Re: [PATCH v3 1/3] hwmon: (ads7871) Fix endianness bug in 16-bit register reads

[sashiko-bot@kernel.org]

Sashiko AI review found 1 potential issue(s):
- [High] Passing a pointer to an `int` variable (`&reg`) to `spi_write_then_read()` for a 1-byte transmission causes the wrong byte to be sent on Big-Endian architectures.
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commit 4f7407427267b9ca42b213449235bc095cff57b3
Author: Tabrez Ahmed <tabreztalks@gmail.com>

hwmon: (ads7871) Fix endianness bug in 16-bit register reads

This commit replaces spi_w8r16() with a manual spi_write_then_read() into
a byte array to correctly handle 16-bit sensor reads. It then reconstructs
the integer using get_unaligned_le16() to ensure correct behavior across
all architectures.

Closes: https://sashiko.dev/#/patchset/20260329073352.270451-1-tabreztalks%40gmail.com

> diff --git a/drivers/hwmon/ads7871.c b/drivers/hwmon/ads7871.c
> index 9bfdf9e6bcd77..9b52aa496d522 100644
> --- a/drivers/hwmon/ads7871.c
> +++ b/drivers/hwmon/ads7871.c

[ ... ]

> @@ -77,9 +78,14 @@ static int ads7871_read_reg8(struct spi_device *spi, int reg)
>  static int ads7871_read_reg16(struct spi_device *spi, int reg)
>  {
>  	int ret;
> +	u8 buf[2];
> +
>  	reg = reg | INST_READ_BM | INST_16BIT_BM;
> -	ret = spi_w8r16(spi, reg);
> -	return ret;
> +	ret = spi_write_then_read(spi, &reg, 1, buf, 2);

Will this transmit the wrong instruction byte on Big-Endian architectures?

The variable reg is a 32-bit int, and spi_write_then_read() reads
sequentially from the memory address provided. On Little-Endian
architectures, the lowest memory address correctly contains the least
significant byte, which is the intended 8-bit command.

However, on Big-Endian architectures, the lowest memory address holds the
most significant byte of the int, which is 0x00. Does this result in 0x00
being transmitted over the SPI bus instead of the correct register
instruction?

> +	if (ret < 0)
> +		return ret;
> +
> +	return get_unaligned_le16(buf);
>  }

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Sashiko AI review · https://sashiko.dev/#/patchset/20260418034601.90226-1-tabreztalks@gmail.com?part=1