UBI: fix mean EC calculation
(a + b) / (c + d) != a / c + b / d. The old code errornously
assumed this incorrect formuld. Instead, just sum all erase
counters in a 64-bit variable and divide to the number of EBs
at the end.
Thanks to Adrian Hunter for pointing this out.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/drivers/mtd/ubi/scan.c b/drivers/mtd/ubi/scan.c
index 05aa3e7..96d410e 100644
--- a/drivers/mtd/ubi/scan.c
+++ b/drivers/mtd/ubi/scan.c
@@ -42,6 +42,7 @@
#include <linux/err.h>
#include <linux/crc32.h>
+#include <asm/div64.h>
#include "ubi.h"
#ifdef CONFIG_MTD_UBI_DEBUG_PARANOID
@@ -92,27 +93,6 @@
}
/**
- * commit_to_mean_value - commit intermediate results to the final mean erase
- * counter value.
- * @si: scanning information
- *
- * This is a helper function which calculates partial mean erase counter mean
- * value and adds it to the resulting mean value. As we can work only in
- * integer arithmetic and we want to calculate the mean value of erase counter
- * accurately, we first sum erase counter values in @si->ec_sum variable and
- * count these components in @si->ec_count. If this temporary @si->ec_sum is
- * going to overflow, we calculate the partial mean value
- * (@si->ec_sum/@si->ec_count) and add it to @si->mean_ec.
- */
-static void commit_to_mean_value(struct ubi_scan_info *si)
-{
- si->ec_sum /= si->ec_count;
- if (si->ec_sum % si->ec_count >= si->ec_count / 2)
- si->mean_ec += 1;
- si->mean_ec += si->ec_sum;
-}
-
-/**
* validate_vid_hdr - check that volume identifier header is correct and
* consistent.
* @vid_hdr: the volume identifier header to check
@@ -901,15 +881,8 @@
adjust_mean_ec:
if (!ec_corr) {
- if (si->ec_sum + ec < ec) {
- commit_to_mean_value(si);
- si->ec_sum = 0;
- si->ec_count = 0;
- } else {
- si->ec_sum += ec;
- si->ec_count += 1;
- }
-
+ si->ec_sum += ec;
+ si->ec_count += 1;
if (ec > si->max_ec)
si->max_ec = ec;
if (ec < si->min_ec)
@@ -965,9 +938,11 @@
dbg_msg("scanning is finished");
- /* Finish mean erase counter calculations */
- if (si->ec_count)
- commit_to_mean_value(si);
+ /* Calculate mean erase counter */
+ if (si->ec_count) {
+ do_div(si->ec_sum, si->ec_count);
+ si->mean_ec = si->ec_sum;
+ }
if (si->is_empty)
ubi_msg("empty MTD device detected");