[PATCH] UBI: compatible fallback in absense of sequence numbers

[Artem Bityutskiy]

From: Adrian Hunter <Adrian.Hunter at nokia.com>

Fall back onto thinking everything's OK if either of the sequence
numbers we are asked to compare is zero, which is what was used
before sequence numbers were introduced.

[ Artem: modified the patch to be applicable to upstream UBI, added
        big comment ]

Signed-off-by: Adrian Hunter <ext-adrian.hunter at nokia.com>
Signed-off-by: Phil Carmody <ext-phil.2.carmody at nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy at nokia.com>
---
 drivers/mtd/ubi/scan.c |   13 ++++++++++++-
 1 files changed, 12 insertions(+), 1 deletions(-)

diff --git a/drivers/mtd/ubi/scan.c b/drivers/mtd/ubi/scan.c
index be4994b..e7161ad 100644
--- a/drivers/mtd/ubi/scan.c
+++ b/drivers/mtd/ubi/scan.c
@@ -782,11 +782,22 @@ static int process_eb(struct ubi_device *ubi, struct ubi_scan_info *si,
 			return -EINVAL;
 		}
 
+		/*
+		 * Make sure that all PEBs have the same image sequence number.
+		 * This allows us to detect situations when users flash UBI
+		 * images incorrectly, so that the flash has the new UBI image
+		 * and leftovers from the old one. This feature was added
+		 * relatively recently, and the sequence number was always
+		 * zero, because old UBI implementations always set it to zero.
+		 * For this reasons, we do not panic if some PEBs have zero
+		 * sequence number, while other PEBs have non-zero sequence
+		 * number.
+		 */
 		image_seq = be32_to_cpu(ech->image_seq);
 		if (!si->image_seq_set) {
 			ubi->image_seq = image_seq;
 			si->image_seq_set = 1;
-		} else if (ubi->image_seq != image_seq) {
+		} else if (ubi->image_seq && ubi->image_seq != image_seq) {
 			ubi_err("bad image sequence number %d in PEB %d, "
 				"expected %d", image_seq, pnum, ubi->image_seq);
 			ubi_dbg_dump_ec_hdr(ech);
-- 
1.6.0.6