drm: Deprecate non-atomic mode sets

We've been keeping these poorly tested paths around long enough, it's time
to deprecate them, to be dropped after the next major release.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>

libweston/backend-drm/kms.c

@@ -2069,6 +2069,15 @@ init_kms_caps(struct drm_device *device)
 	weston_log("DRM: %s atomic modesetting\n",
 		   device->atomic_modeset ? "supports" : "does not support");
 
+	if (!device->atomic_modeset) {
+#ifdef ALLOW_DEPRECATED_MODESET
+		weston_log("DRM Warning: Non-atomic modeset support is deprecated and will be removed.\n");
+#else
+		weston_log("Error: Kernel DRM KMS does not support DRM_CLIENT_CAP_ATOMIC.\n");
+		return -1;
+#endif
+	}
+
 	if (!getenv("WESTON_DISABLE_GBM_MODIFIERS")) {
 		ret = drmGetCap(device->kms_device->fd, DRM_CAP_ADDFB2_MODIFIERS, &cap);
 		if (ret == 0)

meson.build

@@ -221,6 +221,9 @@ else
 	prog_glslang = find_program('', required : false)
 endif
 
+if get_option('allow-deprecated-modeset')
+	config_h.set('ALLOW_DEPRECATED_MODESET', '1')
+endif
 
 subdir('include')
 subdir('protocol')

meson_options.txt

@@ -209,3 +209,10 @@ option(
 	value : false,
 	description : 'Enable performance analysis with Perfetto.'
 )
+
+option(
+	'allow-deprecated-modeset',
+	type: 'boolean',
+	value: false,
+	description: 'DEPRECATED: Non-atomic modeset paths will be removed'
+)