drm/backend: add support for cursor hotspots

Enables DRM_CLIENT_CAP_CURSOR_PLANE_HOTSPOT and sets the cursor hotspots on the
cursor plane so virtualized drivers can accelerate mouse movement.

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3776

backend/drm/atomic.c

@@ -353,6 +353,10 @@ static void set_plane_props(struct atomic *atom, struct wlr_drm_backend *drm,
 	atomic_add(atom, id, props->crtc_y, (uint64_t)y);
 }
 
+static bool supports_cursor_hotspots(const struct wlr_drm_plane* plane) {
+	return plane->props.hotspot_x && plane->props.hotspot_y;
+}
+
 static void atomic_connector_add(struct atomic *atom,
 		const struct wlr_drm_connector_state *state, bool modeset) {
 	struct wlr_drm_connector *conn = state->connector;
@@ -391,6 +395,12 @@ static void atomic_connector_add(struct atomic *atom,
 			if (drm_connector_is_cursor_visible(conn)) {
 				set_plane_props(atom, drm, crtc->cursor, state->cursor_fb,
 					crtc->id, conn->cursor_x, conn->cursor_y);
+				if (supports_cursor_hotspots(crtc->cursor)) {
+					atomic_add(atom, crtc->cursor->id,
+						crtc->cursor->props.hotspot_x, conn->cursor_hotspot_x);
+					atomic_add(atom, crtc->cursor->id,
+						crtc->cursor->props.hotspot_y, conn->cursor_hotspot_y);
+				}
 			} else {
 				plane_disable(atom, crtc->cursor);
 			}

backend/drm/drm.c

@@ -111,6 +111,11 @@ bool check_drm_features(struct wlr_drm_backend *drm) {
 		wlr_log(WLR_DEBUG, "Using atomic DRM interface");
 		drm->iface = &atomic_iface;
 	}
+#ifdef DRM_CLIENT_CAP_CURSOR_PLANE_HOTSPOT
+	if (drm->iface == &atomic_iface && drmSetClientCap(drm->fd, DRM_CLIENT_CAP_CURSOR_PLANE_HOTSPOT, 1) == 0) {
+		wlr_log(WLR_INFO, "DRM_CLIENT_CAP_CURSOR_PLANE_HOTSPOT supported");
+	}
+#endif
 
 	if (drm->iface == &legacy_iface) {
 		drm->supports_tearing_page_flips = drmGetCap(drm->fd, DRM_CAP_ASYNC_PAGE_FLIP, &cap) == 0 && cap == 1;

backend/drm/properties.c

@@ -53,6 +53,8 @@ static const struct prop_info plane_info[] = {
 	{ "CRTC_Y", INDEX(crtc_y) },
 	{ "FB_DAMAGE_CLIPS", INDEX(fb_damage_clips) },
 	{ "FB_ID", INDEX(fb_id) },
+	{ "HOTSPOT_X", INDEX(hotspot_x) },
+	{ "HOTSPOT_Y", INDEX(hotspot_y) },
 	{ "IN_FORMATS", INDEX(in_formats) },
 	{ "SRC_H", INDEX(src_h) },
 	{ "SRC_W", INDEX(src_w) },

include/backend/drm/properties.h

@@ -65,8 +65,10 @@ union wlr_drm_plane_props {
 		uint32_t fb_id;
 		uint32_t crtc_id;
 		uint32_t fb_damage_clips;
+		uint32_t hotspot_x;
+		uint32_t hotspot_y;
 	};
-	uint32_t props[14];
+	uint32_t props[16];
 };
 
 bool get_drm_connector_props(int fd, uint32_t id,