lib/bch: Allow easy bit swapping

It seems that several hardware ECC engine use a swapped representation
of bytes compared to software. This might having to do with how the
ECC engine is wired to the NAND controller or the order the bits are
passed to the hardware BCH logic.

This means that when the software BCH engine is working in conjunction
with data generated with hardware, sometimes we might need to swap the
bits inside bytes, eg:

    0x0A = b0000_1010 -> b0101_0000 = 0x50

Make it possible by adding a boolean to the BCH initialization routine.

Regarding the implementation itself, this is a rather simple approach
that can probably be enhanced in the future by preparing the
->a_{mod,pow}_tab tables with the swapping in mind.
Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200519074549.23673-3-miquel.raynal@bootlin.com

drivers/mtd/devices/docg3.c

@@ -1985,7 +1985,7 @@ static int __init docg3_probe(struct platform_device *pdev)
 	cascade->base = base;
 	mutex_init(&cascade->lock);
 	cascade->bch = bch_init(DOC_ECC_BCH_M, DOC_ECC_BCH_T,
-				DOC_ECC_BCH_PRIMPOLY);
+				DOC_ECC_BCH_PRIMPOLY, false);
 	if (!cascade->bch)
 		return ret;
 

drivers/mtd/nand/raw/nand_bch.c

@@ -130,7 +130,7 @@ struct nand_bch_control *nand_bch_init(struct mtd_info *mtd)
 	if (!nbc)
 		goto fail;
 
-	nbc->bch = bch_init(m, t, 0);
+	nbc->bch = bch_init(m, t, 0, false);
 	if (!nbc->bch)
 		goto fail;
 

include/linux/bch.h

@@ -33,6 +33,7 @@
  * @cache:      log-based polynomial representation buffer
  * @elp:        error locator polynomial
  * @poly_2t:    temporary polynomials of degree 2t
+ * @swap_bits:  swap bits within data and syndrome bytes
  */
 struct bch_control {
 	unsigned int    m;
@@ -51,9 +52,11 @@ struct bch_control {
 	int            *cache;
 	struct gf_poly *elp;
 	struct gf_poly *poly_2t[4];
+	bool		swap_bits;
 };
 
-struct bch_control *bch_init(int m, int t, unsigned int prim_poly);
+struct bch_control *bch_init(int m, int t, unsigned int prim_poly,
+			     bool swap_bits);
 
 void bch_free(struct bch_control *bch);
 

lib/bch.c

@@ -114,6 +114,49 @@ struct gf_poly_deg1 {
 	unsigned int   c[2];
 };
 
