backend/drm: steal CRTCs from disabled outputs

This commit allows outputs that need a CRTC to steal it from
user-disabled outputs. Note that in the case there are enough
CRTCs, disabled outputs don't loose it (so there's no modeset
and plane initialization needed after DPMS). CRTC allocation
still prefers to keep the old configuration, even if that means
allocating an extra CRTC to a disabled output.

CRTC reallocation now happen when enabling/disabling an output as
well as when trying to modeset. When enabling an output without a
CRTC, we realloc to try to steal a CRTC from a disabled output
(that doesn't really need the CRTC). When disabling an output, we
try to give our CRTC to an output that needs one. Modesetting is
similar to enabling.

A new DRM connector field has been added: `desired_enabled`.
Outputs without CRTCs get automatically disabled. This field keeps
track of the state desired by the user, allowing to automatically
re-enable outputs when a CRTC becomes free.

This required some changes to the allocation algorithm. Previously,
the algorithm tried to keep the previous configuration even if a
new configuration with a better score was possible (it only changed
configuration when the old one didn't work anymore). This is now
changed and the old configuration (still preferred) is only
retained without considering new possibilities when it's perfect
(all outputs have CRTCs).

User-disabled outputs now have `possible_crtcs` set to 0, meaning
they can only retain a previous CRTC (not acquire a new one). The
allocation algorithm has been updated to do not bump the score
when assigning a CRTC to a disabled output.

3 files changed, 108 insertions, 55 deletions

diff --git a/backend/drm/atomic.c b/backend/drm/atomic.c
index 335c9de1..90acac25 100644
--- a/backend/drm/atomic.c
+++ b/backend/drm/atomic.c
@@ -129,6 +129,9 @@ static bool atomic_crtc_pageflip(struct wlr_drm_backend *drm,
 static bool atomic_conn_enable(struct wlr_drm_backend *drm,
 		struct wlr_drm_connector *conn, bool enable) {
 	struct wlr_drm_crtc *crtc = conn->crtc;
+	if (crtc == NULL) {
+		return !enable;
+	}
 
 	struct atomic atom;
 	atomic_begin(crtc, &atom);
diff --git a/backend/drm/drm.c b/backend/drm/drm.c
index e61e624d..eac3a312 100644
--- a/backend/drm/drm.c
+++ b/backend/drm/drm.c
@@ -342,14 +342,39 @@ static void drm_connector_start_renderer(struct wlr_drm_connector *conn) {
 	}
 }
 
+static bool drm_connector_set_mode(struct wlr_output *output,
+	struct wlr_output_mode *mode);
+
+static void realloc_crtcs(struct wlr_drm_backend *drm, bool *changed_outputs);
+
+static void attempt_enable_needs_modeset(struct wlr_drm_backend *drm) {
+	// Try to modeset any output that has a desired mode and a CRTC (ie. was
+	// lacking a CRTC on last modeset)
+	struct wlr_drm_connector *conn;
+	wl_list_for_each(conn, &drm->outputs, link) {
+		if (conn->state == WLR_DRM_CONN_NEEDS_MODESET &&
+				conn->crtc != NULL && conn->desired_mode != NULL &&
+				conn->desired_enabled) {
+			drm_connector_set_mode(&conn->output, conn->desired_mode);
+		}
+	}
+}
+
 bool enable_drm_connector(struct wlr_output *output, bool enable) {
 	struct wlr_drm_connector *conn = (struct wlr_drm_connector *)output;
+	struct wlr_drm_backend *drm = (struct wlr_drm_backend *)output->backend;
 	if (conn->state != WLR_DRM_CONN_CONNECTED
 			&& conn->state != WLR_DRM_CONN_NEEDS_MODESET) {
 		return false;
 	}
 
