Re: [PATCH v5 1/3] hwmon: (ads7871) Fix endianness bug in 16-bit register reads

[Guenter Roeck]

On 4/30/26 19:35, Tabrez Ahmed wrote:
> The ads7871_read_reg16() function relies on spi_w8r16() to read the
> 16-bit sensor output. The ADS7871 device transmits the Least Significant
> Byte (LSB) first.
>
> On Little-Endian architectures, spi_w8r16() correctly reconstructs the
> 16-bit value. However, on Big-Endian architectures, the byte swapping
> causes the first received byte (LSB) to be placed in the most significant
> byte of the u16, resulting in corrupted voltage readings.
>
> Replace spi_w8r16() with a manual spi_write_then_read() into a byte array,
> and safely reconstruct the integer using get_unaligned_le16() to ensure
> correct behavior across all architectures. Additionally, use a u8
> variable for the command byte to ensure the correct instruction is
> transmitted on Big-Endian systems.
>
> Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> Closes: https://sashiko.dev/#/patchset/20260418034601.90226-1-tabreztalks@xxxxxxxxx
> Signed-off-by: Tabrez Ahmed <tabreztalks@xxxxxxxxx>
> ---
>   drivers/hwmon/ads7871.c | 14 +++++++++++---
>   1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/hwmon/ads7871.c b/drivers/hwmon/ads7871.c
> index 9bfdf9e6bcd77..d77eff430935b 100644
> --- a/drivers/hwmon/ads7871.c
> +++ b/drivers/hwmon/ads7871.c
> @@ -59,9 +59,9 @@
>   #include <linux/hwmon-sysfs.h>
>   #include <linux/err.h>
>   #include <linux/delay.h>
> +#include <linux/unaligned.h>
>   
>   #define DEVICE_NAME	"ads7871"
> -
>   struct ads7871_data {
>   	struct spi_device *spi;
>   };
> @@ -77,9 +77,17 @@ 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 tx_cmd;
> +	u8 rx_buf[2];
> +
>   	reg = reg | INST_READ_BM | INST_16BIT_BM;
> -	ret = spi_w8r16(spi, reg);
> -	return ret;
> +	tx_cmd = reg;

This can be combined into

	tx_cmd = reg | INST_READ_BM | INST_16BIT_BM;

Thanks,
Guenter

> +
> +	ret = spi_write_then_read(spi, &tx_cmd, 1, rx_buf, 2);
> +	if (ret < 0)
> +		return ret;
> +
> +	return get_unaligned_le16(rx_buf);
>   }
>   
>   static int ads7871_write_reg8(struct spi_device *spi, int reg, u8 val)