[U-Boot] [PATCH v2 4/4] mtd: nand: omap: optimized chip->ecc.correct() for H/W ECC schemes

[Pekon Gupta]

chip->ecc.correct() is used for detecting and correcting bit-flips during read
operations. In omap-nand driver it implemented as:
(a) omap_correct_data(): for h/w based ECC_HAM1 scheme
(b) omap_correct_data_bch() + CONFIG_NAND_OMAP_ECC_BCH8_CODE_HW_DETECTION_SW
	for ECC_BCH8 scheme using GPMC and software lib/bch.c
(c) omap_correct_data_bch() + CONFIG_NAND_OMAP_ECC_BCH8_CODE_HW
	for ECC_BCH8 scheme using GPMC and ELM

This patch updates (c)
- checks for calc_ecc[]==0x00 so that error_correction is not required for
  known good pages.
- adds scalability for other ECC_BCHx scheme by merging following
  omap_rotate_ecc_bch() + omap_fix_errors_bch() => omap_correct_data_bch()
- fixing logic for bit-flip correction based on error_loc[count]

Signed-off-by: Pekon Gupta <pekon at ti.com>
---
 drivers/mtd/nand/omap_gpmc.c | 124 +++++++++++++++----------------------------
 1 file changed, 44 insertions(+), 80 deletions(-)

diff --git a/drivers/mtd/nand/omap_gpmc.c b/drivers/mtd/nand/omap_gpmc.c
index fa3a82a..8c2ba53 100644
--- a/drivers/mtd/nand/omap_gpmc.c
+++ b/drivers/mtd/nand/omap_gpmc.c
@@ -19,6 +19,8 @@
 
 
 #define SECTOR_BYTES		512
+/* 4 bit padding to make byte aligned, 56 = 52 + 4 */
+#define BCH4_BIT_PAD		4
 
 static uint8_t cs;
 static __maybe_unused struct nand_ecclayout hw_nand_oob =
@@ -319,77 +321,6 @@ static int omap_calculate_ecc_bch(struct mtd_info *mtd, const uint8_t *dat,
 }
 
 /*
- * omap_rotate_ecc_bch - Rotate the syndrome bytes
- *
- * @mtd:	MTD device structure
- * @calc_ecc:	ECC read from ECC registers
- * @syndrome:	Rotated syndrome will be retuned in this array
- *
- */
-static void omap_rotate_ecc_bch(struct mtd_info *mtd, uint8_t *calc_ecc,
-		uint8_t *syndrome)
-{
-	struct nand_chip *chip = mtd->priv;
-	struct nand_bch_priv *bch = chip->priv;
-	uint8_t n_bytes = 0;
-	int8_t i, j;
-
-	switch (bch->type) {
-	case ECC_BCH4:
-		n_bytes = 8;
-		break;
-
-	case ECC_BCH16:
-		n_bytes = 28;
-		break;
-
-	case ECC_BCH8:
-	default:
-		n_bytes = 13;
-		break;
-	}
-
-	for (i = 0, j = (n_bytes-1); i < n_bytes; i++, j--)
-		syndrome[i] =  calc_ecc[j];
-}
-
-/*
- * omap_fix_errors_bch - Correct bch error in the data
- *
- * @mtd:	MTD device structure
- * @data:	Data read from flash
- * @error_count:Number of errors in data
- * @error_loc:	Locations of errors in the data
- *
- */
-static void omap_fix_errors_bch(struct mtd_info *mtd, uint8_t *data,
-		uint32_t error_count, uint32_t *error_loc)
-{
-	struct nand_chip *chip = mtd->priv;
-	struct nand_bch_priv *bch = chip->priv;
-	uint8_t count = 0;
-	uint32_t error_byte_pos;
-	uint32_t error_bit_mask;
-	uint32_t last_bit = (bch->nibbles * 4) - 1;
-
-	/* Flip all bits as specified by the error location array. */
-	/* FOR( each found error location flip the bit ) */
-	for (count = 0; count < error_count; count++) {
-		if (error_loc[count] > last_bit) {
-			/* Remove the ECC spare bits from correction. */
-			error_loc[count] -= (last_bit + 1);
-			/* Offset bit in data region */
-			error_byte_pos = ((512 * 8) -
-					(error_loc[count]) - 1) / 8;
-			/* Error Bit mask */
-			error_bit_mask = 0x1 << (error_loc[count] % 8);
-			/* Toggle the error bit to make the correction. */
-			data[error_byte_pos] ^= error_bit_mask;
-		}
-	}
-}
-
-/*
  * omap_correct_data_bch - Compares the ecc read from nand spare area
  * with ECC registers values and corrects one bit error if it has occured
  *
@@ -405,16 +336,26 @@ static int omap_correct_data_bch(struct mtd_info *mtd, uint8_t *dat,
 {
 	struct nand_chip *chip = mtd->priv;
 	struct nand_bch_priv *bch = chip->priv;
+	uint32_t eccbytes = chip->ecc.bytes;
 	uint8_t syndrome[28];
-	uint32_t error_count = 0;
+	uint32_t error_count = 0, error_max;
 	uint32_t error_loc[8];
-	uint32_t i, ecc_flag;
+	uint32_t i, j, ecc_flag = 0;
+	uint8_t count, ret = 0;
+	uint32_t byte_pos, bit_pos;
+
+	/* check calculated ecc */
+	for (i = 0; i < chip->ecc.bytes && !ecc_flag; i++)
+		if (calc_ecc[i] != 0x00)
+			ecc_flag = 1;
+	if (!ecc_flag)
+		return 0;
 
