mtd: nand: add software BCH ECC support

This patch adds software BCH ECC support to mtd, in order to handle recent
NAND device ecc requirements (4 bits or more).

It does so by adding a new ecc mode (NAND_ECC_SOFT_BCH) for use by board
drivers, and a new Kconfig option to enable BCH support. It relies on the
generic BCH library introduced in a previous patch.

When a board driver uses mode NAND_ECC_SOFT_BCH, it should also set fields
chip->ecc.size and chip->ecc.bytes to select BCH ecc data size and required
error correction capability. See nand_bch_init() documentation for details.

It has been tested on the following platforms using mtd-utils, UBI and
UBIFS: x86 (with nandsim), arm926ejs.

Signed-off-by: Ivan Djelic <ivan.djelic@parrot.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

1 files changed, 243 insertions, 0 deletions

diff --git a/drivers/mtd/nand/nand_bch.c b/drivers/mtd/nand/nand_bch.c
new file mode 100644
index 000000000000..0f931e757116
--- /dev/null
+++ b/drivers/mtd/nand/nand_bch.c
@@ -0,0 +1,243 @@
+/*
+ * This file provides ECC correction for more than 1 bit per block of data,
+ * using binary BCH codes. It relies on the generic BCH library lib/bch.c.
+ *
+ * Copyright © 2011 Ivan Djelic <ivan.djelic@parrot.com>
+ *
+ * This file is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 or (at your option) any
+ * later version.
+ *
+ * This file is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this file; if not, write to the Free Software Foundation, Inc.,
+ * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
+ */
+
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/bitops.h>
+#include <linux/mtd/mtd.h>
+#include <linux/mtd/nand.h>
+#include <linux/mtd/nand_bch.h>
+#include <linux/bch.h>
+
+/**
+ * struct nand_bch_control - private NAND BCH control structure
+ * @bch:       BCH control structure
+ * @ecclayout: private ecc layout for this BCH configuration
+ * @errloc:    error location array
+ * @eccmask:   XOR ecc mask, allows erased pages to be decoded as valid
+ */
+struct nand_bch_control {
+	struct bch_control   *bch;
+	struct nand_ecclayout ecclayout;
+	unsigned int         *errloc;
+	unsigned char        *eccmask;
+};
+
+/**
+ * nand_bch_calculate_ecc - [NAND Interface] Calculate ECC for data block
+ * @mtd:	MTD block structure
+ * @buf:	input buffer with raw data
+ * @code:	output buffer with ECC
+ */
+int nand_bch_calculate_ecc(struct mtd_info *mtd, const unsigned char *buf,
+			   unsigned char *code)
+{
+	const struct nand_chip *chip = mtd->priv;
+	struct nand_bch_control *nbc = chip->ecc.priv;
+	unsigned int i;
+
+	memset(code, 0, chip->ecc.bytes);
+	encode_bch(nbc->bch, buf, chip->ecc.size, code);
+
+	/* apply mask so that an erased page is a valid codeword */
+	for (i = 0; i < chip->ecc.bytes; i++)
+		code[i] ^= nbc->eccmask[i];
+
+	return 0;
+}
+EXPORT_SYMBOL(nand_bch_calculate_ecc);
+
+/**
+ * nand_bch_correct_data - [NAND Interface] Detect and correct bit error(s)
+ * @mtd:	MTD block structure
+ * @buf:	raw data read from the chip
+ * @read_ecc:	ECC from the chip
+ * @calc_ecc:	the ECC calculated from raw data
+ *
+ * Detect and correct bit errors for a data byte block
+ */
+int nand_bch_correct_data(struct mtd_info *mtd, unsigned char *buf,
+			  unsigned char *read_ecc, unsigned char *calc_ecc)
+{
+	const struct nand_chip *chip = mtd->priv;
+	struct nand_bch_control *nbc = chip->ecc.priv;
+	unsigned int *errloc = nbc->errloc;
+	int i, count;
+
+	count = decode_bch(nbc->bch, NULL, chip->ecc.size, read_ecc, calc_ecc,
+			   NULL, errloc);
+	if (count > 0) {
+		for (i = 0; i < count; i++) {
+			if (errloc[i] < (chip->ecc.size*8))
+				/* error is located in data, correct it */
+				buf[errloc[i] >> 3] ^= (1 << (errloc[i] & 7));
+			/* else error in ecc, no action needed */
+
+			DEBUG(MTD_DEBUG_LEVEL0, "%s: corrected bitflip %u\n",
+			      __func__, errloc[i]);
+		}
+	} else if (count < 0) {
+		printk(KERN_ERR "ecc unrecoverable error\n");
