drm: add fallback drmModeAddFB for drivers which do not support drmModeAddFB2

This makes wlroots able to run on some big-endian machines like G4 and
G5 systems with ATI Radeon 7500 AGP graphics.

backend/drm/util.c

@@ -170,6 +170,35 @@ const char *conn_get_name(uint32_t type_id) {
 	}
 }
 
+static uint32_t get_fb_for_bo_legacy(struct gbm_bo *bo) {
+	struct gbm_device *gbm = gbm_bo_get_device(bo);
+
+	/* We only support this as a fallback of last resort for ARGB8888 visuals,
+	 * like xf86-video-modesetting does. This is necessary on BE machines. */
+	if (gbm_bo_get_format(bo) != GBM_FORMAT_ARGB8888 ||
+			gbm_bo_get_plane_count(bo) != 1) {
+		wlr_log(WLR_DEBUG,
+			"Invalid visual %x (%d planes) requested for legacy DRM framebuffer",
+			gbm_bo_get_format(bo), gbm_bo_get_plane_count(bo));
+		return 0;
+	}
+
+	int fd = gbm_device_get_fd(gbm);
+	uint32_t width = gbm_bo_get_width(bo);
+	uint32_t height = gbm_bo_get_height(bo);
+	uint32_t depth = 32;
+	uint32_t bpp = gbm_bo_get_bpp(bo);
+	uint32_t pitch = gbm_bo_get_stride(bo);
+	uint32_t handle = gbm_bo_get_handle(bo).u32;
+
+	uint32_t id = 0;
+	if (drmModeAddFB(fd, width, height, depth, bpp, pitch, handle, &id)) {
+		wlr_log_errno(WLR_ERROR, "Unable to add DRM framebuffer");
+	}
+
+	return id;
+}
+
 uint32_t get_fb_for_bo(struct gbm_bo *bo, bool with_modifiers) {
 	struct gbm_device *gbm = gbm_bo_get_device(bo);
 
@@ -199,7 +228,12 @@ uint32_t get_fb_for_bo(struct gbm_bo *bo, bool with_modifiers) {
 	} else {
 		if (drmModeAddFB2(fd, width, height, format, handles, strides,
 				offsets, &id, 0)) {
-			wlr_log_errno(WLR_ERROR, "Unable to add DRM framebuffer");
+			wlr_log_errno(WLR_DEBUG,
+				"Unable to add DRM framebuffer, trying legacy method");
+			id = get_fb_for_bo_legacy(bo);
+			if (id == 0) {
+				wlr_log(WLR_ERROR, "Unable to add DRM framebuffer");
+			}
 		}
 	}