mtd: spi-nor: core: Allow specifying the byte order in Octal DTR mode

Macronix swaps bytes on a 16-bit boundary when configured in Octal DTR.
The byte order of 16-bit words is swapped when read or written in 8D-8D-8D
mode compared to STR modes. Allow operations to specify the byte order in
DTR mode, so that controllers can swap the bytes back at run-time to
address the flash's endianness requirements, if they are capable. If the
controller is not capable of swapping the bytes, the protocol is downgrade
via spi_nor_spimem_adjust_hwcaps(). When available, the swapping of the
bytes is always done regardless if it's a data or register access, so that
it comply with the JESD216 requirements: "Byte order of 16-bit words is
swapped when read in 8D-8D-8D mode compared to 1-1-1".

Merge Tudor's patch and add modifications for suiting newer version
of Linux kernel.

Suggested-by: Michael Walle <mwalle@kernel.org>
Signed-off-by: JaimeLiao <jaimeliao@mxic.com.tw>
Signed-off-by: AlvinZhou <alvinzhou@mxic.com.tw>
Link: https://lore.kernel.org/r/20240926141956.2386374-4-alvinzhou.tw@gmail.com
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>

Author: ambarus

drivers/mtd/spi-nor/core.c

@@ -113,6 +113,9 @@ void spi_nor_spimem_setup_op(const struct spi_nor *nor,
 		op->cmd.opcode = (op->cmd.opcode << 8) | ext;
 		op->cmd.nbytes = 2;
 	}
+
+	if (proto == SNOR_PROTO_8_8_8_DTR && nor->flags & SNOR_F_SWAP16)
+		op->data.swap16 = true;
 }
 
 /**

drivers/mtd/spi-nor/core.h

@@ -140,6 +140,7 @@ enum spi_nor_option_flags {
 	SNOR_F_RWW		= BIT(14),
 	SNOR_F_ECC		= BIT(15),
 	SNOR_F_NO_WP		= BIT(16),
+	SNOR_F_SWAP16		= BIT(17),
 };
 
 struct spi_nor_read_command {