spi: spi-mem: Allow specifying the byte order in DTR mode

There are NOR flashes (Macronix) that swap the bytes on a 16-bit boundary
when configured in DTR mode. The byte order of 16-bit words is swapped
when read or written in Double Transfer Rate (DTR) mode compared to
Single Transfer Rate (STR) mode. If one writes D0 D1 D2 D3 bytes using
1-1-1 mode, and uses 8D-8D-8D SPI mode for reading, it will read back
D1 D0 D3 D2. Swapping the bytes is a bad design decision because this may
introduce some endianness problems. It can affect the boot sequence if the
entire boot sequence is not handled in either 8D-8D-8D mode or 1-1-1 mode.
Fortunately there are controllers that can swap back the bytes at runtime,
fixing the endiannesses. Provide a way for the upper layers to specify the
byte order in DTR mode.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>

Author: ambarus

include/linux/spi/spi-mem.h

@@ -89,6 +89,8 @@ enum spi_mem_data_dir {
  * @dummy.dtr: whether the dummy bytes should be sent in DTR mode or not
  * @data.buswidth: number of IO lanes used to send/receive the data
  * @data.dtr: whether the data should be sent in DTR mode or not
+ * @data.dtr_bswap16: whether the byte order of 16-bit words is swapped when
+ *		      read or written in DTR mode compared to STR mode.
  * @data.dir: direction of the transfer
  * @data.nbytes: number of data bytes to send/receive. Can be zero if the
  *		 operation does not involve transferring data
@@ -119,6 +121,7 @@ struct spi_mem_op {
 	struct {
 		u8 buswidth;
 		u8 dtr : 1;
+		u8 dtr_bswap16 : 1;
 		enum spi_mem_data_dir dir;
 		unsigned int nbytes;
 		union {