[MTD] Improve software ECC calculation

Unrolling the loops produces denser and much faster code.
Add a config switch which allows to select the byte order of the
resulting ecc code. The current Linux implementation has a byte
swap versus the SmartMedia specification

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 96 insertions, 134 deletions

diff --git a/drivers/mtd/nand/Kconfig b/drivers/mtd/nand/Kconfig
index 2d0ebad55a49..4d235b91267d 100644
--- a/drivers/mtd/nand/Kconfig
+++ b/drivers/mtd/nand/Kconfig
@@ -23,6 +23,14 @@ config MTD_NAND_VERIFY_WRITE
           device thinks the write was successful, a bit could have been
           flipped accidentaly due to device wear or something else.
 
+config MTD_NAND_ECC_SMC
+        bool "NAND ECC Smart Media byte order"
+        depends on MTD_NAND
+        default n
+        help
+          Software ECC according to the Smart Media Specification.
+          The original Linux implementation had byte 0 and 1 swapped.
+
 config MTD_NAND_AUTCPU12
         tristate "SmartMediaCard on autronix autcpu12 board"
         depends on MTD_NAND && ARCH_AUTCPU12
diff --git a/drivers/mtd/nand/nand_ecc.c b/drivers/mtd/nand/nand_ecc.c
index 101892985b02..2a163e4084df 100644
--- a/drivers/mtd/nand/nand_ecc.c
+++ b/drivers/mtd/nand/nand_ecc.c
@@ -7,6 +7,8 @@
  * Copyright (C) 2000-2004 Steven J. Hill (sjhill@realitydiluted.com)
  *                         Toshiba America Electronics Components, Inc.
  *
+ * Copyright (C) 2006 Thomas Gleixner <tglx@linutronix.de>
+ *
  * $Id: nand_ecc.c,v 1.15 2005/11/07 11:14:30 gleixner Exp $
  *
  * This file is free software; you can redistribute it and/or modify it
@@ -63,87 +65,75 @@ static const u_char nand_ecc_precalc_table[] = {
 };
 
 /**
- * nand_trans_result - [GENERIC] create non-inverted ECC
- * @reg2:       line parity reg 2
- * @reg3:       line parity reg 3
- * @ecc_code:   ecc
- *
- * Creates non-inverted ECC code from line parity
- */
-static void nand_trans_result(u_char reg2, u_char reg3, u_char *ecc_code)
-{
-        u_char a, b, i, tmp1, tmp2;
-
-        /* Initialize variables */
-        a = b = 0x80;
-        tmp1 = tmp2 = 0;
-
-        /* Calculate first ECC byte */
-        for (i = 0; i < 4; i++) {
-                if (reg3 & a)   /* LP15,13,11,9 --> ecc_code[0] */
-                        tmp1 |= b;
-                b >>= 1;
-                if (reg2 & a)   /* LP14,12,10,8 --> ecc_code[0] */
-                        tmp1 |= b;
-                b >>= 1;
-                a >>= 1;
-        }
-
-        /* Calculate second ECC byte */
-        b = 0x80;
-        for (i = 0; i < 4; i++) {
-                if (reg3 & a)   /* LP7,5,3,1 --> ecc_code[1] */
-                        tmp2 |= b;
-                b >>= 1;
-                if (reg2 & a)   /* LP6,4,2,0 --> ecc_code[1] */
-                        tmp2 |= b;
-                b >>= 1;
-                a >>= 1;
-        }
-
-        /* Store two of the ECC bytes */
-        ecc_code[0] = tmp1;
-        ecc_code[1] = tmp2;
-}
-
-/**
- * nand_calculate_ecc - [NAND Interface] Calculate 3 byte ECC code for 256 byte block
+ * nand_calculate_ecc - [NAND Interface] Calculate 3 byte ECC code
+ *                      for 256 byte block
  * @mtd:        MTD block structure
  * @dat:        raw data
  * @ecc_code:   buffer for ECC
  */
-int nand_calculate_ecc(struct mtd_info *mtd, const u_char *dat, u_char *ecc_code)
+int nand_calculate_ecc(struct mtd_info *mtd, const u_char *dat,
+                       u_char *ecc_code)
 {
-        u_char idx, reg1, reg2, reg3;
-        int j;
+        uint8_t idx, reg1, reg2, reg3, tmp1, tmp2;
+        int i;
 
