boot: pxe_utils: Add fallback support

When configured correctly, we can detect when boot fails after the boot
process has been handed over to the kernel through the use of U-Boot's
bootcount support. In some instances, such as when we are performing
atomic updates via a system such as OSTree, it is desirable to provide a
fallback option so that we can return to a previous (hopefully working)
state.

Add a "fallback" option to the supported extlinux configuration options
that points to a label like "default" so that we can utilise this in
later commits.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

boot/pxe_utils.c

@@ -781,6 +781,7 @@ enum token_type {
 	T_IPAPPEND,
 	T_BACKGROUND,
 	T_KASLRSEED,
+	T_FALLBACK,
 	T_INVALID
 };
 
@@ -814,6 +815,7 @@ static const struct token keywords[] = {
 	{"ipappend", T_IPAPPEND,},
 	{"background", T_BACKGROUND,},
 	{"kaslrseed", T_KASLRSEED,},
+	{"fallback", T_FALLBACK,},
 	{NULL, T_INVALID}
 };
 
@@ -1356,6 +1358,18 @@ static int parse_pxefile_top(struct pxe_context *ctx, char *p, unsigned long bas
 
 			break;
 
+		case T_FALLBACK:
+			err = parse_sliteral(&p, &label_name);
+
+			if (label_name) {
+				if (cfg->fallback_label)
+					free(cfg->fallback_label);
+
+				cfg->fallback_label = label_name;
+			}
+
+			break;
+
 		case T_INCLUDE:
 			err = handle_include(ctx, &p,
 					     base + ALIGN(strlen(b), 4), cfg,
@@ -1395,6 +1409,7 @@ void destroy_pxe_menu(struct pxe_menu *cfg)
 
 	free(cfg->title);
 	free(cfg->default_label);
+	free(cfg->fallback_label);
 
 	list_for_each_safe(pos, n, &cfg->labels) {
 		label = list_entry(pos, struct pxe_label, list);

doc/README.pxe

@@ -120,6 +120,11 @@ Unrecognized commands are ignored.
 default <label>	    - the label named here is treated as the default and is
 		      the first label 'pxe boot' attempts to boot.
 
+fallback <label>    - the label named here is treated as a fallback option that
+		      may be attempted should it be detected that booting of
+		      the default has failed to complete, for example via
+		      U-Boot's boot count limit functionality.
+
 menu title <string> - sets a title for the menu of labels being displayed.
 
 menu include <path> - use tftp to retrieve the pxe file at <path>, which

include/pxe_utils.h

@@ -62,6 +62,7 @@ struct pxe_label {
  *
  * title - the name of the menu as given by a 'menu title' line.
  * default_label - the name of the default label, if any.
+ * fallback_label - the name of the fallback label, if any.
  * bmp - the bmp file name which is displayed in background
  * timeout - time in tenths of a second to wait for a user key-press before
  *           booting the default label.
@@ -73,6 +74,7 @@ struct pxe_label {
 struct pxe_menu {
 	char *title;
 	char *default_label;
+	char *fallback_label;
 	char *bmp;
 	int timeout;
 	int prompt;