range(drm->fd, wlr_conn->props.max_bpc,
+						NULL, &wlr_conn->max_bpc)) {
+					wlr_log(WLR_ERROR, "Failed to introspect 'max bpc' property");
+				}
+			}
+
 			size_t edid_len = 0;
 			uint8_t *edid = get_drm_prop_blob(drm->fd,
 				wlr_conn->id, wlr_conn->props.edid, &edid_len);

backend/drm/properties.c

@@ -1,4 +1,5 @@
 #define _POSIX_C_SOURCE 200809L
+#include <assert.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
@@ -26,6 +27,7 @@ static const struct prop_info connector_info[] = {
 	{ "PATH", INDEX(path) },
 	{ "content type", INDEX(content_type) },
 	{ "link-status", INDEX(link_status) },
+	{ "max bpc", INDEX(max_bpc) },
 	{ "non-desktop", INDEX(non_desktop) },
 	{ "panel orientation", INDEX(panel_orientation) },
 	{ "subconnector", INDEX(subconnector) },
@@ -181,3 +183,28 @@ char *get_drm_prop_enum(int fd, uint32_t obj, uint32_t prop_id) {
 
 	return str;
 }
+
+bool introspect_drm_prop_range(int fd, uint32_t prop_id,
+		uint64_t *min, uint64_t *max) {
+	drmModePropertyRes *prop = drmModeGetProperty(fd, prop_id);
+	if (!prop) {
+		return false;
+	}
+
+	if (drmModeGetPropertyType(prop) != DRM_MODE_PROP_RANGE) {
+		drmModeFreeProperty(prop);
+		return false;
+	}
+
+	assert(prop->count_values == 2);
+
+	if (min != NULL) {
+		*min = prop->values[0];
+	}
+	if (max != NULL) {
+		*max = prop->values[1];
+	}
+
+	drmModeFreeProperty(prop);
+	return true;
+}

include/backend/drm/drm.h

@@ -119,6 +119,7 @@ struct wlr_drm_connector {
 	enum wlr_drm_connector_status status;
 	bool desired_enabled;
 	uint32_t id;
+	uint64_t max_bpc;
 	struct wlr_drm_lease *lease;
 
 	struct wlr_drm_crtc *crtc;

include/backend/drm/properties.h

@@ -21,6 +21,7 @@ union wlr_drm_connector_props {
 		uint32_t non_desktop;
 		uint32_t panel_orientation; // not guaranteed to exist
 		uint32_t content_type; // not guaranteed to exist
+		uint32_t max_bpc; // not guaranteed to exist
 
 		// atomic-modesetting only
 
@@ -76,4 +77,7 @@ bool get_drm_prop(int fd, uint32_t obj, uint32_t prop, uint64_t *ret);
 void *get_drm_prop_blob(int fd, uint32_t obj, uint32_t prop, size_t *ret_len);
 char *get_drm_prop_enum(int fd, uint32_t obj, uint32_t prop);
 
+bool introspect_drm_prop_range(int fd, uint32_t prop_id,
+	uint64_t *min, uint64_t *max);
+
 #endif