backend/drm: better hotplug handling

This commit handles better situations in which the number of
connected outputs is greater than the number of available CRTCs.
It'll enable as many outputs as possible, and transfer CRTCs to
outputs that need one on unplug.

This changes CRTC and plane reallocation to happen after scanning
DRM connectors instead of on modeset.

This cleanups CRTCs and planes on unplug to allow them to be
re-used for other outputs.

On modeset, if an output doesn't have a CRTC, the desired mode is
saved and used later when the output gains a CRTC.

Future work includes giving priority to enabled outputs over
disabled ones for CRTC allocation. This requires the compositor to
know about all outputs (even outputs without CRTCs) to properly
modeset outputs enabled in the compositor config file and disable
outputs disabled in the config file.

backend/drm/drm.c

@@ -404,9 +404,11 @@ static void realloc_planes(struct wlr_drm_backend *drm, const uint32_t *crtc_in,
 			struct wlr_drm_plane *new = &drm->type_planes[type][crtc_res[i]];
 
 			if (*old != new) {
-				wlr_log(WLR_DEBUG, "Assigning plane %d -> %d to CRTC %d",
+				wlr_log(WLR_DEBUG,
+					"Assigning plane %d -> %d (type %zu) to CRTC %d",
 					*old ? (int)(*old)->id : -1,
 					new ? (int)new->id : -1,
+					type,
 					c->id);
 
 				changed_outputs[crtc_res[i]] = true;
@@ -420,192 +422,41 @@ static void realloc_planes(struct wlr_drm_backend *drm, const uint32_t *crtc_in,
 	}
 }
 
-static void realloc_crtcs(struct wlr_drm_backend *drm,
-		struct wlr_drm_connector *conn, bool *changed_outputs) {
-	uint32_t crtc[drm->num_crtcs];
-	uint32_t crtc_res[drm->num_crtcs];
-	ssize_t num_outputs = wl_list_length(&drm->outputs);
-	uint32_t possible_crtc[num_outputs];
-
-	for (size_t i = 0; i < drm->num_crtcs; ++i) {
-		crtc[i] = UNMATCHED;
-	}
-
-	memset(possible_crtc, 0, sizeof(possible_crtc));
-
-	wlr_log(WLR_DEBUG, "Reallocating CRTCs for output '%s'", conn->output.name);
-
-	ssize_t index = -1, i = -1;
-	struct wlr_drm_connector *c;
-	wl_list_for_each(c, &drm->outputs, link) {
-		i++;
-		if (c == conn) {
-			index = i;
-		}
-
-		wlr_log(WLR_DEBUG, "output '%s' crtc=%p state=%d",
-			c->output.name, c->crtc, c->state);
-
-		if (c->crtc) {
-			crtc[c->crtc - drm->crtcs] = i;
-		}
-
-		if (c->state == WLR_DRM_CONN_CONNECTED) {
-			possible_crtc[i] = c->possible_crtc;
-		}
-	}
-	assert(index != -1);
-
-	possible_crtc[index] = conn->possible_crtc;
-	match_obj(wl_list_length(&drm->outputs), possible_crtc,
-		drm->num_crtcs, crtc, crtc_res);
-
-	bool matched[num_outputs];
-	memset(matched, false, sizeof(matched));
-	for (size_t i = 0; i < drm->num_crtcs; ++i) {
-		if (crtc_res[i] != UNMATCHED) {
-			matched[crtc_res[i]] = true;
-		}
-	}
-
-	// There is no point doing anything if this monitor doesn't get activated
-	if (!matched[index]) {
-		wlr_log(WLR_DEBUG, "Could not match a CRTC for this output");
-		return;
-	}
-
-	for (size_t i = 0; i < drm->num_crtcs; ++i) {
-		// We don't want any of the current monitors to be deactivated.
-		if (crtc[i] != UNMATCHED && !matched[crtc[i]]) {
-			wlr_log(WLR_DEBUG, "Could not match a CRTC for other output %d",
-				crtc[i]);
-			return;
-		}
-	}
-
-	changed_outputs[index] = true;
-
-	for (size_t i = 0; i < drm->num_crtcs; ++i) {
-		if (crtc_res[i] == UNMATCHED) {
-			continue;
-		}
-
-		if (crtc_res[i] != crtc[i]) {
-			changed_outputs[crtc_res[i]] = true;
-			struct wlr_drm_connector *c;
-			size_t pos = 0;
-			wl_list_for_each(c, &drm->outputs, link) {
-				if (pos == crtc_res[i]) {
-					break;
-				}
-				pos++;
-			}
-			c->crtc = &drm->crtcs[i];
-
-			wlr_log(WLR_DEBUG, "Assigning CRTC %d to output '%s'",
-				drm->crtcs[i].id, c->output.name);
-		}
-	}
-
-	realloc_planes(drm, crtc_res, changed_outputs);
-}
-
-static uint32_t get_possible_crtcs(int fd, uint32_t conn_id) {
-	drmModeConnector *conn = drmModeGetConnector(fd, conn_id);
-	if (!conn) {
-		wlr_log_errno(WLR_ERROR, "Failed to get DRM connector");
-		return 0;
-	}
-
-	if (conn->connection != DRM_MODE_CONNECTED || conn->count_modes == 0) {
-		wlr_log(WLR_ERROR, "Output is not connected");
-		goto error_conn;
-	}
-
-	drmModeEncoder *enc = NULL;
-	for (int i = 0; !enc && i < conn->count_encoders; ++i) {
-		enc = drmModeGetEncoder(fd, conn->encoders[i]);
-	}
-
-	if (!enc) {
-		wlr_log(WLR_ERROR, "Failed to get DRM encoder");
-		goto error_conn;
-	}
-
-	uint32_t ret = enc->possible_crtcs;
-	drmModeFreeEncoder(enc);
-	drmModeFreeConnector(conn);
-	return ret;
-
-error_conn:
-	drmModeFreeConnector(conn);
-	return 0;
-}
-
 static void drm_connector_cleanup(struct wlr_drm_connector *conn);
 
