bootcount_ext: Add flag to enable/disable bootcount

After a successful upgrade, multiple problem during boot sequence may
trigger the altbootcmd process.
This patch adds a version and an upgrade_available entries to the
bootcount file to enable/disable the bootcount check.
When failing to read the bootcount file it will consider that bootcount is
enabled, acting as previously, and update the file accordingly.

The bootcount file is only saved when `upgrade_available` is true, this
allows to save writes to the filesystem.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/README.bootcount

@@ -0,0 +1,51 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Boot Count Limit
+================
+
+This allows to detect multiple failed attempts to boot Linux.
+
+After a power-on reset, "bootcount" variable will be initialized with 1, and
+each reboot will increment the value by 1.
+
+If, after a reboot, the new value of "bootcount" exceeds the value of
+"bootlimit", then instead of the standard boot action (executing the contents of
+"bootcmd") an alternate boot action will be performed, and the contents of
+"altbootcmd" will be executed.
+
+If the variable "bootlimit" is not defined in the environment, the Boot Count
+Limit feature is disabled. If it is enabled, but "altbootcmd" is not defined,
+then U-Boot will drop into interactive mode and remain there.
+
+It is the responsibility of some application code (typically a Linux
+application) to reset the variable "bootcount", thus allowing for more boot
+cycles.
+
+BOOTCOUNT_EXT
+-------------
+
+This adds support for maintaining boot count in a file on an EXT filesystem.
+The file to use is define by:
+
+SYS_BOOTCOUNT_EXT_INTERFACE
+SYS_BOOTCOUNT_EXT_DEVPART
+SYS_BOOTCOUNT_EXT_NAME
+
+The format of the file is:
+
+==== =================
+type entry
+==== =================
+u8   magic
+u8   version
+u8   bootcount
+u8   upgrade_available
+==== =================
+
+To prevent unattended usage of "altbootcmd" the "upgrade_available" variable is
+used.
+If "upgrade_available" is 0, "bootcount" is not saved, if "upgrade_available" is
+1 "bootcount" is save.
+So the Userspace Application must set the "upgrade_available" and "bootcount"
+variables to 0, if a boot was successfully.
+This also prevents writes on all reboots.

drivers/bootcount/bootcount_ext.c

@@ -7,11 +7,21 @@
 #include <fs.h>
 #include <mapmem.h>
 
-#define BC_MAGIC	0xbc
+#define BC_MAGIC	0xbd
+#define BC_VERSION	1
+
+typedef struct {
+	u8 magic;
+	u8 version;
+	u8 bootcount;
+	u8 upgrade_available;
+} bootcount_ext_t;
+
+static u8 upgrade_available = 1;
 
 void bootcount_store(ulong a)
 {
-	u8 *buf;
+	bootcount_ext_t *buf;
 	loff_t len;
 	int ret;
 
@@ -21,20 +31,27 @@ void bootcount_store(ulong a)
 		return;
 	}
 
-	buf = map_sysmem(CONFIG_SYS_BOOTCOUNT_ADDR, 2);
-	buf[0] = BC_MAGIC;
-	buf[1] = (a & 0xff);
+	/* Only update bootcount during upgrade process */
+	if (!upgrade_available)
+		return;
+
+	buf = map_sysmem(CONFIG_SYS_BOOTCOUNT_ADDR, sizeof(bootcount_ext_t));
+	buf->magic = BC_MAGIC;
+	buf->version = BC_VERSION;
+	buf->bootcount = (a & 0xff);
+	buf->upgrade_available = upgrade_available;
 	unmap_sysmem(buf);
 
 	ret = fs_write(CONFIG_SYS_BOOTCOUNT_EXT_NAME,
-		       CONFIG_SYS_BOOTCOUNT_ADDR, 0, 2, &len);
+		       CONFIG_SYS_BOOTCOUNT_ADDR, 0, sizeof(bootcount_ext_t),
+		       &len);
 	if (ret != 0)
 		puts("Error storing bootcount\n");
 }
 
 ulong bootcount_load(void)
 {
-	u8 *buf;
+	bootcount_ext_t *buf;
 	loff_t len_read;
 	int ret;
 
@@ -45,15 +62,20 @@ ulong bootcount_load(void)
 	}
 
 	ret = fs_read(CONFIG_SYS_BOOTCOUNT_EXT_NAME, CONFIG_SYS_BOOTCOUNT_ADDR,
-		      0, 2, &len_read);
-	if (ret != 0 || len_read != 2) {
+		      0, sizeof(bootcount_ext_t), &len_read);
+	if (ret != 0 || len_read != sizeof(bootcount_ext_t)) {
 		puts("Error loading bootcount\n");
 		return 0;
 	}
 
-	buf = map_sysmem(CONFIG_SYS_BOOTCOUNT_ADDR, 2);
-	if (buf[0] == BC_MAGIC)
-		ret = buf[1];
+	buf = map_sysmem(CONFIG_SYS_BOOTCOUNT_ADDR, sizeof(bootcount_ext_t));
+	if (buf->magic == BC_MAGIC && buf->version == BC_VERSION) {
+		upgrade_available = buf->upgrade_available;
+		if (upgrade_available)
+			ret = buf->bootcount;
+	} else {
+		puts("Incorrect bootcount file\n");
+	}
 
 	unmap_sysmem(buf);