mtd: nand: relax ecc.read_page() return value for uncorrectable ECC

The comment for ecc.read_page() requires that it should return
"0 if bitflips uncorrectable".

Actually, drivers could return positive values when uncorrectable
bitflips occur.  For example, nand_read_page_swecc() is the case.
If ecc.correct() returns -EBADMSG for the first ECC sector, and
a positive value for the second one, nand_read_page_swecc() returns
a positive max_bitflips and increments ecc_stats.failed for the same
page.

The requirement can be relaxed by tweaking nand_do_read_ops().
Move the max_bitflips calculation below the retry.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Suggested-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>

drivers/mtd/nand/nand_base.c

@@ -1993,8 +1993,6 @@ static int nand_do_read_ops(struct mtd_info *mtd, loff_t from,
 				break;
 			}
 
-			max_bitflips = max_t(unsigned int, max_bitflips, ret);
-
 			/* Transfer not aligned data */
 			if (use_bufpoi) {
 				if (!NAND_HAS_SUBPAGE_READ(chip) && !oob &&
@@ -2045,6 +2043,7 @@ static int nand_do_read_ops(struct mtd_info *mtd, loff_t from,
 			}
 
 			buf += bytes;
+			max_bitflips = max_t(unsigned int, max_bitflips, ret);
 		} else {
 			memcpy(buf, chip->buffers->databuf + col, bytes);
 			buf += bytes;

include/linux/mtd/nand.h

@@ -516,7 +516,7 @@ static inline void nand_hw_control_init(struct nand_hw_control *nfc)
  *			out-of-band data).
  * @read_page:	function to read a page according to the ECC generator
  *		requirements; returns maximum number of bitflips corrected in
- *		any single ECC step, 0 if bitflips uncorrectable, -EIO hw error
+ *		any single ECC step, -EIO hw error
  * @read_subpage:	function to read parts of the page covered by ECC;
  *			returns same as read_page()
  * @write_subpage:	function to write parts of the page covered by ECC.