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Commit 4bc1dca4 authored by Artem Bityutskiy's avatar Artem Bityutskiy
Browse files

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: default avatarArtem Bityutskiy <Artem.Bityutskiy@nokia.com>
parent cbd8a9d2

drivers/mtd/ubi/scan.c

+8 −33
Original line number Diff line number Diff line
@@ -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;

		}



		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");

drivers/mtd/ubi/scan.h

+1 −1
Original line number Diff line number Diff line
@@ -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;

};