[PATCH v2 4/6] can: mcp251xfd: mcp251xfd_regmap_crc_write(): workaround for errata 5

[Gregor Herburger <gregor.herburger@xxxxxxxxxxxxxxx>]

According to Errata DS80000789E 5 writing IOCON register using one SPI
write command clears LAT0/LAT1.

Errata Fix/Work Around suggests to write registers with single byte write
instructions. However, it seems that every write to the second byte
causes the overwrite of LAT0/LAT1.

Never write byte 2 of IOCON register to avoid clearing of LAT0/LAT1.

Signed-off-by: Gregor Herburger <gregor.herburger@xxxxxxxxxxxxxxx>
---
 drivers/net/can/spi/mcp251xfd/mcp251xfd-regmap.c | 29 +++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/drivers/net/can/spi/mcp251xfd/mcp251xfd-regmap.c b/drivers/net/can/spi/mcp251xfd/mcp251xfd-regmap.c
index 65150e762007..43fcf7f50591 100644
--- a/drivers/net/can/spi/mcp251xfd/mcp251xfd-regmap.c
+++ b/drivers/net/can/spi/mcp251xfd/mcp251xfd-regmap.c
@@ -229,14 +229,41 @@ mcp251xfd_regmap_crc_gather_write(void *context,
 	return spi_sync_transfer(spi, xfer, ARRAY_SIZE(xfer));
 }
 
+static int mcp251xfd_regmap_crc_write_iocon(void *context, const void *data)
+{
+	u16 reg = MCP251XFD_REG_IOCON;
+
+	/* Never write to bits 16..23 of IOCON register to avoid clearing of LAT0/LAT1
+	 *
+	 * According to Errata DS80000789E 5 writing IOCON register using one
+	 * SPI write command clears LAT0/LAT1.
+	 *
+	 * Errata Fix/Work Around suggests to write registers with single byte
+	 * write instructions. However, it seems that the byte at 0xe06(IOCON[23:16])
+	 * is for read-only access and writing to it causes the clearing of LAT0/LAT1.
+	 */
+
+	/* Write IOCON[15:0] */
+	mcp251xfd_regmap_crc_gather_write(context, &reg, 1, data, 2);
+	reg += 3;
+	/* Write IOCON[31:24] */
+	mcp251xfd_regmap_crc_gather_write(context, &reg, 1, data + 3, 1);
+
+	return 0;
+}
+
 static int
 mcp251xfd_regmap_crc_write(void *context,
 			   const void *data, size_t count)
 {
 	const size_t data_offset = sizeof(__be16) +
 		mcp251xfd_regmap_crc.pad_bits / BITS_PER_BYTE;
+	u16 reg = *(u16 *)data;
 
-	return mcp251xfd_regmap_crc_gather_write(context,
+	if (reg == MCP251XFD_REG_IOCON)
+		return mcp251xfd_regmap_crc_write_iocon(context, data + data_offset);
+	else
+		return mcp251xfd_regmap_crc_gather_write(context,
 						 data, data_offset,
 						 data + data_offset,
 						 count - data_offset);

-- 
2.34.1