build: deprecate fbdev backend

Rename the build option to "deprecated-backend-fbdev" so that a
previously configured build dir doesn't retain the old setting.
This is consistent with the existing "deprecated-wl-shell" option.
Make the option default to "false".

Print a warning when fbdev is force-enabled.

Signed-off-by: Simon Ser <contact@emersion.fr>
References: https://gitlab.freedesktop.org/wayland/weston/-/issues/581

.gitlab-ci.yml

@@ -283,6 +283,7 @@ aarch64-debian-container_prep:
       -Dwerror=true
       -Dtest-skip-is-failure=true
       -Dlauncher-libseat=true
+      -Ddeprecated-backend-fbdev=true
   after_script:
   - ninja -C "$BUILDDIR" coverage-html > "$BUILDDIR/meson-logs/ninja-coverage-html.txt"
   - ninja -C "$BUILDDIR" coverage-xml

libweston/backend-fbdev/fbdev.c

@@ -893,6 +893,8 @@ fbdev_backend_create(struct weston_compositor *compositor,
 		seat_id = param->seat_id;
 
 	weston_log("initializing fbdev backend\n");
+	weston_log("warning: the fbdev backend is deprecated, please migrate "
+		   "to the DRM backend\n");
 
 	backend = zalloc(sizeof *backend);
 	if (backend == NULL)

libweston/backend-fbdev/meson.build

@@ -1,7 +1,10 @@
-if not get_option('backend-fbdev')
+if not get_option('deprecated-backend-fbdev')
 	subdir_done()
 endif
 
+warning('Support for the deprecated fbdev backend is enabled.')
+warning('This feature will be removed in a future version.')
+
 config_h.set('BUILD_FBDEV_COMPOSITOR', '1')
 
 srcs_fbdev = [

meson_options.txt

@@ -45,10 +45,10 @@ option(
 	description: 'Weston backend: X11 (nested)'
 )
 option(
-	'backend-fbdev',
+	'deprecated-backend-fbdev',
 	type: 'boolean',
-	value: true,
+	value: false,
-	description: 'Weston backend: fbdev'
+	description: 'Weston backend: fbdev (deprecated)'
 )
 option(
 	'backend-default',