+static u8 swap_bits_table[] = {
+	0x00, 0x80, 0x40, 0xc0, 0x20, 0xa0, 0x60, 0xe0,
+	0x10, 0x90, 0x50, 0xd0, 0x30, 0xb0, 0x70, 0xf0,
+	0x08, 0x88, 0x48, 0xc8, 0x28, 0xa8, 0x68, 0xe8,
+	0x18, 0x98, 0x58, 0xd8, 0x38, 0xb8, 0x78, 0xf8,
+	0x04, 0x84, 0x44, 0xc4, 0x24, 0xa4, 0x64, 0xe4,
+	0x14, 0x94, 0x54, 0xd4, 0x34, 0xb4, 0x74, 0xf4,
+	0x0c, 0x8c, 0x4c, 0xcc, 0x2c, 0xac, 0x6c, 0xec,
+	0x1c, 0x9c, 0x5c, 0xdc, 0x3c, 0xbc, 0x7c, 0xfc,
+	0x02, 0x82, 0x42, 0xc2, 0x22, 0xa2, 0x62, 0xe2,
+	0x12, 0x92, 0x52, 0xd2, 0x32, 0xb2, 0x72, 0xf2,
+	0x0a, 0x8a, 0x4a, 0xca, 0x2a, 0xaa, 0x6a, 0xea,
+	0x1a, 0x9a, 0x5a, 0xda, 0x3a, 0xba, 0x7a, 0xfa,
+	0x06, 0x86, 0x46, 0xc6, 0x26, 0xa6, 0x66, 0xe6,
+	0x16, 0x96, 0x56, 0xd6, 0x36, 0xb6, 0x76, 0xf6,
+	0x0e, 0x8e, 0x4e, 0xce, 0x2e, 0xae, 0x6e, 0xee,
+	0x1e, 0x9e, 0x5e, 0xde, 0x3e, 0xbe, 0x7e, 0xfe,
+	0x01, 0x81, 0x41, 0xc1, 0x21, 0xa1, 0x61, 0xe1,
+	0x11, 0x91, 0x51, 0xd1, 0x31, 0xb1, 0x71, 0xf1,
+	0x09, 0x89, 0x49, 0xc9, 0x29, 0xa9, 0x69, 0xe9,
+	0x19, 0x99, 0x59, 0xd9, 0x39, 0xb9, 0x79, 0xf9,
+	0x05, 0x85, 0x45, 0xc5, 0x25, 0xa5, 0x65, 0xe5,
+	0x15, 0x95, 0x55, 0xd5, 0x35, 0xb5, 0x75, 0xf5,
+	0x0d, 0x8d, 0x4d, 0xcd, 0x2d, 0xad, 0x6d, 0xed,
+	0x1d, 0x9d, 0x5d, 0xdd, 0x3d, 0xbd, 0x7d, 0xfd,
+	0x03, 0x83, 0x43, 0xc3, 0x23, 0xa3, 0x63, 0xe3,
+	0x13, 0x93, 0x53, 0xd3, 0x33, 0xb3, 0x73, 0xf3,
+	0x0b, 0x8b, 0x4b, 0xcb, 0x2b, 0xab, 0x6b, 0xeb,
+	0x1b, 0x9b, 0x5b, 0xdb, 0x3b, 0xbb, 0x7b, 0xfb,
+	0x07, 0x87, 0x47, 0xc7, 0x27, 0xa7, 0x67, 0xe7,
+	0x17, 0x97, 0x57, 0xd7, 0x37, 0xb7, 0x77, 0xf7,
+	0x0f, 0x8f, 0x4f, 0xcf, 0x2f, 0xaf, 0x6f, 0xef,
+	0x1f, 0x9f, 0x5f, 0xdf, 0x3f, 0xbf, 0x7f, 0xff,
+};
+
+static u8 swap_bits(struct bch_control *bch, u8 in)
+{
+	if (!bch->swap_bits)
+		return in;
+
+	return swap_bits_table[in];
+}
+
 /*
  * same as bch_encode(), but process input data one byte at a time
  */
@@ -126,7 +169,9 @@ static void bch_encode_unaligned(struct bch_control *bch,
 	const int l = BCH_ECC_WORDS(bch)-1;
 
 	while (len--) {
-		p = bch->mod8_tab + (l+1)*(((ecc[0] >> 24)^(*data++)) & 0xff);
+		u8 tmp = swap_bits(bch, *data++);
+
+		p = bch->mod8_tab + (l+1)*(((ecc[0] >> 24)^(tmp)) & 0xff);
 
 		for (i = 0; i < l; i++)
 			ecc[i] = ((ecc[i] << 8)|(ecc[i+1] >> 24))^(*p++);
@@ -145,10 +190,16 @@ static void load_ecc8(struct bch_control *bch, uint32_t *dst,
 	unsigned int i, nwords = BCH_ECC_WORDS(bch)-1;
 
 	for (i = 0; i < nwords; i++, src += 4)
-		dst[i] = (src[0] << 24)|(src[1] << 16)|(src[2] << 8)|src[3];
+		dst[i] = ((u32)swap_bits(bch, src[0]) << 24) |
+			((u32)swap_bits(bch, src[1]) << 16) |
+			((u32)swap_bits(bch, src[2]) << 8) |
+			swap_bits(bch, src[3]);
 
 	memcpy(pad, src, BCH_ECC_BYTES(bch)-4*nwords);
-	dst[nwords] = (pad[0] << 24)|(pad[1] << 16)|(pad[2] << 8)|pad[3];
+	dst[nwords] = ((u32)swap_bits(bch, pad[0]) << 24) |
+		((u32)swap_bits(bch, pad[1]) << 16) |
+		((u32)swap_bits(bch, pad[2]) << 8) |
+		swap_bits(bch, pad[3]);
 }
 