 static bool drm_connector_set_mode(struct wlr_output *output,
 		struct wlr_output_mode *mode) {
 	struct wlr_drm_connector *conn = (struct wlr_drm_connector *)output;
 	struct wlr_drm_backend *drm = (struct wlr_drm_backend *)output->backend;
-	bool changed_outputs[wl_list_length(&drm->outputs)];
+	if (conn->crtc == NULL) {
-
+		wlr_log(WLR_ERROR, "Cannot modeset '%s': no CRTC for this connector",
-	wlr_log(WLR_INFO, "Modesetting '%s' with '%ux%u@%u mHz'", conn->output.name,
+			conn->output.name);
-			mode->width, mode->height, mode->refresh);
+		// Save the desired mode for later, when we'll get a proper CRTC
-
+		conn->desired_mode = mode;
-	conn->possible_crtc = get_possible_crtcs(drm->fd, conn->id);
+		return false;
-	if (conn->possible_crtc == 0) {
-		goto error_conn;
 	}
 
-	memset(changed_outputs, false, sizeof(changed_outputs));
+	wlr_log(WLR_INFO, "Modesetting '%s' with '%ux%u@%u mHz'",
-	realloc_crtcs(drm, conn, changed_outputs);
+		conn->output.name, mode->width, mode->height, mode->refresh);
 
-	struct wlr_drm_crtc *crtc = conn->crtc;
+	if (!init_drm_plane_surfaces(conn->crtc->primary, drm,
-	if (!crtc) {
+			mode->width, mode->height, GBM_FORMAT_XRGB8888)) {
-		wlr_log(WLR_ERROR, "Unable to match %s with a CRTC", conn->output.name);
+		wlr_log(WLR_ERROR, "Failed to initialize renderer for plane");
 		return false;
 	}
-	wlr_log(WLR_DEBUG, "%s: crtc=%td ovr=%td pri=%td cur=%td", conn->output.name,
-		crtc - drm->crtcs,
-		crtc->overlay ? crtc->overlay - drm->overlay_planes : -1,
-		crtc->primary ? crtc->primary - drm->primary_planes : -1,
-		crtc->cursor ? crtc->cursor - drm->cursor_planes : -1);
 
 	conn->state = WLR_DRM_CONN_CONNECTED;
+	conn->desired_mode = NULL;
 	wlr_output_update_mode(&conn->output, mode);
+	wlr_output_update_enabled(&conn->output, true);
+
+	drm_connector_start_renderer(conn);
 
 	// When switching VTs, the mode is not updated but the buffers become
 	// invalid, so we need to manually damage the output here
 	wlr_output_damage_whole(&conn->output);
 
