android_ab: Try backup booloader_message

Some devices keep 2 copies of the bootloader_message in the misc
partition and write each in sequence when updating. This ensures that
there is always one valid copy of the bootloader_message. Teach u-boot
to optionally try a backup bootloader_message from a specified offset if
the primary one fails its CRC check.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>

boot/android_ab.c

@@ -85,11 +85,13 @@ static int ab_control_default(struct bootloader_control *abc)
  */
 static int ab_control_create_from_disk(struct blk_desc *dev_desc,
 				       const struct disk_partition *part_info,
-				       struct bootloader_control **abc)
+				       struct bootloader_control **abc,
+				       ulong offset)
 {
 	ulong abc_offset, abc_blocks, ret;
 
-	abc_offset = offsetof(struct bootloader_message_ab, slot_suffix);
+	abc_offset = offset +
+		     offsetof(struct bootloader_message_ab, slot_suffix);
 	if (abc_offset % part_info->blksz) {
 		log_err("ANDROID: Boot control block not block aligned.\n");
 		return -EINVAL;
@@ -135,11 +137,12 @@ static int ab_control_create_from_disk(struct blk_desc *dev_desc,
  */
 static int ab_control_store(struct blk_desc *dev_desc,
 			    const struct disk_partition *part_info,
-			    struct bootloader_control *abc)
+			    struct bootloader_control *abc, ulong offset)
 {
 	ulong abc_offset, abc_blocks, ret;
 
-	abc_offset = offsetof(struct bootloader_message_ab, slot_suffix) /
+	abc_offset = offset +
+		     offsetof(struct bootloader_message_ab, slot_suffix) /
 		     part_info->blksz;
 	abc_blocks = DIV_ROUND_UP(sizeof(struct bootloader_control),
 				  part_info->blksz);
@@ -189,8 +192,11 @@ int ab_select_slot(struct blk_desc *dev_desc, struct disk_partition *part_info,
 	int slot, i, ret;
 	bool store_needed = false;
 	char slot_suffix[4];
+#if ANDROID_AB_BACKUP_OFFSET
+	struct bootloader_control *backup_abc = NULL;
+#endif
 
-	ret = ab_control_create_from_disk(dev_desc, part_info, &abc);
+	ret = ab_control_create_from_disk(dev_desc, part_info, &abc, 0);
 	if (ret < 0) {
 		/*
 		 * This condition represents an actual problem with the code or
@@ -200,22 +206,53 @@ int ab_select_slot(struct blk_desc *dev_desc, struct disk_partition *part_info,
 		return ret;
 	}
 
+#if ANDROID_AB_BACKUP_OFFSET
+	ret = ab_control_create_from_disk(dev_desc, part_info, &backup_abc,
+					  ANDROID_AB_BACKUP_OFFSET);
+	if (ret < 0) {
+		free(abc);
+		return ret;
+	}
+#endif
+
 	crc32_le = ab_control_compute_crc(abc);
 	if (abc->crc32_le != crc32_le) {
 		log_err("ANDROID: Invalid CRC-32 (expected %.8x, found %.8x),",
 			crc32_le, abc->crc32_le);
-		log_err("re-initializing A/B metadata.\n");
+#if ANDROID_AB_BACKUP_OFFSET
+		crc32_le = ab_control_compute_crc(backup_abc);
+		if (backup_abc->crc32_le != crc32_le) {
+			log_err("ANDROID: Invalid backup CRC-32 ")
+			log_err("expected %.8x, found %.8x),",
+				crc32_le, backup_abc->crc32_le);
+#endif
 
-		ret = ab_control_default(abc);
+			log_err("re-initializing A/B metadata.\n");
-		if (ret < 0) {
+
-			free(abc);
+			ret = ab_control_default(abc);
-			return -ENODATA;
+			if (ret < 0) {
+#if ANDROID_AB_BACKUP_OFFSET
+				free(backup_abc);
+#endif
+				free(abc);
+				return -ENODATA;
+			}
+#if ANDROID_AB_BACKUP_OFFSET
+		} else {
+			/*
+			 * Backup is valid. Copy it to the primary
+			 */
+			memcpy(abc, backup_abc, sizeof(*abc));
 		}
+#endif
 		store_needed = true;
 	}
 
 	if (abc->magic != BOOT_CTRL_MAGIC) {
 		log_err("ANDROID: Unknown A/B metadata: %.8x\n", abc->magic);
+#if ANDROID_AB_BACKUP_OFFSET
+		free(backup_abc);
+#endif
 		free(abc);
 		return -ENODATA;
 	}
@@ -223,6 +260,9 @@ int ab_select_slot(struct blk_desc *dev_desc, struct disk_partition *part_info,
 	if (abc->version > BOOT_CTRL_VERSION) {
 		log_err("ANDROID: Unsupported A/B metadata version: %.8x\n",
 			abc->version);
+#if ANDROID_AB_BACKUP_OFFSET
+		free(backup_abc);
+#endif
 		free(abc);
 		return -ENODATA;
 	}
@@ -297,8 +337,21 @@ int ab_select_slot(struct blk_desc *dev_desc, struct disk_partition *part_info,
 
 	if (store_needed) {
 		abc->crc32_le = ab_control_compute_crc(abc);
-		ab_control_store(dev_desc, part_info, abc);
+		ab_control_store(dev_desc, part_info, abc, 0);
 	}
+
+#if ANDROID_AB_BACKUP_OFFSET
+	/*
+	 * If the backup doesn't match the primary, write the primary
+	 * to the backup offset
+	 */
+	if (memcmp(backup_abc, abc, sizeof(*abc)) != 0) {
+		ab_control_store(dev_desc, part_info, abc,
+				 ANDROID_AB_BACKUP_OFFSET);
+	}
+	free(backup_abc);
+#endif
+
 	free(abc);
 
 	if (slot < 0)

common/Kconfig

@@ -936,6 +936,15 @@ config ANDROID_AB
 	  allows a bootloader to try a new version of the system but roll back
 	  to previous version if the new one didn't boot all the way.
 
+config ANDROID_AB_BACKUP_OFFSET
+	hex "Offset of backup bootloader control"
+	depends on ANDROID_AB
+	default 0x0
+	help
+	  If non-zero, a backup bootloader message starting at this offset in
+	  the partition will tried in the event that the primary one (starting
+	  at offset 0) fails its checksum.
+
 endmenu
 
 menu "Blob list"

doc/android/ab.rst

@@ -31,6 +31,12 @@ boot script. This command analyzes and processes A/B metadata stored on a
 special partition (e.g. ``misc``) and determines which slot should be used for
 booting up.
 
+If the A/B metadata partition has a backup bootloader_message block that is used
+to ensure one is always valid even in the event of interruption when writing, it
+can be enabled in your board configuration file::
+
+    CONFIG_ANDROID_AB_BACKUP_OFFSET=0x1000
+
 Command usage
 -------------