-	struct wlr_drm_backend *drm = (struct wlr_drm_backend *)output->backend;
+	conn->desired_enabled = enable;
+
+	if (enable && conn->crtc == NULL) {
+		// Maybe we can steal a CRTC from a disabled output
+		realloc_crtcs(drm, NULL);
+	}
+
 	bool ok = drm->iface->conn_enable(drm, conn, enable);
 	if (!ok) {
 		return false;
@@ -357,6 +382,10 @@ bool enable_drm_connector(struct wlr_output *output, bool enable) {
 
 	if (enable) {
 		drm_connector_start_renderer(conn);
+	} else {
+		realloc_crtcs(drm, NULL);
+
+		attempt_enable_needs_modeset(drm);
 	}
 
 	wlr_output_update_enabled(&conn->output, enable);
@@ -430,6 +459,10 @@ static bool drm_connector_set_mode(struct wlr_output *output,
 	struct wlr_drm_connector *conn = (struct wlr_drm_connector *)output;
 	struct wlr_drm_backend *drm = (struct wlr_drm_backend *)output->backend;
 	if (conn->crtc == NULL) {
+		// Maybe we can steal a CRTC from a disabled output
+		realloc_crtcs(drm, NULL);
+	}
+	if (conn->crtc == NULL) {
 		wlr_log(WLR_ERROR, "Cannot modeset '%s': no CRTC for this connector",
 			conn->output.name);
 		// Save the desired mode for later, when we'll get a proper CRTC
@@ -450,6 +483,7 @@ static bool drm_connector_set_mode(struct wlr_output *output,
 	conn->desired_mode = NULL;
 	wlr_output_update_mode(&conn->output, mode);
 	wlr_output_update_enabled(&conn->output, true);
+	conn->desired_enabled = true;
 
 	drm_connector_start_renderer(conn);
 
@@ -731,17 +765,27 @@ static void dealloc_crtc(struct wlr_drm_connector *conn) {
 	conn->crtc = NULL;
 }
 
-void realloc_crtcs(struct wlr_drm_backend *drm, bool *changed_outputs) {
+static void realloc_crtcs(struct wlr_drm_backend *drm, bool *changed_outputs) {
 	size_t num_outputs = wl_list_length(&drm->outputs);
 
+	if (changed_outputs == NULL) {
+		changed_outputs = calloc(num_outputs, sizeof(bool));
+		if (changed_outputs == NULL) {
+			wlr_log(WLR_ERROR, "Allocation failed");
+			return;
+		}
+	}
+
 	wlr_log(WLR_DEBUG, "Reallocating CRTCs");
 
-	uint32_t crtc[drm->num_crtcs];
+	uint32_t crtc[drm->num_crtcs + 1];
 	for (size_t i = 0; i < drm->num_crtcs; ++i) {
 		crtc[i] = UNMATCHED;
 	}
 
-	uint32_t possible_crtc[num_outputs];
+	struct wlr_drm_connector *connectors[num_outputs + 1];
+
+	uint32_t possible_crtc[num_outputs + 1];
 	memset(possible_crtc, 0, sizeof(possible_crtc));
 
 	wlr_log(WLR_DEBUG, "State before reallocation:");
@@ -749,25 +793,31 @@ void realloc_crtcs(struct wlr_drm_backend *drm, bool *changed_outputs) {
 	struct wlr_drm_connector *conn;
 	wl_list_for_each(conn, &drm->outputs, link) {
 		i++;
+		connectors[i] = conn;
 
-		wlr_log(WLR_DEBUG, "  '%s' crtc=%d state=%d", conn->output.name,
-			conn->crtc ? (int)(conn->crtc - drm->crtcs) : -1, conn->state);
+		wlr_log(WLR_DEBUG, "  '%s' crtc=%d state=%d desired_enabled=%d",
+			conn->output.name,
+			conn->crtc ? (int)(conn->crtc - drm->crtcs) : -1,
+			conn->state, conn->desired_enabled);
 
 		if (conn->crtc) {
 			crtc[conn->crtc - drm->crtcs] = i;
 		}
 
-		if (conn->state == WLR_DRM_CONN_CONNECTED ||
-				conn->state == WLR_DRM_CONN_NEEDS_MODESET) {
+		// Only search CRTCs for user-enabled outputs (that are already
+		// connected or in need of a modeset)
+		if ((conn->state == WLR_DRM_CONN_CONNECTED ||
+				conn->state == WLR_DRM_CONN_NEEDS_MODESET) &&
+				conn->desired_enabled) {
 			possible_crtc[i] = conn->possible_crtc;
 		}
 	}
 