+	/* check for whether its a erased-page */
 	ecc_flag = 0;
-	for (i = 0; i < chip->ecc.bytes; i++)
+	for (i = 0; i < chip->ecc.bytes && !ecc_flag; i++)
 		if (read_ecc[i] != 0xff)
 			ecc_flag = 1;
-
 	if (!ecc_flag)
 		return 0;
 
@@ -425,20 +366,43 @@ static int omap_correct_data_bch(struct mtd_info *mtd, uint8_t *dat,
 	 * while reading ECC result we read it in big endian.
 	 * Hence while loading to ELM we have rotate to get the right endian.
 	 */
-	omap_rotate_ecc_bch(mtd, calc_ecc, syndrome);
+	for (i = 0, j = (eccbytes-1); i < eccbytes; i++, j--)
+		syndrome[i] =  calc_ecc[j];
 
 	/* use elm module to check for errors */
 	if (elm_check_error(syndrome, bch->nibbles, &error_count,
 				error_loc) != 0) {
-		printf("ECC: uncorrectable.\n");
+		printf("nand: uncorrectable ECC errors\n");
 		return -1;
 	}
 
 	/* correct bch error */
-	if (error_count > 0)
-		omap_fix_errors_bch(mtd, dat, error_count, error_loc);
+	for (count = 0; count < error_count; count++) {
+		switch (bch->type) {
+		case ECC_BCH4:
+			error_max = SECTOR_BYTES + (eccbytes - 1);
+			/* add 4 to take care 4 bit padding */
+			error_loc[count] += BCH4_BIT_PAD;
+			break;
+		case ECC_BCH8:
+			/* 14th byte in ECC is reserved to match ROM layout */
+			error_max = SECTOR_BYTES + (eccbytes - 1);
+			break;
+		default:
+			return -1;
+		}
+		byte_pos = error_max - ((error_loc[count] - 1) / 8);
+		bit_pos  = error_loc[count] % 8;
 
-	return 0;
+		if (byte_pos < SECTOR_BYTES)
+			dat[byte_pos] ^= 1 << bit_pos;
+		else if (byte_pos < error_max)
+			read_ecc[byte_pos - SECTOR_BYTES] = 1 << bit_pos;
+		else
+			ret = -1;
+	}
+
+	return ret;
 }
 
 /**
-- 
1.8.1