mtd: rawnand: Add nand_extract_bits()

There are cases where ECC bytes are not byte-aligned. Indeed, BCH
implies using a number of ECC bits, which are not always a multiple of
8. We then need a helper like nand_extract_bits() to extract these
syndromes from a buffer.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200519074549.23673-5-miquel.raynal@bootlin.com

drivers/mtd/nand/raw/nand_base.c

@@ -274,6 +274,50 @@ static int check_offs_len(struct nand_chip *chip, loff_t ofs, uint64_t len)
 	return ret;
 }
 
+/**
+ * nand_extract_bits - Copy unaligned bits from one buffer to another one
+ * @dst: destination buffer
+ * @dst_off: bit offset at which the writing starts
+ * @src: source buffer
+ * @src_off: bit offset at which the reading starts
+ * @nbits: number of bits to copy from @src to @dst
+ *
+ * Copy bits from one memory region to another (overlap authorized).
+ */
+void nand_extract_bits(u8 *dst, unsigned int dst_off, const u8 *src,
+		       unsigned int src_off, unsigned int nbits)
+{
+	unsigned int tmp, n;
+
+	dst += dst_off / 8;
+	dst_off %= 8;
+	src += src_off / 8;
+	src_off %= 8;
+
+	while (nbits) {
+		n = min3(8 - dst_off, 8 - src_off, nbits);
+
+		tmp = (*src >> src_off) & GENMASK(n - 1, 0);
+		*dst &= ~GENMASK(n - 1 + dst_off, dst_off);
+		*dst |= tmp << dst_off;
+
+		dst_off += n;
+		if (dst_off >= 8) {
+			dst++;
+			dst_off -= 8;
+		}
+
+		src_off += n;
+		if (src_off >= 8) {
+			src++;
+			src_off -= 8;
+		}
+
+		nbits -= n;
+	}
+}
+EXPORT_SYMBOL_GPL(nand_extract_bits);
+
 /**
  * nand_select_target() - Select a NAND target (A.K.A. die)
  * @chip: NAND chip object

include/linux/mtd/rawnand.h

@@ -1414,6 +1414,10 @@ int nand_gpio_waitrdy(struct nand_chip *chip, struct gpio_desc *gpiod,
 void nand_select_target(struct nand_chip *chip, unsigned int cs);
 void nand_deselect_target(struct nand_chip *chip);
 
+/* Bitops */
+void nand_extract_bits(u8 *dst, unsigned int dst_off, const u8 *src,
+		       unsigned int src_off, unsigned int nbits);
+
 /**
  * nand_get_data_buf() - Get the internal page buffer
  * @chip: NAND chip object