can: tcan4x5x: fix data length in regmap write path

In regmap_spi_gather_write() the "addr" is prepared. The chip expects
the number of 32 bit words to write in the lower 8 bits of addr. However
the number of byte to write in shifted left by 3 (== divided by 8).

The function tcan4x5x_regmap_write() is called with a data buffer, which
holds the register information in the first 32 bits, followed by the
actual data. tcan4x5x_regmap_write() calls regmap_spi_gather_write()
with the val pointer pointing to the actual data (i.e. the original
pointer is incremented by 4 bytes), but without decrementing the count.

If the regmap framework only calls tcan4x5x_regmap_write() to read
single 32 bit registers these two bugs cancel each other.

This patch fixes the code.

Fixes: 5443c226 ("can: tcan4x5x: Add tcan4x5x driver to the kernel")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

drivers/net/can/m_can/tcan4x5x.c

@@ -178,7 +178,7 @@ static int regmap_spi_gather_write(void *context, const void *reg,
 		{ .tx_buf = val, .len = val_len, },
 	};
 
-	addr = TCAN4X5X_WRITE_CMD | (*((u16 *)reg) << 8) | val_len >> 3;
+	addr = TCAN4X5X_WRITE_CMD | (*((u16 *)reg) << 8) | val_len >> 2;
 
 	spi_message_init(&m);
 	spi_message_add_tail(&t[0], &m);
@@ -192,7 +192,7 @@ static int tcan4x5x_regmap_write(void *context, const void *data, size_t count)
 	u16 *reg = (u16 *)(data);
 	const u32 *val = data + 4;
 
-	return regmap_spi_gather_write(context, reg, 4, val, count);
+	return regmap_spi_gather_write(context, reg, 4, val, count - 4);
 }
 
 static int regmap_spi_async_write(void *context,