backend/drm: Add async page flip support to legacy

Atomic doesn't support such flags yet.

backend/drm/drm.c

@@ -106,6 +106,10 @@ bool check_drm_features(struct wlr_drm_backend *drm) {
 		drm->iface = &atomic_iface;
 	}
 
+	if (drm->iface == &legacy_iface) {
+		drm->supports_tearing_page_flips = drmGetCap(drm->fd, DRM_CAP_ASYNC_PAGE_FLIP, &cap) == 0 && cap == 1;
+	}
+
 	int ret = drmGetCap(drm->fd, DRM_CAP_TIMESTAMP_MONOTONIC, &cap);
 	drm->clock = (ret == 0 && cap == 1) ? CLOCK_MONOTONIC : CLOCK_REALTIME;
 
@@ -650,6 +654,11 @@ static bool drm_connector_test(struct wlr_output *output,
 		if (!drm_connector_state_update_primary_fb(conn, &pending)) {
 			goto out;
 		}
+
+		if (pending.base->tearing_page_flip && !conn->backend->supports_tearing_page_flips) {
+			wlr_log(WLR_ERROR, "Attempted to submit a tearing page flip to an unsupported backend!");
+			goto out;
+		}
 	}
 	if (state->committed & WLR_OUTPUT_STATE_LAYERS) {
 		if (!drm_connector_set_pending_layer_fbs(conn, pending.base)) {
@@ -725,7 +734,7 @@ bool drm_connector_commit_state(struct wlr_drm_connector *conn,
 		// 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) {
+		if (conn->pending_page_flip_crtc && !pending.modeset && !pending.base->tearing_page_flip) {
 			wlr_drm_conn_log(conn, WLR_ERROR, "Failed to page-flip output: "
 				"a page-flip is already pending");
 			goto out;
@@ -748,6 +757,9 @@ bool drm_connector_commit_state(struct wlr_drm_connector *conn,
 	}
 
 	uint32_t flags = pending.active ? DRM_MODE_PAGE_FLIP_EVENT : 0;
+	if (pending.base->tearing_page_flip) {
+		flags |= DRM_MODE_PAGE_FLIP_ASYNC;
+	}
 
 	ok = drm_crtc_commit(conn, &pending, flags, false);
 	if (!ok) {

backend/drm/legacy.c

@@ -175,8 +175,13 @@ static bool legacy_crtc_commit(struct wlr_drm_connector *conn,
 	}
 
 	if (flags & DRM_MODE_PAGE_FLIP_EVENT) {
+		uint32_t page_flags = DRM_MODE_PAGE_FLIP_EVENT;
+		if (flags & DRM_MODE_PAGE_FLIP_ASYNC) {
+			page_flags |= DRM_MODE_PAGE_FLIP_ASYNC;
+		}
+
 		if (drmModePageFlip(drm->fd, crtc->id, fb_id,
-				DRM_MODE_PAGE_FLIP_EVENT, drm)) {
+				page_flags, drm)) {
 			wlr_drm_conn_log_errno(conn, WLR_ERROR, "drmModePageFlip failed");
 			return false;
 		}

include/backend/drm/drm.h

@@ -114,6 +114,8 @@ struct wlr_drm_backend {
 	uint64_t cursor_width, cursor_height;
 
 	struct wlr_drm_format_set mgpu_formats;
+
+	bool supports_tearing_page_flips;
 };
 
 struct wlr_drm_mode {

include/wlr/types/wlr_output.h

@@ -89,6 +89,12 @@ struct wlr_output_state {
 	enum wl_output_subpixel subpixel;
 
 	struct wlr_buffer *buffer;
+	/* Request a tearing page-flip. When enabled, this may cause the output to
+	 * display a part of the previous buffer and a part of the current buffer at
+	 * the same time. The backend may reject the commit if a tearing page-flip
+	 * cannot be performed, in which case the caller should fall back to a
+	 * regular page-flip at the next wlr_output.frame event. */
+	bool tearing_page_flip;
 
 	enum wlr_output_state_mode_type mode_type;
 	struct wlr_output_mode *mode;

types/output/output.c

@@ -665,6 +665,9 @@ static bool output_basic_test(struct wlr_output *output,
 			wlr_log(WLR_DEBUG, "Primary buffer size mismatch");
 			return false;
 		}
+	} else if (state->tearing_page_flip) {
+		wlr_log(WLR_ERROR, "Trying to commit a tearing page flip without a buffer?");
+		return false;
 	}
 
 	if (state->committed & WLR_OUTPUT_STATE_RENDER_FORMAT) {