         /* Initialize variables */
         reg1 = reg2 = reg3 = 0;
-        ecc_code[0] = ecc_code[1] = ecc_code[2] = 0;
 
         /* Build up column parity */
-        for (j = 0; j < 256; j++) {
-
+        for(i = 0; i < 256; i++) {
                 /* Get CP0 - CP5 from table */
-                idx = nand_ecc_precalc_table[dat[j]];
+                idx = nand_ecc_precalc_table[*dat++];
                 reg1 ^= (idx & 0x3f);
 
                 /* All bit XOR = 1 ? */
                 if (idx & 0x40) {
-                        reg3 ^= (u_char) j;
-                        reg2 ^= ~((u_char) j);
+                        reg3 ^= (uint8_t) i;
+                        reg2 ^= ~((uint8_t) i);
                 }
         }
 
         /* Create non-inverted ECC code from line parity */
-        nand_trans_result(reg2, reg3, ecc_code);
+        tmp1  = (reg3 & 0x80) >> 0; /* B7 -> B7 */
+        tmp1 |= (reg2 & 0x80) >> 1; /* B7 -> B6 */
+        tmp1 |= (reg3 & 0x40) >> 1; /* B6 -> B5 */
+        tmp1 |= (reg2 & 0x40) >> 2; /* B6 -> B4 */
+        tmp1 |= (reg3 & 0x20) >> 2; /* B5 -> B3 */
+        tmp1 |= (reg2 & 0x20) >> 3; /* B5 -> B2 */
+        tmp1 |= (reg3 & 0x10) >> 3; /* B4 -> B1 */
+        tmp1 |= (reg2 & 0x10) >> 4; /* B4 -> B0 */
+
+        tmp2  = (reg3 & 0x08) << 4; /* B3 -> B7 */
+        tmp2 |= (reg2 & 0x08) << 3; /* B3 -> B6 */
+        tmp2 |= (reg3 & 0x04) << 3; /* B2 -> B5 */
+        tmp2 |= (reg2 & 0x04) << 2; /* B2 -> B4 */
+        tmp2 |= (reg3 & 0x02) << 2; /* B1 -> B3 */
+        tmp2 |= (reg2 & 0x02) << 1; /* B1 -> B2 */
+        tmp2 |= (reg3 & 0x01) << 1; /* B0 -> B1 */
+        tmp2 |= (reg2 & 0x01) << 0; /* B7 -> B0 */
 
         /* Calculate final ECC code */
-        ecc_code[0] = ~ecc_code[0];
-        ecc_code[1] = ~ecc_code[1];
+#ifdef CONFIG_NAND_ECC_SMC
+        ecc_code[0] = ~tmp2;
+        ecc_code[1] = ~tmp1;
+#else
+        ecc_code[0] = ~tmp1;
+        ecc_code[1] = ~tmp2;
+#endif
         ecc_code[2] = ((~reg1) << 2) | 0x03;
+
         return 0;
 }
+EXPORT_SYMBOL(nand_calculate_ecc);
+
+static inline int countbits(uint32_t byte)
+{
+        int res = 0;
+
+        for (;byte; byte >>= 1)
+                res += byte & 0x01;
+        return res;
+}
 
 /**
  * nand_correct_data - [NAND Interface] Detect and correct bit error(s)
@@ -154,90 +144,54 @@ int nand_calculate_ecc(struct mtd_info *mtd, const u_char *dat, u_char *ecc_code
  *
  * Detect and correct a 1 bit error for 256 byte block
  */
-int nand_correct_data(struct mtd_info *mtd, u_char *dat, u_char *read_ecc, u_char *calc_ecc)
+int nand_correct_data(struct mtd_info *mtd, u_char *dat,
+                      u_char *read_ecc, u_char *calc_ecc)
 {
-        u_char a, b, c, d1, d2, d3, add, bit, i;
+        uint8_t s0, s1, s2;
+
+#ifdef CONFIG_NAND_ECC_SMC
+        s0 = calc_ecc[0] ^ read_ecc[0];
+        s1 = calc_ecc[1] ^ read_ecc[1];
+        s2 = calc_ecc[2] ^ read_ecc[2];
+#else
+        s1 = calc_ecc[0] ^ read_ecc[0];
+        s0 = calc_ecc[1] ^ read_ecc[1];
+        s2 = calc_ecc[2] ^ read_ecc[2];
+#endif
+        if ((s0 | s1 | s2) == 0)
+                return 0;
 
