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>

1 files changed, 12 insertions, 2 deletions

diff --git a/fs/ubifs/super.c b/fs/ubifs/super.c
index 372c7fb66531..302a2056422e 100644
--- a/fs/ubifs/super.c
+++ b/fs/ubifs/super.c
@@ -1351,8 +1351,9 @@ static int mount_ubifs(struct ubifs_info *c)
         x = (long long)c->log_lebs * c->leb_size + c->max_bud_bytes;
         ubifs_msg("journal size:       %lld bytes (%lld KiB, %lld MiB, %d "
                   "LEBs)", x, x >> 10, x >> 20, c->log_lebs + c->max_bud_cnt);
-        ubifs_msg("media format:       %d (latest is %d)",
-                  c->fmt_version, UBIFS_FORMAT_VERSION);
+        ubifs_msg("media format:       w%d/r%d (latest is w%d/r%d)",
+                  c->fmt_version, c->ro_compat_version,
+                  UBIFS_FORMAT_VERSION, UBIFS_RO_COMPAT_VERSION);
         ubifs_msg("default compressor: %s", ubifs_compr_name(c->default_compr));
         ubifs_msg("reserved for root:  %llu bytes (%llu KiB)",
                 c->report_rp_size, c->report_rp_size >> 10);
@@ -1492,6 +1493,15 @@ static int ubifs_remount_rw(struct ubifs_info *c)
 {
         int err, lnum;
 
+        if (c->rw_incompat) {
+                ubifs_err("the file-system is not R/W-compatible");
+                ubifs_msg("on-flash format version is w%d/r%d, but software "
+                          "only supports up to version w%d/r%d", c->fmt_version,
+                          c->ro_compat_version, UBIFS_FORMAT_VERSION,
+                          UBIFS_RO_COMPAT_VERSION);
+                return -EROFS;
+        }
+
         mutex_lock(&c->umount_mutex);
         dbg_save_space_info(c);
         c->remounting_rw = 1;