mtd: check for max_bitflips in mtd_read_oob()

mtd_read_oob() has some unexpected similarities to mtd_read(). For
instance, when ops->datbuf != NULL, nand_base.c might return max_bitflips;
however, when ops->datbuf == NULL, nand_base's code potentially could
return -EUCLEAN (no in-tree drivers do this yet). In any case where the
driver might return max_bitflips, we should translate this into an
appropriate return code using the bitflip_threshold.

Essentially, mtd_read_oob() duplicates the logic from mtd_read().

This prevents users of mtd_read_oob() from receiving a positive return
value (i.e., from max_bitflips) and interpreting it as an unknown error.

Artem: amend comments.
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Mike Dunn <mikedunn@newsguy.com>
Reviewed-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/mtdcore.c

@@ -860,10 +860,22 @@ EXPORT_SYMBOL_GPL(mtd_panic_write);
 
 int mtd_read_oob(struct mtd_info *mtd, loff_t from, struct mtd_oob_ops *ops)
 {
+	int ret_code;
 	ops->retlen = ops->oobretlen = 0;
 	if (!mtd->_read_oob)
 		return -EOPNOTSUPP;
-	return mtd->_read_oob(mtd, from, ops);
+	/*
+	 * In cases where ops->datbuf != NULL, mtd->_read_oob() has semantics
+	 * similar to mtd->_read(), returning a non-negative integer
+	 * representing max bitflips. In other cases, mtd->_read_oob() may
+	 * return -EUCLEAN. In all cases, perform similar logic to mtd_read().
+	 */
+	ret_code = mtd->_read_oob(mtd, from, ops);
+	if (unlikely(ret_code < 0))
+		return ret_code;
+	if (mtd->ecc_strength == 0)
+		return 0;	/* device lacks ecc */
+	return ret_code >= mtd->bitflip_threshold ? -EUCLEAN : 0;
 }
 EXPORT_SYMBOL_GPL(mtd_read_oob);