-	uint32_t crtc_res[drm->num_crtcs];
+	uint32_t crtc_res[drm->num_crtcs + 1];
 	match_obj(wl_list_length(&drm->outputs), possible_crtc,
 		drm->num_crtcs, crtc, crtc_res);
 
-	bool matched[num_outputs];
+	bool matched[num_outputs + 1];
 	memset(matched, false, sizeof(matched));
 	for (size_t i = 0; i < drm->num_crtcs; ++i) {
 		if (crtc_res[i] != UNMATCHED) {
@@ -777,34 +827,30 @@ void realloc_crtcs(struct wlr_drm_backend *drm, bool *changed_outputs) {
 
 	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]]) {
+		if (crtc[i] != UNMATCHED && !matched[crtc[i]] &&
+				connectors[crtc[i]]->desired_enabled) {
 			wlr_log(WLR_DEBUG, "Could not match a CRTC for connected output %d",
 				crtc[i]);
 			return;
 		}
 	}
 
-	struct wlr_drm_connector *connectors[num_outputs];
-	i = 0;
-	wl_list_for_each(conn, &drm->outputs, link) {
-		connectors[i] = conn;
-		i++;
-	}
-
 	for (size_t i = 0; i < drm->num_crtcs; ++i) {
-		if (crtc_res[i] == UNMATCHED) {
-			// De-allocate CRTCs we don't use anymore
-			if (crtc[i] != UNMATCHED) {
-				dealloc_crtc(connectors[crtc[i]]);
-			}
+		if (crtc_res[i] == crtc[i]) {
 			continue;
 		}
 
-		if (crtc_res[i] != crtc[i]) {
+		// De-allocate this CRTC on previous output
+		if (crtc[i] != UNMATCHED) {
+			changed_outputs[crtc[i]] = true;
+			dealloc_crtc(connectors[crtc[i]]);
+		}
+
+		// Assign this CRTC to next output
+		if (crtc_res[i] != UNMATCHED) {
 			changed_outputs[crtc_res[i]] = true;
 
 			struct wlr_drm_connector *conn = connectors[crtc_res[i]];
-
 			dealloc_crtc(conn);
 			conn->crtc = &drm->crtcs[i];
 
@@ -815,8 +861,10 @@ void realloc_crtcs(struct wlr_drm_backend *drm, bool *changed_outputs) {
 
 	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);
+		wlr_log(WLR_DEBUG, "  '%s' crtc=%d state=%d desired_enabled=%d",
+			conn->output.name,
+			conn->crtc ? (int)(conn->crtc - drm->crtcs) : -1,
+			conn->state, conn->desired_enabled);
 	}
 
 	realloc_planes(drm, crtc_res, changed_outputs);
@@ -827,10 +875,10 @@ void realloc_crtcs(struct wlr_drm_backend *drm, bool *changed_outputs) {
 		i++;
 		struct wlr_output_mode *mode = conn->output.current_mode;
 
-		if (conn->state != WLR_DRM_CONN_CONNECTED || !changed_outputs[i]) {
+		if (conn->state != WLR_DRM_CONN_CONNECTED || !changed_outputs[i]
+				|| conn->crtc == NULL) {
 			continue;
 		}
-		assert(conn->crtc);
 
 		if (!init_drm_plane_surfaces(conn->crtc->primary, drm,
 				mode->width, mode->height, GBM_FORMAT_XRGB8888)) {
@@ -1004,6 +1052,7 @@ void scan_drm_connectors(struct wlr_drm_backend *drm) {
 			}
 
 			wlr_output_update_enabled(&wlr_conn->output, wlr_conn->crtc != NULL);
+			wlr_conn->desired_enabled = true;
 
 			wlr_conn->state = WLR_DRM_CONN_NEEDS_MODESET;
 			new_outputs[new_outputs_len++] = wlr_conn;
@@ -1066,14 +1115,7 @@ void scan_drm_connectors(struct wlr_drm_backend *drm) {
 			&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);
-		}
-	}
+	attempt_enable_needs_modeset(drm);
 }
 
 static void page_flip_handler(int fd, unsigned seq,
@@ -1082,7 +1124,7 @@ static void page_flip_handler(int fd, unsigned seq,
 	struct wlr_drm_backend *drm = (struct wlr_drm_backend *)conn->output.backend;
 
 	conn->pageflip_pending = false;
-	if (conn->state != WLR_DRM_CONN_CONNECTED) {
+	if (conn->state != WLR_DRM_CONN_CONNECTED || conn->crtc == NULL) {
 		return;
 	}
 