 /*
@@ -161,15 +212,15 @@ static void store_ecc8(struct bch_control *bch, uint8_t *dst,
 	unsigned int i, nwords = BCH_ECC_WORDS(bch)-1;
 
 	for (i = 0; i < nwords; i++) {
-		*dst++ = (src[i] >> 24);
-		*dst++ = (src[i] >> 16) & 0xff;
-		*dst++ = (src[i] >>  8) & 0xff;
-		*dst++ = (src[i] >>  0) & 0xff;
+		*dst++ = swap_bits(bch, src[i] >> 24);
+		*dst++ = swap_bits(bch, src[i] >> 16);
+		*dst++ = swap_bits(bch, src[i] >> 8);
+		*dst++ = swap_bits(bch, src[i]);
 	}
-	pad[0] = (src[nwords] >> 24);
-	pad[1] = (src[nwords] >> 16) & 0xff;
-	pad[2] = (src[nwords] >>  8) & 0xff;
-	pad[3] = (src[nwords] >>  0) & 0xff;
+	pad[0] = swap_bits(bch, src[nwords] >> 24);
+	pad[1] = swap_bits(bch, src[nwords] >> 16);
+	pad[2] = swap_bits(bch, src[nwords] >> 8);
+	pad[3] = swap_bits(bch, src[nwords]);
 	memcpy(dst, pad, BCH_ECC_BYTES(bch)-4*nwords);
 }
 
@@ -240,7 +291,13 @@ void bch_encode(struct bch_control *bch, const uint8_t *data,
 	 */
 	while (mlen--) {
 		/* input data is read in big-endian format */
-		w = r[0]^cpu_to_be32(*pdata++);
+		w = cpu_to_be32(*pdata++);
+		if (bch->swap_bits)
+			w = (u32)swap_bits(bch, w) |
+			    ((u32)swap_bits(bch, w >> 8) << 8) |
+			    ((u32)swap_bits(bch, w >> 16) << 16) |
+			    ((u32)swap_bits(bch, w >> 24) << 24);
+		w ^= r[0];
 		p0 = tab0 + (l+1)*((w >>  0) & 0xff);
 		p1 = tab1 + (l+1)*((w >>  8) & 0xff);
 		p2 = tab2 + (l+1)*((w >> 16) & 0xff);
@@ -1048,7 +1105,9 @@ int bch_decode(struct bch_control *bch, const uint8_t *data, unsigned int len,
 				break;
 			}
 			errloc[i] = nbits-1-errloc[i];
-			errloc[i] = (errloc[i] & ~7)|(7-(errloc[i] & 7));
+			if (!bch->swap_bits)
+				errloc[i] = (errloc[i] & ~7) |
+					    (7-(errloc[i] & 7));
 		}
 	}
 	return (err >= 0) ? err : -EBADMSG;
@@ -1240,6 +1299,7 @@ static uint32_t *compute_generator_polynomial(struct bch_control *bch)
  * @m:          Galois field order, should be in the range 5-15
  * @t:          maximum error correction capability, in bits
  * @prim_poly:  user-provided primitive polynomial (or 0 to use default)
+ * @swap_bits:  swap bits within data and syndrome bytes
  *
  * Returns:
  *  a newly allocated BCH control structure if successful, NULL otherwise
@@ -1256,7 +1316,8 @@ static uint32_t *compute_generator_polynomial(struct bch_control *bch)
  * BCH control structure, ecc length in bytes is given by member @ecc_bytes of
  * the structure.
  */
-struct bch_control *bch_init(int m, int t, unsigned int prim_poly)
+struct bch_control *bch_init(int m, int t, unsigned int prim_poly,
+			     bool swap_bits)
 {
 	int err = 0;
 	unsigned int i, words;
@@ -1321,6 +1382,7 @@ struct bch_control *bch_init(int m, int t, unsigned int prim_poly)
 	bch->syn       = bch_alloc(2*t*sizeof(*bch->syn), &err);
 	bch->cache     = bch_alloc(2*t*sizeof(*bch->cache), &err);
 	bch->elp       = bch_alloc((t+1)*sizeof(struct gf_poly_deg1), &err);
+	bch->swap_bits = swap_bits;
 
 	for (i = 0; i < ARRAY_SIZE(bch->poly_2t); i++)
 		bch->poly_2t[i] = bch_alloc(GF_POLY_SZ(2*t), &err);