[MTD] [NAND] S3C2410: Hardware ECC correction code

Add support for correcting errors detected by the
hardware ECC.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>

2 files changed, 62 insertions, 13 deletions

diff --git a/drivers/mtd/nand/Kconfig b/drivers/mtd/nand/Kconfig
index 9326a56f0fbc..143a7f048257 100644
--- a/drivers/mtd/nand/Kconfig
+++ b/drivers/mtd/nand/Kconfig
@@ -126,10 +126,6 @@ config MTD_NAND_S3C2410_HWECC
           incorrect ECC generation, and if using these, the default of
           software ECC is preferable.
 
-          If you lay down a device with the hardware ECC, then you will
-          currently not be able to switch to software, as there is no
-          implementation for ECC method used by the S3C2410
-
 config MTD_NAND_NDFC
         tristate "NDFC NanD Flash Controller"
         depends on MTD_NAND && 44x
diff --git a/drivers/mtd/nand/s3c2410.c b/drivers/mtd/nand/s3c2410.c
index 8b3203571eeb..e8e030171ec4 100644
--- a/drivers/mtd/nand/s3c2410.c
+++ b/drivers/mtd/nand/s3c2410.c
@@ -337,17 +337,69 @@ static int s3c2412_nand_devready(struct mtd_info *mtd)
 static int s3c2410_nand_correct_data(struct mtd_info *mtd, u_char *dat,
                                      u_char *read_ecc, u_char *calc_ecc)
 {
-        pr_debug("s3c2410_nand_correct_data(%p,%p,%p,%p)\n", mtd, dat, read_ecc, calc_ecc);
+        struct s3c2410_nand_info *info = s3c2410_nand_mtd_toinfo(mtd);
+        unsigned int diff0, diff1, diff2;
+        unsigned int bit, byte;
 
-        pr_debug("eccs: read %02x,%02x,%02x vs calc %02x,%02x,%02x\n",
-                 read_ecc[0], read_ecc[1], read_ecc[2], calc_ecc[0], calc_ecc[1], calc_ecc[2]);
+        pr_debug("%s(%p,%p,%p,%p)\n", __func__, mtd, dat, read_ecc, calc_ecc);
 
-        if (read_ecc[0] == calc_ecc[0] && read_ecc[1] == calc_ecc[1] && read_ecc[2] == calc_ecc[2])
-                return 0;
+        diff0 = read_ecc[0] ^ calc_ecc[0];
+        diff1 = read_ecc[1] ^ calc_ecc[1];
+        diff2 = read_ecc[2] ^ calc_ecc[2];
+
+        pr_debug("%s: rd %02x%02x%02x calc %02x%02x%02x diff %02x%02x%02x\n",
+                 __func__,
+                 read_ecc[0], read_ecc[1], read_ecc[2],
+                 calc_ecc[0], calc_ecc[1], calc_ecc[2],
+                 diff0, diff1, diff2);
+
+        if (diff0 == 0 && diff1 == 0 && diff2 == 0)
+                return 0;               /* ECC is ok */
+
+        /* Can we correct this ECC (ie, one row and column change).
+         * Note, this is similar to the 256 error code on smartmedia */
+
+        if (((diff0 ^ (diff0 >> 1)) & 0x55) == 0x55 &&
+            ((diff1 ^ (diff1 >> 1)) & 0x55) == 0x55 &&
+            ((diff2 ^ (diff2 >> 1)) & 0x55) == 0x55) {
+                /* calculate the bit position of the error */
+
+                bit  = (diff2 >> 2) & 1;
+                bit |= (diff2 >> 3) & 2;
+                bit |= (diff2 >> 4) & 4;
+
+                /* calculate the byte position of the error */
+
+                byte  = (diff1 << 1) & 0x80;
+                byte |= (diff1 << 2) & 0x40;
+                byte |= (diff1 << 3) & 0x20;
+                byte |= (diff1 << 4) & 0x10;
 
-        /* we curently have no method for correcting the error */
+                byte |= (diff0 >> 3) & 0x08;
+                byte |= (diff0 >> 2) & 0x04;
+                byte |= (diff0 >> 1) & 0x02;
+                byte |= (diff0 >> 0) & 0x01;
 
-        return -1;
+                byte |= (diff2 << 8) & 0x100;
+
+                dev_dbg(info->device, "correcting error bit %d, byte %d\n",
+                        bit, byte);
+
+                dat[byte] ^= (1 << bit);
+                return 1;
+        }
+
+        /* if there is only one bit difference in the ECC, then
+         * one of only a row or column parity has changed, which
+         * means the error is most probably in the ECC itself */
+
+        diff0 |= (diff1 << 8);
+        diff0 |= (diff2 << 16);
+
+        if ((diff0 & ~(1<<fls(diff0))) == 0)
+                return 1;
+
+        return 0;
 }
 
 /* ECC functions
@@ -383,7 +435,8 @@ static int s3c2410_nand_calculate_ecc(struct mtd_info *mtd, const u_char *dat, u
         ecc_code[1] = readb(info->regs + S3C2410_NFECC + 1);
         ecc_code[2] = readb(info->regs + S3C2410_NFECC + 2);
 
-        pr_debug("calculate_ecc: returning ecc %02x,%02x,%02x\n", ecc_code[0], ecc_code[1], ecc_code[2]);
+        pr_debug("%s: returning ecc %02x%02x%02x\n", __func__,
+                 ecc_code[0], ecc_code[1], ecc_code[2]);
 
         return 0;
 }
@@ -397,7 +450,7 @@ static int s3c2440_nand_calculate_ecc(struct mtd_info *mtd, const u_char *dat, u
         ecc_code[1] = ecc >> 8;
         ecc_code[2] = ecc >> 16;
 
-        pr_debug("calculate_ecc: returning ecc %02x,%02x,%02x\n", ecc_code[0], ecc_code[1], ecc_code[2]);
+        pr_debug("%s: returning ecc %06x\n", __func__, ecc);
 
         return 0;
 }