@@ -1155,16 +1197,18 @@ static void drm_connector_cleanup(struct wlr_drm_connector *conn) {
 	case WLR_DRM_CONN_CONNECTED:
 	case WLR_DRM_CONN_CLEANUP:;
 		struct wlr_drm_crtc *crtc = conn->crtc;
-		for (int i = 0; i < 3; ++i) {
-			if (!crtc->planes[i]) {
-				continue;
-			}
+		if (crtc != NULL) {
+			for (int i = 0; i < 3; ++i) {
+				if (!crtc->planes[i]) {
+					continue;
+				}
 
-			finish_drm_surface(&crtc->planes[i]->surf);
-			finish_drm_surface(&crtc->planes[i]->mgpu_surf);
-			if (crtc->planes[i]->id == 0) {
-				free(crtc->planes[i]);
-				crtc->planes[i] = NULL;
+				finish_drm_surface(&crtc->planes[i]->surf);
+				finish_drm_surface(&crtc->planes[i]->mgpu_surf);
+				if (crtc->planes[i]->id == 0) {
+					free(crtc->planes[i]);
+					crtc->planes[i] = NULL;
+				}
 			}
 		}
 
diff --git a/backend/drm/util.c b/backend/drm/util.c
index 050da2a6..f9637c33 100644
--- a/backend/drm/util.c
+++ b/backend/drm/util.c
@@ -223,7 +223,8 @@ struct match_state {
 static bool match_obj_(struct match_state *st, size_t skips, size_t score, size_t replaced, size_t i) {
 	// Finished
 	if (i >= st->num_res) {
-		if (score > st->score || (score == st->score && replaced < st->replaced)) {
+		if (score > st->score ||
+				(score == st->score && replaced < st->replaced)) {
 			st->score = score;
 			st->replaced = replaced;
 			memcpy(st->best, st->res, sizeof(st->best[0]) * st->num_res);
@@ -243,29 +244,33 @@ static bool match_obj_(struct match_state *st, size_t skips, size_t score, size_
 		return match_obj_(st, skips + 1, score, replaced, i + 1);
 	}
 
+	bool has_best = false;
+
 	/*
 	 * Attempt to use the current solution first, to try and avoid
 	 * recalculating everything
 	 */
 	if (st->orig[i] != UNMATCHED && !is_taken(i, st->res, st->orig[i])) {
 		st->res[i] = st->orig[i];
-		if (match_obj_(st, skips, score + 1, replaced, i + 1)) {
-			return true;
+		size_t obj_score = st->objs[st->res[i]] != 0 ? 1 : 0;
+		if (match_obj_(st, skips, score + obj_score, replaced, i + 1)) {
+			has_best = true;
 		}
 	}
 	if (st->orig[i] == UNMATCHED) {
 		st->res[i] = UNMATCHED;
-		match_obj_(st, skips, score, replaced, i + 1);
-		if (st->exit_early) {
-			return true;
+		if (match_obj_(st, skips, score, replaced, i + 1)) {
+			has_best = true;
 		}
 	}
+	if (st->exit_early) {
+		return true;
+	}
 
 	if (st->orig[i] != UNMATCHED) {
 		++replaced;
 	}
 
-	bool has_best = false;
 	for (size_t candidate = 0; candidate < st->num_objs; ++candidate) {
 		// We tried this earlier
 		if (candidate == st->orig[i]) {
@@ -283,7 +288,8 @@ static bool match_obj_(struct match_state *st, size_t skips, size_t score, size_
 		}
 
 		st->res[i] = candidate;
-		if (match_obj_(st, skips, score + 1, replaced, i + 1)) {
+		size_t obj_score = st->objs[candidate] != 0 ? 1 : 0;
+		if (match_obj_(st, skips, score + obj_score, replaced, i + 1)) {
 			has_best = true;
 		}