-	// Since realloc_crtcs can deallocate planes on OTHER outputs,
-	// we actually need to reinitialize any that has changed
-	ssize_t output_index = -1;
-	wl_list_for_each(conn, &drm->outputs, link) {
-		output_index++;
-		struct wlr_output_mode *mode = conn->output.current_mode;
-		struct wlr_drm_crtc *crtc = conn->crtc;
-
-		if (conn->state != WLR_DRM_CONN_CONNECTED ||
-				!changed_outputs[output_index]) {
-			continue;
-		}
-
-		if (!init_drm_plane_surfaces(crtc->primary, drm,
-				mode->width, mode->height, GBM_FORMAT_XRGB8888)) {
-			wlr_log(WLR_ERROR, "Failed to initialize renderer for plane");
-			goto error_conn;
-		}
-
-		drm_connector_start_renderer(conn);
-	}
-
 	return true;
-
-error_conn:
-	drm_connector_cleanup(conn);
-	return false;
 }
 
 bool wlr_drm_connector_add_mode(struct wlr_output *output,
@@ -855,6 +706,167 @@ static const int32_t subpixel_map[] = {
 	[DRM_MODE_SUBPIXEL_NONE] = WL_OUTPUT_SUBPIXEL_NONE,
 };
 
+static void dealloc_crtc(struct wlr_drm_connector *conn) {
+	struct wlr_drm_backend *drm = (struct wlr_drm_backend *)conn->output.backend;
+	if (conn->crtc == NULL) {
+		return;
+	}
+
+	for (size_t type = 0; type < 3; ++type) {
+		struct wlr_drm_plane *plane = conn->crtc->planes[type];
+		if (plane == NULL) {
+			continue;
+		}
+
+		finish_drm_surface(&plane->surf);
+		conn->crtc->planes[type] = NULL;
+	}
+
+	drm->iface->conn_enable(drm, conn, false);
+
+	conn->crtc = NULL;
+}
+
+void realloc_crtcs(struct wlr_drm_backend *drm, bool *changed_outputs) {
+	size_t num_outputs = wl_list_length(&drm->outputs);
+
+	wlr_log(WLR_DEBUG, "Reallocating CRTCs");
+
+	uint32_t crtc[drm->num_crtcs];
+	for (size_t i = 0; i < drm->num_crtcs; ++i) {
+		crtc[i] = UNMATCHED;
+	}
+
+	uint32_t possible_crtc[num_outputs];
+	memset(possible_crtc, 0, sizeof(possible_crtc));
+
+	wlr_log(WLR_DEBUG, "State before reallocation:");
+	ssize_t i = -1;
+	struct wlr_drm_connector *conn;
+	wl_list_for_each(conn, &drm->outputs, link) {
+		i++;
+
+		wlr_log(WLR_DEBUG, "  '%s' crtc=%d state=%d", conn->output.name,
+			conn->crtc ? (int)(conn->crtc - drm->crtcs) : -1, conn->state);
+
+		if (conn->crtc) {
+			crtc[conn->crtc - drm->crtcs] = i;
+		}
+
+		if (conn->state == WLR_DRM_CONN_CONNECTED ||
+				conn->state == WLR_DRM_CONN_NEEDS_MODESET) {
+			possible_crtc[i] = conn->possible_crtc;
+		}
+	}
+
+	uint32_t crtc_res[drm->num_crtcs];
+	match_obj(wl_list_length(&drm->outputs), possible_crtc,
+		drm->num_crtcs, crtc, crtc_res);
+
+	bool matched[num_outputs];
+	memset(matched, false, sizeof(matched));
+	for (size_t i = 0; i < drm->num_crtcs; ++i) {
+		if (crtc_res[i] != UNMATCHED) {
+			matched[crtc_res[i]] = true;
+		}
+	}
+
+	for (size_t i = 0; i < drm->num_crtcs; ++i) {
+		// We don't want any of the current monitors to be deactivated
+		if (crtc[i] != UNMATCHED && !matched[crtc[i]]) {
+			wlr_log(WLR_DEBUG, "Could not match a CRTC for connected output %d",
+				crtc[i]);
+			return;
+		}
+	}
+
+	for (size_t i = 0; i < drm->num_crtcs; ++i) {
+		if (crtc_res[i] == UNMATCHED) {
+			continue;
+		}
+
+		if (crtc_res[i] != crtc[i]) {
+			changed_outputs[crtc_res[i]] = true;
+
+			struct wlr_drm_connector *c;
+			size_t pos = 0;
+			wl_list_for_each(c, &drm->outputs, link) {
+				if (pos == crtc_res[i]) {
+					break;
+				}
+				pos++;
+			}
+
+			dealloc_crtc(c);
+			c->crtc = &drm->crtcs[i];
+
+			wlr_log(WLR_DEBUG, "Assigning CRTC %zu to output %d -> %d '%s'",
+				i, crtc[i], crtc_res[i], c->output.name);
+		}
+	}
+
+	wlr_log(WLR_DEBUG, "State after reallocation:");
+	wl_list_for_each(conn, &drm->outputs, link) {
+		wlr_log(WLR_DEBUG, "  '%s' crtc=%d state=%d", conn->output.name,
+			conn->crtc ? (int)(conn->crtc - drm->crtcs) : -1, conn->state);
+	}
+
+	realloc_planes(drm, crtc_res, changed_outputs);
+
+	// We need to reinitialize any plane that has changed
+	i = -1;
+	wl_list_for_each(conn, &drm->outputs, link) {
+		i++;
+		struct wlr_output_mode *mode = conn->output.current_mode;
+
+		if (conn->state != WLR_DRM_CONN_CONNECTED || !changed_outputs[i]) {
+			continue;
+		}
+		assert(conn->crtc);
+
+		if (!init_drm_plane_surfaces(conn->crtc->primary, drm,
+				mode->width, mode->height, GBM_FORMAT_XRGB8888)) {
+			wlr_log(WLR_ERROR, "Failed to initialize renderer for plane");
+			drm_connector_cleanup(conn);
+			break;
+		}
+
+		drm_connector_start_renderer(conn);
+	}
+}
+
+static uint32_t get_possible_crtcs(int fd, uint32_t conn_id) {
+	drmModeConnector *conn = drmModeGetConnector(fd, conn_id);
+	if (!conn) {
+		wlr_log_errno(WLR_ERROR, "Failed to get DRM connector");
+		return 0;
+	}
+
+	if (conn->connection != DRM_MODE_CONNECTED || conn->count_modes == 0) {
+		wlr_log(WLR_ERROR, "Output is not connected");
+		goto error_conn;
+	}
+
+	drmModeEncoder *enc = NULL;
+	for (int i = 0; !enc && i < conn->count_encoders; ++i) {
+		enc = drmModeGetEncoder(fd, conn->encoders[i]);
+	}
+
+	if (!enc) {
+		wlr_log(WLR_ERROR, "Failed to get DRM encoder");
+		goto error_conn;
+	}
+
+	uint32_t ret = enc->possible_crtcs;
+	drmModeFreeEncoder(enc);
+	drmModeFreeConnector(conn);
+	return ret;
+
+error_conn:
+	drmModeFreeConnector(conn);
+	return 0;
+}
+
 void scan_drm_connectors(struct wlr_drm_backend *drm) {
 	wlr_log(WLR_INFO, "Scanning DRM connectors");
 