+		count = -1;
+	}
+	return count;
+}
+EXPORT_SYMBOL(nand_bch_correct_data);
+
+/**
+ * nand_bch_init - [NAND Interface] Initialize NAND BCH error correction
+ * @mtd:	MTD block structure
+ * @eccsize:	ecc block size in bytes
+ * @eccbytes:	ecc length in bytes
+ * @ecclayout:	output default layout
+ *
+ * Returns:
+ *  a pointer to a new NAND BCH control structure, or NULL upon failure
+ *
+ * Initialize NAND BCH error correction. Parameters @eccsize and @eccbytes
+ * are used to compute BCH parameters m (Galois field order) and t (error
+ * correction capability). @eccbytes should be equal to the number of bytes
+ * required to store m*t bits, where m is such that 2^m-1 > @eccsize*8.
+ *
+ * Example: to configure 4 bit correction per 512 bytes, you should pass
+ * @eccsize = 512  (thus, m=13 is the smallest integer such that 2^m-1 > 512*8)
+ * @eccbytes = 7   (7 bytes are required to store m*t = 13*4 = 52 bits)
+ */
+struct nand_bch_control *
+nand_bch_init(struct mtd_info *mtd, unsigned int eccsize, unsigned int eccbytes,
+	      struct nand_ecclayout **ecclayout)
+{
+	unsigned int m, t, eccsteps, i;
+	struct nand_ecclayout *layout;
+	struct nand_bch_control *nbc = NULL;
+	unsigned char *erased_page;
+
+	if (!eccsize || !eccbytes) {
+		printk(KERN_WARNING "ecc parameters not supplied\n");
+		goto fail;
+	}
+
+	m = fls(1+8*eccsize);
+	t = (eccbytes*8)/m;
+
+	nbc = kzalloc(sizeof(*nbc), GFP_KERNEL);
+	if (!nbc)
+		goto fail;
+
+	nbc->bch = init_bch(m, t, 0);
+	if (!nbc->bch)
+		goto fail;
+
+	/* verify that eccbytes has the expected value */
+	if (nbc->bch->ecc_bytes != eccbytes) {
+		printk(KERN_WARNING "invalid eccbytes %u, should be %u\n",
+		       eccbytes, nbc->bch->ecc_bytes);
+		goto fail;
+	}
+
+	eccsteps = mtd->writesize/eccsize;
+
+	/* if no ecc placement scheme was provided, build one */
+	if (!*ecclayout) {
+
+		/* handle large page devices only */
+		if (mtd->oobsize < 64) {
+			printk(KERN_WARNING "must provide an oob scheme for "
+			       "oobsize %d\n", mtd->oobsize);
+			goto fail;
+		}
+
+		layout = &nbc->ecclayout;
+		layout->eccbytes = eccsteps*eccbytes;
+
+		/* reserve 2 bytes for bad block marker */
+		if (layout->eccbytes+2 > mtd->oobsize) {
+			printk(KERN_WARNING "no suitable oob scheme available "
+			       "for oobsize %d eccbytes %u\n", mtd->oobsize,
+			       eccbytes);
+			goto fail;
+		}
+		/* put ecc bytes at oob tail */
+		for (i = 0; i < layout->eccbytes; i++)
+			layout->eccpos[i] = mtd->oobsize-layout->eccbytes+i;
+
+		layout->oobfree[0].offset = 2;
+		layout->oobfree[0].length = mtd->oobsize-2-layout->eccbytes;
+
+		*ecclayout = layout;
+	}
+
+	/* sanity checks */
+	if (8*(eccsize+eccbytes) >= (1 << m)) {
+		printk(KERN_WARNING "eccsize %u is too large\n", eccsize);
+		goto fail;
+	}
+	if ((*ecclayout)->eccbytes != (eccsteps*eccbytes)) {
+		printk(KERN_WARNING "invalid ecc layout\n");
+		goto fail;
+	}
+
+	nbc->eccmask = kmalloc(eccbytes, GFP_KERNEL);
+	nbc->errloc = kmalloc(t*sizeof(*nbc->errloc), GFP_KERNEL);
+	if (!nbc->eccmask || !nbc->errloc)
+		goto fail;
+	/*
+	 * compute and store the inverted ecc of an erased ecc block
+	 */
+	erased_page = kmalloc(eccsize, GFP_KERNEL);
+	if (!erased_page)
+		goto fail;
+
+	memset(erased_page, 0xff, eccsize);
+	memset(nbc->eccmask, 0, eccbytes);
+	encode_bch(nbc->bch, erased_page, eccsize, nbc->eccmask);
+	kfree(erased_page);
+
+	for (i = 0; i < eccbytes; i++)
+		nbc->eccmask[i] ^= 0xff;
+
+	return nbc;
+fail:
+	nand_bch_free(nbc);
+	return NULL;
+}
+EXPORT_SYMBOL(nand_bch_init);
+
+/**
+ * nand_bch_free - [NAND Interface] Release NAND BCH ECC resources
+ * @nbc:	NAND BCH control structure
+ */
+void nand_bch_free(struct nand_bch_control *nbc)
+{
+	if (nbc) {
+		free_bch(nbc->bch);
+		kfree(nbc->errloc);
+		kfree(nbc->eccmask);
+		kfree(nbc);
+	}
+}
+EXPORT_SYMBOL(nand_bch_free);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Ivan Djelic <ivan.djelic@parrot.com>");
+MODULE_DESCRIPTION("NAND software BCH ECC support");