UBIFS: add R/O compatibility
Now UBIFS is supported by u-boot. If we ever decide to change the
media format, then people will have to upgrade their u-boots to
mount new format images. However, very often it is possible to
preserve R/O forward-compatibility, even though the write
forward-compatibility is not preserved.
This patch introduces a new super-block field which stores the
R/O compatibility version.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Acked-by: Adrian Hunter <Adrian.Hunter@nokia.com>
diff --git a/fs/ubifs/ubifs-media.h b/fs/ubifs/ubifs-media.h
index b25fc36..3eee07e 100644
--- a/fs/ubifs/ubifs-media.h
+++ b/fs/ubifs/ubifs-media.h
@@ -36,9 +36,31 @@
/* UBIFS node magic number (must not have the padding byte first or last) */
#define UBIFS_NODE_MAGIC 0x06101831
-/* UBIFS on-flash format version */
+/*
+ * UBIFS on-flash format version. This version is increased when the on-flash
+ * format is changing. If this happens, UBIFS is will support older versions as
+ * well. But older UBIFS code will not support newer formats. Format changes
+ * will be rare and only when absolutely necessary, e.g. to fix a bug or to add
+ * a new feature.
+ *
+ * UBIFS went into mainline kernel with format version 4. The older formats
+ * were development formats.
+ */
#define UBIFS_FORMAT_VERSION 4
+/*
+ * Read-only compatibility version. If the UBIFS format is changed, older UBIFS
+ * implementations will not be able to mount newer formats in read-write mode.
+ * However, depending on the change, it may be possible to mount newer formats
+ * in R/O mode. This is indicated by the R/O compatibility version which is
+ * stored in the super-block.
+ *
+ * This is needed to support boot-loaders which only need R/O mounting. With
+ * this flag it is possible to do UBIFS format changes without a need to update
+ * boot-loaders.
+ */
+#define UBIFS_RO_COMPAT_VERSION 0
+
/* Minimum logical eraseblock size in bytes */
#define UBIFS_MIN_LEB_SZ (15*1024)
@@ -53,7 +75,7 @@
/*
* If compressed data length is less than %UBIFS_MIN_COMPRESS_DIFF bytes
- * shorter than uncompressed data length, UBIFS preferes to leave this data
+ * shorter than uncompressed data length, UBIFS prefers to leave this data
* node uncompress, because it'll be read faster.
*/
#define UBIFS_MIN_COMPRESS_DIFF 64
@@ -586,6 +608,7 @@
* @padding2: reserved for future, zeroes
* @time_gran: time granularity in nanoseconds
* @uuid: UUID generated when the file system image was created
+ * @ro_compat_version: UBIFS R/O compatibility version
*/
struct ubifs_sb_node {
struct ubifs_ch ch;
@@ -612,7 +635,8 @@
__le64 rp_size;
__le32 time_gran;
__u8 uuid[16];
- __u8 padding2[3972];
+ __le32 ro_compat_version;
+ __u8 padding2[3968];
} __attribute__ ((packed));
/**