[RFC/PATCH] mtd: mtd_read: Fix bitflips_threshold comparison to allow max bitflips

[Stefan Roese]

On a custom AM335x based platform with a Toshiba NAND device
(TC58NVG1S3H) we are currently seeing quite a few of these UBI messages:

[   18.044967] UBI: fixable bit-flip detected at PEB 50
[   18.050252] UBI: schedule PEB 50 for scrubbing
...

After a bit debugging I found that those messages are only printed when
the OMAP NAND driver has detected 8 (corrected) bitflips / 512 bytes on
a read. We're using HW BCH8 and the Toshiba chip supports 8 bit ECC for
each 512Byte. I was wondering why 8 bitflips resulted in these UBI
messages and e.g. 7 bitflips didn't. Hence I discovered the comparison
"ret_code >= mtd->bitflip_threshold" in mtd_read().

With this patch applied all tests (UBIFS) I've done so far didn't produce
any of these "UBI: fixable bit-flip" messages any more.

Note that I'm sending this patch as RFC for now. To get some feedback
from other MTD / NAND developers on this issue. The main question is:
Should mtd_read() return -EUCLEAN if the corrected bitflips are equal to
the bitflip-threshold value? Or should it return 0 since the bitflips
have been corrected?

Signed-off-by: Stefan Roese <sr at denx.de>
Cc: Brian Norris <computersforpeace at gmail.com>
Cc: Pekon Gupta <pekon at ti.com>
---
 drivers/mtd/mtdcore.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mtd/mtdcore.c b/drivers/mtd/mtdcore.c
index 92311a5..28500a1 100644
--- a/drivers/mtd/mtdcore.c
+++ b/drivers/mtd/mtdcore.c
@@ -824,7 +824,7 @@ int mtd_read(struct mtd_info *mtd, loff_t from, size_t len, size_t *retlen,
 		return ret_code;
 	if (mtd->ecc_strength == 0)
 		return 0;	/* device lacks ecc */
-	return ret_code >= mtd->bitflip_threshold ? -EUCLEAN : 0;
+	return ret_code > mtd->bitflip_threshold ? -EUCLEAN : 0;
 }
 EXPORT_SYMBOL_GPL(mtd_read);
 
-- 
1.8.5.2