Re: [PATCH v5 03/12] iio: dac: ad5686: acquire lock when doing powerdown control

[Jonathan Cameron]

On Fri, 01 May 2026 10:14:56 +0100
Rodrigo Alencar via B4 Relay <devnull+rodrigo.alencar.analog.com@xxxxxxxxxx> wrote:

> From: Rodrigo Alencar <rodrigo.alencar@xxxxxxxxxx>
> 
> Protect write access of pwr_down_mode and pwr_down_mask fields with
> existing mutex lock. Each channel exposes their own attributes for
> controlling powerdown modes and powerdown state. This fixes potential
> race conditions as those functions perform non-atomic read-modify-write
> operations to those pwr_down_* fields. This issue exists since the
> ad5686 driver was first introduced.
> 
> Fixes: c2f37c8dcadc ("iio: dac: New driver for AD5686R, AD5685R, AD5684R Digital to analog converters")
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxx>
> Signed-off-by: Rodrigo Alencar <rodrigo.alencar@xxxxxxxxxx>
https://sashiko.dev/#/patchset/20260501-ad5686-fixes-v5-0-0b2f45488418%40analog.com

Reasonable concern about possible races in the read path leading to
-EPERM rather than simply the wrong value.

The other bit about not updating hardware is true but I think it's fine not to change
what powerdown mode we are in whilst powered down.
 
> ---
>  drivers/iio/dac/ad5686.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/iio/dac/ad5686.c b/drivers/iio/dac/ad5686.c
> index 076fe8b8bd85..69358dd66cbc 100644
> --- a/drivers/iio/dac/ad5686.c
> +++ b/drivers/iio/dac/ad5686.c
> @@ -39,6 +39,8 @@ static int ad5686_set_powerdown_mode(struct iio_dev *indio_dev,
>  {
>  	struct ad5686_state *st = iio_priv(indio_dev);
>  
> +	guard(mutex)(&st->lock);
> +
>  	st->pwr_down_mode &= ~(0x3 << (chan->channel * 2));
>  	st->pwr_down_mode |= ((mode + 1) << (chan->channel * 2));
>  
> @@ -77,6 +79,8 @@ static ssize_t ad5686_write_dac_powerdown(struct iio_dev *indio_dev,
>  	if (ret)
>  		return ret;
>  
> +	guard(mutex)(&st->lock);
> +
>  	if (readin)
>  		st->pwr_down_mask |= (0x3 << (chan->channel * 2));
>  	else
>