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From:   Tudor Ambarus <tudor.ambarus@microchip.com>
To:     <p.yadav@ti.com>, <michael@walle.cc>, <broonie@kernel.org>
CC:     <miquel.raynal@bootlin.com>, <richard@nod.at>, <vigneshr@ti.com>,
        <linux-mtd@lists.infradead.org>, <linux-kernel@vger.kernel.org>,
        <linux-spi@vger.kernel.org>, <nicolas.ferre@microchip.com>,
        <zhengxunli@mxic.com.tw>, <jaimeliao@mxic.com.tw>,
        Tudor Ambarus <tudor.ambarus@microchip.com>
Subject: [PATCH 1/4] spi: spi-mem: Allow specifying the byte order in DTR mode
Date:   Fri, 18 Feb 2022 16:58:57 +0200
Message-ID: <20220218145900.1440045-2-tudor.ambarus@microchip.com>
In-Reply-To: <20220218145900.1440045-1-tudor.ambarus@microchip.com>
References: <20220218145900.1440045-1-tudor.ambarus@microchip.com>
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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>
---
 include/linux/spi/spi-mem.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/spi/spi-mem.h b/include/linux/spi/spi-mem.h
index 85e2ff7b840d..e1878417420c 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_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 {
-- 
2.25.1