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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>
This commit is contained in:
Artem Bityutskiy 2008-04-19 20:44:31 +03:00
parent cbd8a9d2cd
commit 4bc1dca4b0
2 changed files with 9 additions and 34 deletions
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41
drivers/mtd/ubi/scan.c
View File

@ -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
@ -91,27 +92,6 @@ static int add_to_list(struct ubi_scan_info *si, int pnum, int ec,
return 0;
}
/**
* 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.
@ -901,15 +881,8 @@ static int process_eb(struct ubi_device *ubi, struct ubi_scan_info *si, int pnum
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 @@ struct ubi_scan_info *ubi_scan(struct ubi_device *ubi)
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");

2
drivers/mtd/ubi/scan.h
View File

@ -124,7 +124,7 @@ struct ubi_scan_info {
int max_ec;
unsigned long long max_sqnum;
int mean_ec;
int ec_sum;
uint64_t ec_sum;
int ec_count;
};