@@ -910,17 +922,16 @@ void scan_drm_connectors(struct wlr_drm_backend *drm) {
 			wlr_conn->state = WLR_DRM_CONN_DISCONNECTED;
 			wlr_conn->id = drm_conn->connector_id;
 
+			snprintf(wlr_conn->output.name, sizeof(wlr_conn->output.name),
+				"%s-%"PRIu32, conn_get_name(drm_conn->connector_type),
+				drm_conn->connector_type_id);
+
 			if (curr_enc) {
 				wlr_conn->old_crtc = drmModeGetCrtc(drm->fd, curr_enc->crtc_id);
 			}
 
-			snprintf(wlr_conn->output.name, sizeof(wlr_conn->output.name),
-				"%s-%"PRIu32,
-				 conn_get_name(drm_conn->connector_type),
-				 drm_conn->connector_type_id);
-
 			wl_list_insert(&drm->outputs, &wlr_conn->link);
-			wlr_log(WLR_INFO, "Found display '%s'", wlr_conn->output.name);
+			wlr_log(WLR_INFO, "Found connector '%s'", wlr_conn->output.name);
 		} else {
 			seen[index] = true;
 		}
@@ -976,6 +987,12 @@ void scan_drm_connectors(struct wlr_drm_backend *drm) {
 				wl_list_insert(&wlr_conn->output.modes, &mode->wlr_mode.link);
 			}
 
