[PATCH v2 1/6] spi: spi-mem: Allow specifying the byte order in DTR mode

[Tudor Ambarus <tudor.ambarus@xxxxxxxxxxxxx>]

There are NOR flashes (Macronix) that swap the bytes on a 16-bit boundary
when configured in Octal DTR mode. The byte order of 16-bit words is
swapped when read or written in Octal Double Transfer Rate (DTR) mode
compared to Single Transfer Rate (STR) modes. 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
it 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. So we must swap the bytes back to have the same byte order as in STR
modes. Fortunately there are controllers that can swap the bytes back at
runtime, addressing the flash's endiannesses requirements. Provide a way
for the upper layers to specify the byte order in Octal DTR mode.

Signed-off-by: Tudor Ambarus <tudor.ambarus@xxxxxxxxxxxxx>
---
 drivers/spi/spi-mem.c       | 4 ++++
 include/linux/spi/spi-mem.h | 6 ++++++
 2 files changed, 10 insertions(+)

diff --git a/drivers/spi/spi-mem.c b/drivers/spi/spi-mem.c
index f38ac31961c9..3b2e586bb6a6 100644
--- a/drivers/spi/spi-mem.c
+++ b/drivers/spi/spi-mem.c
@@ -171,6 +171,10 @@ bool spi_mem_default_supports_op(struct spi_mem *mem,
 		if (!spi_mem_controller_is_capable(ctlr, dtr))
 			return false;
 
+		if (op->data.dtr_swab16 &&
+		    !(spi_mem_controller_is_capable(ctlr, dtr_swab16)))
+			return false;
+
 		if (op->cmd.nbytes != 2)
 			return false;
 	} else {
diff --git a/include/linux/spi/spi-mem.h b/include/linux/spi/spi-mem.h
index 2ba044d0d5e5..a1bf51eda31f 100644
--- a/include/linux/spi/spi-mem.h
+++ b/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_swab16: whether the byte order of 16-bit words is swapped when read
+ *		     or written in Octal DTR mode compared to STR mode.
  * @data.ecc: whether error correction is required or not
  * @data.dir: direction of the transfer
  * @data.nbytes: number of data bytes to send/receive. Can be zero if the
@@ -120,6 +122,7 @@ struct spi_mem_op {
 	struct {
 		u8 buswidth;
 		u8 dtr : 1;
+		u8 dtr_swab16 : 1;
 		u8 ecc : 1;
 		enum spi_mem_data_dir dir;
 		unsigned int nbytes;
@@ -290,10 +293,13 @@ struct spi_controller_mem_ops {
 /**
  * struct spi_controller_mem_caps - SPI memory controller capabilities
  * @dtr: Supports DTR operations
+ * @dtr_swab16: Supports swapping bytes on a 16 bit boundary when configured in
+ *		Octal DTR
  * @ecc: Supports operations with error correction
  */
 struct spi_controller_mem_caps {
 	bool dtr;
+	bool dtr_swab16;
 	bool ecc;
 };
 
-- 
2.25.1