backend/drm: introduce page-flip tracking struct

Introduce a per-page-flip tracking struct passed to the kernel
when we request a page-flip event for an atomic commit. The kernel
will pass us back this pointer when delivering the event.

This eliminates any risk of mixing up events together. In particular,
if two events are pending, or if the CRTC of a connector is swapped,
we no longer blow up in the page-flip event handler.

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3753

1 files changed, 44 insertions, 27 deletions

diff --git a/backend/drm/drm.c b/backend/drm/drm.c
index e8a6f5c6..92feed6c 100644
--- a/backend/drm/drm.c
+++ b/backend/drm/drm.c
@@ -412,15 +412,32 @@ static struct wlr_drm_layer *get_or_create_layer(struct wlr_drm_backend *drm,
 	return layer;
 }
 
+static void drm_connector_set_pending_page_flip(struct wlr_drm_connector *conn,
+		struct wlr_drm_page_flip *page_flip) {
+	if (conn->pending_page_flip != NULL) {
+		conn->pending_page_flip->conn = NULL;
+	}
+	conn->pending_page_flip = page_flip;
+}
+
 static bool drm_crtc_commit(struct wlr_drm_connector *conn,
 		const struct wlr_drm_connector_state *state,
 		uint32_t flags, bool test_only) {
 	// Disallow atomic-only flags
 	assert((flags & ~DRM_MODE_PAGE_FLIP_FLAGS) == 0);
 
+	struct wlr_drm_page_flip *page_flip = NULL;
+	if (flags & DRM_MODE_PAGE_FLIP_EVENT) {
+		page_flip = calloc(1, sizeof(*page_flip));
+		if (page_flip == NULL) {
+			return false;
+		}
+		page_flip->conn = conn;
+	}
+
 	struct wlr_drm_backend *drm = conn->backend;
 	struct wlr_drm_crtc *crtc = conn->crtc;
-	bool ok = drm->iface->crtc_commit(conn, state, flags, test_only);
+	bool ok = drm->iface->crtc_commit(conn, state, page_flip, flags, test_only);
 	if (ok && !test_only) {
 		drm_fb_clear(&crtc->primary->queued_fb);
 		if (state->primary_fb != NULL) {
@@ -434,6 +451,8 @@ static bool drm_crtc_commit(struct wlr_drm_connector *conn,
 		wl_list_for_each(layer, &crtc->layers, link) {
 			drm_fb_move(&layer->queued_fb, &layer->pending_fb);
 		}
+
+		drm_connector_set_pending_page_flip(conn, page_flip);
 	} else {
 		// The set_cursor() hook is a bit special: it's not really synchronized
 		// to commit() or test(). Once set_cursor() returns true, the new
@@ -446,6 +465,8 @@ static bool drm_crtc_commit(struct wlr_drm_connector *conn,
 		wl_list_for_each(layer, &crtc->layers, link) {
 			drm_fb_clear(&layer->pending_fb);
 		}
+
+		free(page_flip);
 	}
 	return ok;
 }
@@ -732,16 +753,6 @@ bool drm_connector_commit_state(struct wlr_drm_connector *conn,
 		if (!drm_connector_state_update_primary_fb(conn, &pending)) {
 			goto out;
 		}
-
-		// wlr_drm_interface.crtc_commit will perform either a non-blocking
-		// page-flip, either a blocking modeset. When performing a blocking modeset
-		// we'll wait for all queued page-flips to complete, so we don't need this
-		// safeguard.
-		if (conn->pending_page_flip_crtc && !pending.modeset) {
-			wlr_drm_conn_log(conn, WLR_ERROR, "Failed to page-flip output: "
-				"a page-flip is already pending");
-			goto out;
-		}
 	}
 	if (pending.base->committed & WLR_OUTPUT_STATE_LAYERS) {
 		if (!drm_connector_set_pending_layer_fbs(conn, pending.base)) {
@@ -759,7 +770,19 @@ bool drm_connector_commit_state(struct wlr_drm_connector *conn,
 		}
 	}
 
-	uint32_t flags = pending.active ? DRM_MODE_PAGE_FLIP_EVENT : 0;
+	uint32_t flags = 0;
+	if (pending.active) {
+		flags |= DRM_MODE_PAGE_FLIP_EVENT;
+		// wlr_drm_interface.crtc_commit will perform either a non-blocking
+		// page-flip, either a blocking modeset. When performing a blocking modeset
+		// we'll wait for all queued page-flips to complete, so we don't need this
+		// safeguard.
+		if (conn->pending_page_flip != NULL && !pending.modeset) {
+			wlr_drm_conn_log(conn, WLR_ERROR, "Failed to page-flip output: "
+				"a page-flip is already pending");
+			goto out;
+		}
+	}
 	if (pending.base->tearing_page_flip) {
 		flags |= DRM_MODE_PAGE_FLIP_ASYNC;
 	}
@@ -777,9 +800,6 @@ bool drm_connector_commit_state(struct wlr_drm_connector *conn,
 		conn->cursor_enabled = false;
 		conn->crtc = NULL;
 	}
-	if (flags & DRM_MODE_PAGE_FLIP_EVENT) {
-		conn->pending_page_flip_crtc = conn->crtc->id;
-	}
 
 out:
 	drm_connector_state_finish(&pending);
@@ -1029,7 +1049,7 @@ static void drm_connector_destroy_output(struct wlr_output *output) {
 	dealloc_crtc(conn);
 
 	conn->status = DRM_MODE_DISCONNECTED;
-	conn->pending_page_flip_crtc = 0;
+	drm_connector_set_pending_page_flip(conn, NULL);
 
 	struct wlr_drm_mode *mode, *mode_tmp;
 	wl_list_for_each_safe(mode, mode_tmp, &conn->output.modes, wlr_mode.link) {
@@ -1660,22 +1680,19 @@ static int mhz_to_nsec(int mhz) {
 
 static void handle_page_flip(int fd, unsigned seq,
 		unsigned tv_sec, unsigned tv_usec, unsigned crtc_id, void *data) {
-	struct wlr_drm_backend *drm = data;
+	struct wlr_drm_page_flip *page_flip = data;
 
-	bool found = false;
-	struct wlr_drm_connector *conn;
-	wl_list_for_each(conn, &drm->connectors, link) {
-		if (conn->pending_page_flip_crtc == crtc_id) {
-			found = true;
-			break;
-		}
+	struct wlr_drm_connector *conn = page_flip->conn;
+	if (conn != NULL) {
+		conn->pending_page_flip = NULL;
 	}
-	if (!found) {
-		wlr_log(WLR_DEBUG, "Unexpected page-flip event for CRTC %u", crtc_id);
+	free(page_flip);
+
+	if (conn == NULL) {
 		return;
 	}
 
-	conn->pending_page_flip_crtc = 0;
+	struct wlr_drm_backend *drm = conn->backend;
 
 	if (conn->status != DRM_MODE_CONNECTED || conn->crtc == NULL) {
 		wlr_drm_conn_log(conn, WLR_DEBUG,