From 6c552da6be027a61e258e2fc604354949c1da786 Mon Sep 17 00:00:00 2001
From: Daniel Mentz <danielmentz@google.com>
Date: Tue, 26 Jan 2016 14:40:29 -0800
Subject: [PATCH] Don't report negligible number of bit flips

Previously, nanddump would print a message like the following for every
corrected bit flip:

ECC: 1 corrected bitflip(s) at offset 0x01cae000

This creates too much noise when using MLC NAND flash chips. A small
number of bit flips are expected for these chips and is no reason for
concern.  With this change, we only print this message when the number
of bit flips exceeds a certain threshold set by the kernel.
---
 include/libmtd.h |  1 +
 lib/libmtd.c     |  6 ++++++
 lib/libmtd_int.h |  2 ++
 nanddump.c       | 15 ++++++++++++++-
 4 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/include/libmtd.h b/include/libmtd.h
index 07c304a..c0c9365 100644
--- a/include/libmtd.h
+++ b/include/libmtd.h
@@ -87,6 +87,7 @@ struct mtd_dev_info
 	int region_cnt;
 	unsigned int writable:1;
 	unsigned int bb_allowed:1;
+	int bitflip_threshold;
 };
 
 /**
diff --git a/lib/libmtd.c b/lib/libmtd.c
index 9b247ae..54cd4b6 100644
--- a/lib/libmtd.c
+++ b/lib/libmtd.c
@@ -614,6 +614,10 @@ libmtd_t libmtd_open(void)
 	if (!lib->mtd_flags)
 		goto out_error;
 
+	lib->mtd_bitflip_threshold = mkpath(lib->mtd, MTD_BITFLIP_THRESHOLD);
+	if (!lib->mtd_bitflip_threshold)
+		goto out_error;
+
 	lib->sysfs_supported = 1;
 	return lib;
 
@@ -626,6 +630,7 @@ void libmtd_close(libmtd_t desc)
 {
 	struct libmtd *lib = (struct libmtd *)desc;
 
+	free(lib->mtd_bitflip_threshold);
 	free(lib->mtd_flags);
 	free(lib->mtd_region_cnt);
 	free(lib->mtd_oob_size);
@@ -765,6 +770,7 @@ int mtd_get_dev_info1(libmtd_t desc, int mtd_num, struct mtd_dev_info *mtd)
 	if (dev_read_hex_int(lib->mtd_flags, mtd_num, &ret))
 		return -1;
 	mtd->writable = !!(ret & MTD_WRITEABLE);
+	dev_read_pos_int(lib->mtd_bitflip_threshold, mtd_num, &mtd->bitflip_threshold);
 
 	mtd->eb_cnt = mtd->size / mtd->eb_size;
 	mtd->type = type_str2int(mtd->type_str);
diff --git a/lib/libmtd_int.h b/lib/libmtd_int.h
index bb48d35..d49b9e1 100644
--- a/lib/libmtd_int.h
+++ b/lib/libmtd_int.h
@@ -42,6 +42,7 @@ extern "C" {
 #define MTD_OOB_SIZE     "oobsize"
 #define MTD_REGION_CNT   "numeraseregions"
 #define MTD_FLAGS        "flags"
+#define MTD_BITFLIP_THRESHOLD "bitflip_threshold"
 
 #define OFFS64_IOCTLS_UNKNOWN       0
 #define OFFS64_IOCTLS_NOT_SUPPORTED 1
@@ -90,6 +91,7 @@ struct libmtd
 	char *mtd_oob_size;
 	char *mtd_region_cnt;
 	char *mtd_flags;
+	char *mtd_bitflip_threshold;
 	unsigned int sysfs_supported:1;
 	unsigned int offs64_ioctls:2;
 };
diff --git a/nanddump.c b/nanddump.c
index be458c6..eab50a2 100644
--- a/nanddump.c
+++ b/nanddump.c
@@ -427,6 +427,7 @@ int main(int argc, char * const argv[])
 
 		/* ECC stats available ? */
 		if (eccstats) {
+			int bitflip_threshold = mtd.bitflip_threshold ? : 1;
 			if (ioctl(fd, ECCGETSTATS, &stat2)) {
 				perror("ioctl(ECCGETSTATS)");
 				goto closeall;
@@ -435,7 +436,19 @@ int main(int argc, char * const argv[])
 				fprintf(stderr, "ECC: %d uncorrectable bitflip(s)"
 						" at offset 0x%08llx\n",
 						stat2.failed - stat1.failed, ofs);
-			if (stat1.corrected != stat2.corrected)
+			/*
+			 * Report the number of corrected bitflips only if it
+			 * is above the threshold used by the kernel. This
+			 * logic is flawed, though, because the
+			 * bitflip_threshold relates to ecc steps not an entire
+			 * page. For example, a 4kB page might be broken up
+			 * into four ECC steps of 1024 bytes each due to
+			 * limitations of the hardware engine. We would need to
+			 * consider the value referred to as max_bitflips which
+			 * is not exported by the kernel, unfortunately.  As a
+			 * result, we might print this message even if the
+			 * number of bit flips are below the threshold.  */
+			if (stat2.corrected - stat1.corrected >= bitflip_threshold)
 				fprintf(stderr, "ECC: %d corrected bitflip(s) at"
 						" offset 0x%08llx\n",
 						stat2.corrected - stat1.corrected, ofs);
-- 
2.7.0.rc3.207.g0ac5344