+			wlr_conn->possible_crtc = get_possible_crtcs(drm->fd, wlr_conn->id);
+			if (wlr_conn->possible_crtc == 0) {
+				wlr_log(WLR_ERROR, "No CRTC possible for connector '%s'",
+					wlr_conn->output.name);
+			}
+
 			wlr_output_update_enabled(&wlr_conn->output, wlr_conn->crtc != NULL);
 
 			wlr_conn->state = WLR_DRM_CONN_NEEDS_MODESET;
@@ -984,7 +1001,6 @@ void scan_drm_connectors(struct wlr_drm_backend *drm) {
 				drm_conn->connection != DRM_MODE_CONNECTED) {
 			wlr_log(WLR_INFO, "'%s' disconnected", wlr_conn->output.name);
 
-			wlr_output_update_enabled(&wlr_conn->output, false);
 			drm_connector_cleanup(wlr_conn);
 		}
 
@@ -1011,6 +1027,26 @@ void scan_drm_connectors(struct wlr_drm_backend *drm) {
 		free(conn);
 	}
 
+	bool changed_outputs[wl_list_length(&drm->outputs)];
+	memset(changed_outputs, false, sizeof(changed_outputs));
+	for (size_t i = 0; i < new_outputs_len; ++i) {
+		struct wlr_drm_connector *conn = new_outputs[i];
+
+		ssize_t pos = -1;
+		struct wlr_drm_connector *c;
+		wl_list_for_each(c, &drm->outputs, link) {
+			++pos;
+			if (c == conn) {
+				break;
+			}
+		}
+		assert(pos >= 0);
+
+		changed_outputs[pos] = true;
+	}
+
+	realloc_crtcs(drm, changed_outputs);
+
 	for (size_t i = 0; i < new_outputs_len; ++i) {
 		struct wlr_drm_connector *conn = new_outputs[i];
 
@@ -1019,6 +1055,15 @@ void scan_drm_connectors(struct wlr_drm_backend *drm) {
 		wlr_signal_emit_safe(&drm->backend.events.new_output,
 			&conn->output);
 	}
+
+	// Try to modeset any output that has a desired mode and a CRTC (ie. was
+	// lacking a CRTC on last modeset)
+	wl_list_for_each(conn, &drm->outputs, link) {
+		if (conn->state == WLR_DRM_CONN_NEEDS_MODESET && conn->crtc != NULL &&
+				conn->desired_mode != NULL) {
+			drm_connector_set_mode(&conn->output, conn->desired_mode);
+		}
+	}
 }
 
 static void page_flip_handler(int fd, unsigned seq,
@@ -1114,23 +1159,28 @@ static void drm_connector_cleanup(struct wlr_drm_connector *conn) {
 		}
 
 		conn->output.current_mode = NULL;
+		conn->desired_mode = NULL;
 		struct wlr_drm_mode *mode, *tmp;
 		wl_list_for_each_safe(mode, tmp, &conn->output.modes, wlr_mode.link) {
 			wl_list_remove(&mode->wlr_mode.link);
 			free(mode);
 		}
 
+		conn->output.enabled = false;
+		conn->output.width = conn->output.height = conn->output.refresh = 0;
+
 		memset(&conn->output.make, 0, sizeof(conn->output.make));
 		memset(&conn->output.model, 0, sizeof(conn->output.model));
 		memset(&conn->output.serial, 0, sizeof(conn->output.serial));
 
-		conn->crtc = NULL;
-		conn->possible_crtc = 0;
 		conn->pageflip_pending = false;
 		/* Fallthrough */
 	case WLR_DRM_CONN_NEEDS_MODESET:
 		wlr_log(WLR_INFO, "Emitting destruction signal for '%s'",
 			conn->output.name);
+		dealloc_crtc(conn);
+		conn->possible_crtc = 0;
+		conn->desired_mode = NULL;
 		wlr_signal_emit_safe(&conn->output.events.destroy, &conn->output);
 		break;
 	case WLR_DRM_CONN_DISCONNECTED:

include/backend/drm/drm.h

@@ -119,6 +119,7 @@ struct wlr_drm_connector {
 	struct wlr_output output;
 
 	enum wlr_drm_connector_state state;
+	struct wlr_output_mode *desired_mode;
 	uint32_t id;
 
 	struct wlr_drm_crtc *crtc;