[PATCH resend] Newly erased page read workaround

[Viresh Kumar]

From: Vipin Kumar <vipin.kumar at st.com>

A newly erased page contains ff in data as well as spare area. While reading an
erased page, the read out ecc from spare area does not match the ecc generated
by fsmc ecc hardware accelarator. This is because ecc of data ff ff is not ff
ff. This leads to errors when jffs2 fs erases and reads back the pages to
ensure consistency.

This patch adds a software workaround to ensure that the ecc check is not
performed for erased pages. This check is a memory comparison of 512 data bytes
against 0xff. The ecc correction and error reporting algorithm is skipped if the
check passes.
It adds a few extra cycles while reading an erased page but is safer to ensure
that the page is read properly

Signed-off-by: Vipin Kumar <vipin.kumar at st.com>
Signed-off-by: Viresh Kumar <viresh.kumar at st.com>
Acked-by: Linus Walleij <linus.walleij at stericsson.com>
---
 drivers/mtd/nand/fsmc_nand.c |   28 ++++++++++++++++++++++------
 1 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/drivers/mtd/nand/fsmc_nand.c b/drivers/mtd/nand/fsmc_nand.c
index 205b10b..7df270a 100644
--- a/drivers/mtd/nand/fsmc_nand.c
+++ b/drivers/mtd/nand/fsmc_nand.c
@@ -420,7 +420,7 @@ static int fsmc_read_page_hwecc(struct mtd_info *mtd, struct nand_chip *chip,
 	struct fsmc_nand_data *host = container_of(mtd,
 					struct fsmc_nand_data, mtd);
 	struct fsmc_eccplace *ecc_place = host->ecc_place;
-	int i, j, s, stat, eccsize = chip->ecc.size;
+	int i, j, k, s, stat, eccsize = chip->ecc.size;
 	int eccbytes = chip->ecc.bytes;
 	int eccsteps = chip->ecc.steps;
 	uint8_t *p = buf;
@@ -460,11 +460,27 @@ static int fsmc_read_page_hwecc(struct mtd_info *mtd, struct nand_chip *chip,
 		memcpy(&ecc_code[i], oob, 13);
 		chip->ecc.calculate(mtd, p, &ecc_calc[i]);
 
-		stat = chip->ecc.correct(mtd, p, &ecc_code[i], &ecc_calc[i]);
-		if (stat < 0)
-			mtd->ecc_stats.failed++;
-		else
-			mtd->ecc_stats.corrected += stat;
+		/*
+		 * This is a temporary erase check. A newly erased page read
+		 * would result in an ecc error because the oob data is also
+		 * erased to FF and the calculated ecc for an FF data is not
+		 * FF..FF.
+		 * This is a workaround to skip performing correction in case
+		 * data is FF..FF
+		 */
+		for (k = 0; k < eccsize; k++) {
+			if (*(p + k) != 0xff)
+				break;
+		}
+
+		if (k < eccsize) {
+			stat = chip->ecc.correct(mtd, p, &ecc_code[i],
+					&ecc_calc[i]);
+			if (stat < 0)
+				mtd->ecc_stats.failed++;
+			else
+				mtd->ecc_stats.corrected += stat;
+		}
 	}
 
 	return 0;
-- 
1.7.3.4