mtd: track maximum number of bitflips for each read request

mtd_read_oob() callers are currently oblivious to the details of ECC
errors detected during the read operation - they only learn (through the
return value) whether any corrected bitflips or uncorrectable errors
occurred.  More detailed ECC information can be useful to user-space
applications for making better-informed choices about moving data
around.

Extend struct mtd_oob_ops with a pointer to a newly-introduced struct
mtd_req_stats and set its 'max_bitflips' field to the maximum number of
bitflips found in a single ECC step during the read operation performed
by mtd_read_oob().  This is a prerequisite for ultimately passing that
value back to user space.
Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20220629125737.14418-2-kernel@kempniu.pl

drivers/mtd/mtdcore.c

@@ -1624,6 +1624,9 @@ int mtd_read_oob(struct mtd_info *mtd, loff_t from, struct mtd_oob_ops *ops)
 	if (!master->_read_oob && (!master->_read || ops->oobbuf))
 		return -EOPNOTSUPP;
 
+	if (ops->stats)
+		memset(ops->stats, 0, sizeof(*ops->stats));
+
 	if (mtd->flags & MTD_SLC_ON_MLC_EMULATION)
 		ret_code = mtd_io_emulated_slc(mtd, from, true, ops);
 	else
@@ -1641,6 +1644,8 @@ int mtd_read_oob(struct mtd_info *mtd, loff_t from, struct mtd_oob_ops *ops)
 		return ret_code;
 	if (mtd->ecc_strength == 0)
 		return 0;	/* device lacks ecc */
+	if (ops->stats)
+		ops->stats->max_bitflips = ret_code;
 	return ret_code >= mtd->bitflip_threshold ? -EUCLEAN : 0;
 }
 EXPORT_SYMBOL_GPL(mtd_read_oob);

include/linux/mtd/mtd.h

@@ -40,6 +40,10 @@ struct mtd_erase_region_info {
 	unsigned long *lockmap;		/* If keeping bitmap of locks */
 };
 
+struct mtd_req_stats {
+	unsigned int max_bitflips;
+};
+
 /**
  * struct mtd_oob_ops - oob operation operands
  * @mode:	operation mode
@@ -70,6 +74,7 @@ struct mtd_oob_ops {
 	uint32_t	ooboffs;
 	uint8_t		*datbuf;
 	uint8_t		*oobbuf;
+	struct mtd_req_stats *stats;
 };
 
 /**