UBI: fastmap: fix backward compatibility with image_seq

Some old UBI implementations (e.g. U-Boot) have not implemented the image
sequence feature.
So, when erase blocks are written, the image sequence in the ec header
is lost (set to zero).
UBI scan_all() takes this case into account (commits
32bc482 and
2eadaad)

But fastmap scan functions (ubi_scan_fastmap() and scan_pool()) didn't.

This patch fixes the issue.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

drivers/mtd/ubi/fastmap.c

@@ -407,6 +407,7 @@ static int scan_pool(struct ubi_device *ubi, struct ubi_attach_info *ai,
 	 */
 	for (i = 0; i < pool_size; i++) {
 		int scrub = 0;
+		int image_seq;
 
 		pnum = be32_to_cpu(pebs[i]);
 
@@ -425,7 +426,13 @@ static int scan_pool(struct ubi_device *ubi, struct ubi_attach_info *ai,
 		} else if (ret == UBI_IO_BITFLIPS)
 			scrub = 1;
 
-		if (be32_to_cpu(ech->image_seq) != ubi->image_seq) {
+		/*
+		 * Older UBI implementations have image_seq set to zero, so
+		 * we shouldn't fail if image_seq == 0.
+		 */
+		image_seq = be32_to_cpu(ech->image_seq);
+
+		if (image_seq && (image_seq != ubi->image_seq)) {
 			ubi_err("bad image seq: 0x%x, expected: 0x%x",
 				be32_to_cpu(ech->image_seq), ubi->image_seq);
 			ret = UBI_BAD_FASTMAP;
@@ -923,6 +930,8 @@ int ubi_scan_fastmap(struct ubi_device *ubi, struct ubi_attach_info *ai,
 	}
 
 	for (i = 0; i < used_blocks; i++) {
+		int image_seq;
+
 		pnum = be32_to_cpu(fmsb->block_loc[i]);
 
 		if (ubi_io_is_bad(ubi, pnum)) {
@@ -940,10 +949,17 @@ int ubi_scan_fastmap(struct ubi_device *ubi, struct ubi_attach_info *ai,
 		} else if (ret == UBI_IO_BITFLIPS)
 			fm->to_be_tortured[i] = 1;
 
+		image_seq = be32_to_cpu(ech->image_seq);
 		if (!ubi->image_seq)
-			ubi->image_seq = be32_to_cpu(ech->image_seq);
+			ubi->image_seq = image_seq;
 
-		if (be32_to_cpu(ech->image_seq) != ubi->image_seq) {
+		/*
+		 * Older UBI implementations have image_seq set to zero, so
+		 * we shouldn't fail if image_seq == 0.
+		 */
+		if (image_seq && (image_seq != ubi->image_seq)) {
+			ubi_err("wrong image seq:%d instead of %d",
+				be32_to_cpu(ech->image_seq), ubi->image_seq);
 			ret = UBI_BAD_FASTMAP;
 			goto free_hdr;
 		}