-        /* Do error detection */
-        d1 = calc_ecc[0] ^ read_ecc[0];
-        d2 = calc_ecc[1] ^ read_ecc[1];
-        d3 = calc_ecc[2] ^ read_ecc[2];
+        /* Check for a single bit error */
+        if( ((s0 ^ (s0 >> 1)) & 0x55) == 0x55 &&
+            ((s1 ^ (s1 >> 1)) & 0x55) == 0x55 &&
+            ((s2 ^ (s2 >> 1)) & 0x54) == 0x54) {
 
-        if ((d1 | d2 | d3) == 0) {
-                /* No errors */
-                return 0;
-        } else {
-                a = (d1 ^ (d1 >> 1)) & 0x55;
-                b = (d2 ^ (d2 >> 1)) & 0x55;
-                c = (d3 ^ (d3 >> 1)) & 0x54;
-
-                /* Found and will correct single bit error in the data */
-                if ((a == 0x55) && (b == 0x55) && (c == 0x54)) {
-                        c = 0x80;
-                        add = 0;
-                        a = 0x80;
-                        for (i = 0; i < 4; i++) {
-                                if (d1 & c)
-                                        add |= a;
-                                c >>= 2;
-                                a >>= 1;
-                        }
-                        c = 0x80;
-                        for (i = 0; i < 4; i++) {
-                                if (d2 & c)
-                                        add |= a;
-                                c >>= 2;
-                                a >>= 1;
-                        }
-                        bit = 0;
-                        b = 0x04;
-                        c = 0x80;
-                        for (i = 0; i < 3; i++) {
-                                if (d3 & c)
-                                        bit |= b;
-                                c >>= 2;
-                                b >>= 1;
-                        }
-                        b = 0x01;
-                        a = dat[add];
-                        a ^= (b << bit);
-                        dat[add] = a;
-                        return 1;
-                } else {
-                        i = 0;
-                        while (d1) {
-                                if (d1 & 0x01)
-                                        ++i;
-                                d1 >>= 1;
-                        }
-                        while (d2) {
-                                if (d2 & 0x01)
-                                        ++i;
-                                d2 >>= 1;
-                        }
-                        while (d3) {
-                                if (d3 & 0x01)
-                                        ++i;
-                                d3 >>= 1;
-                        }
-                        if (i == 1) {
-                                /* ECC Code Error Correction */
-                                read_ecc[0] = calc_ecc[0];
-                                read_ecc[1] = calc_ecc[1];
-                                read_ecc[2] = calc_ecc[2];
-                                return 2;
-                        } else {
-                                /* Uncorrectable Error */
-                                return -1;
-                        }
-                }
+                uint32_t byteoffs, bitnum;
+
+                byteoffs = (s1 << 0) & 0x80;
+                byteoffs |= (s1 << 1) & 0x40;
+                byteoffs |= (s1 << 2) & 0x20;
+                byteoffs |= (s1 << 3) & 0x10;
+
+                byteoffs |= (s0 >> 4) & 0x08;
+                byteoffs |= (s0 >> 3) & 0x04;
+                byteoffs |= (s0 >> 2) & 0x02;
+                byteoffs |= (s0 >> 1) & 0x01;
+
+                bitnum = (s2 >> 5) & 0x04;
+                bitnum |= (s2 >> 4) & 0x02;
+                bitnum |= (s2 >> 3) & 0x01;
+
+                dat[byteoffs] ^= (1 << bitnum);
+
+                return 1;
         }
 
-        /* Should never happen */
+        if(countbits(s0 | ((uint32_t)s1 << 8) | ((uint32_t)s2 <<16)) == 1)
+                return 1;
+
         return -1;
 }
-
-EXPORT_SYMBOL(nand_calculate_ecc);
 EXPORT_SYMBOL(nand_correct_data);
 
 MODULE_LICENSE("GPL");