mtd: nand: handle ECC errors in OOB

While the standard NAND OOB functions do not do ECC on the spare area,
it is possible for a driver to supply its own OOB ECC functions (e.g., HW
ECC). nand_do_read_oob should act like nand_do_read_ops in checking the
ECC stats and returning -EBADMSG or -EUCLEAN on uncorrectable errors or
correctable bitflips, respectively. These error codes could be used in
flash-based BBT code or in YAFFS, for example.

Doing this, however, messes with the behavior of mtd_do_readoob. Now,
mtd_do_readoob should check whether we had -EUCLEAN or -EBADMSG errors
and discard those as "non-fatal" so that the ioctls can still succeed
with (possibly uncorrected) data.
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/mtdchar.c

@@ -473,6 +473,21 @@ static int mtd_do_readoob(struct mtd_info *mtd, uint64_t start,
 		ret = -EFAULT;
 
 	kfree(ops.oobbuf);
+
+	/*
+	 * NAND returns -EBADMSG on ECC errors, but it returns the OOB
+	 * data. For our userspace tools it is important to dump areas
+	 * with ECC errors!
+	 * For kernel internal usage it also might return -EUCLEAN
+	 * to signal the caller that a bitflip has occured and has
+	 * been corrected by the ECC algorithm.
+	 *
+	 * Note: most NAND ECC algorithms do not calculate ECC
+	 * for the OOB area.
+	 */
+	if (ret == -EUCLEAN || ret == -EBADMSG)
+		return 0;
+
 	return ret;
 }
 

drivers/mtd/nand/nand_base.c

@@ -1750,6 +1750,7 @@ static int nand_do_read_oob(struct mtd_info *mtd, loff_t from,
 {
 	int page, realpage, chipnr, sndcmd = 1;
 	struct nand_chip *chip = mtd->priv;
+	struct mtd_ecc_stats stats;
 	int blkcheck = (1 << (chip->phys_erase_shift - chip->page_shift)) - 1;
 	int readlen = ops->ooblen;
 	int len;
@@ -1758,6 +1759,8 @@ static int nand_do_read_oob(struct mtd_info *mtd, loff_t from,
 	DEBUG(MTD_DEBUG_LEVEL3, "%s: from = 0x%08Lx, len = %i\n",
 			__func__, (unsigned long long)from, readlen);
 
+	stats = mtd->ecc_stats;
+
 	if (ops->mode == MTD_OOB_AUTO)
 		len = chip->ecc.layout->oobavail;
 	else
@@ -1828,7 +1831,11 @@ static int nand_do_read_oob(struct mtd_info *mtd, loff_t from,
 	}
 
 	ops->oobretlen = ops->ooblen;
-	return 0;
+
+	if (mtd->ecc_stats.failed - stats.failed)
+		return -EBADMSG;
+
+	return  mtd->ecc_stats.corrected - stats.corrected ? -EUCLEAN : 0;
 }
 
 /**