Re: [PATCH] staging: iio: adc: ad7816: add mutex to serialize SPI/GPIO operations

[David Lechner <dlechner@baylibre.com>]

On 8/31/25 10:37 AM, Mohammad Amin Hosseini wrote:
> From: mohammad amin hosseini <moahmmad.hosseinii@gmail.com>
> 
> The ad7816 driver was accessing SPI and GPIO lines without
> synchronization, which could lead to race conditions when accessed
> concurrently from multiple contexts. This might result in corrupted
> readings or inconsistent GPIO states.
> 
> Introduce an io_lock mutex in the driver structure to serialize:
> - SPI transactions in ad7816_spi_read() and ad7816_spi_write()
> - GPIO pin toggling sequences
> - Updates to device state via sysfs store functions (mode, channel, oti)
> 
> The mutex ensures proper mutual exclusion and prevents race
> conditions under concurrent access.
> 
> Signed-off-by: Mohammad Amin Hosseini <moahmmad.hosseinii@gmail.com>
> ---
>  drivers/staging/iio/adc/ad7816.c | 29 +++++++++++++++++++----------
>  1 file changed, 19 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/staging/iio/adc/ad7816.c b/drivers/staging/iio/adc/ad7816.c
> index 4774df778de9..06567d048a6d 100644
> --- a/drivers/staging/iio/adc/ad7816.c
> +++ b/drivers/staging/iio/adc/ad7816.c
> @@ -50,6 +50,7 @@ struct ad7816_chip_info {
>  	u8  oti_data[AD7816_CS_MAX + 1];
>  	u8  channel_id;	/* 0 always be temperature */
>  	u8  mode;
> +	struct mutex io_lock;	/* Protects SPI transactions and GPIO toggling */
>  };
>  
>  enum ad7816_type {
> @@ -67,13 +68,13 @@ static int ad7816_spi_read(struct ad7816_chip_info *chip, u16 *data)
>  	int ret;
>  	__be16 buf;
>  
> +	mutex_lock(&chip->io_lock);

Could avoid the gotos by using:

	guard(mutex)(&chip->io_lock);

> +
>  	gpiod_set_value(chip->rdwr_pin, 1);
>  	gpiod_set_value(chip->rdwr_pin, 0);
>  	ret = spi_write(spi_dev, &chip->channel_id, sizeof(chip->channel_id));
> -	if (ret < 0) {
> -		dev_err(&spi_dev->dev, "SPI channel setting error\n");
> -		return ret;
> -	}
> +	if (ret < 0)
> +		goto unlock;
>  	gpiod_set_value(chip->rdwr_pin, 1);
>  
>  	if (chip->mode == AD7816_PD) { /* operating mode 2 */
> @@ -92,13 +93,13 @@ static int ad7816_spi_read(struct ad7816_chip_info *chip, u16 *data)
>  	gpiod_set_value(chip->rdwr_pin, 0);
>  	gpiod_set_value(chip->rdwr_pin, 1);
>  	ret = spi_read(spi_dev, &buf, sizeof(*data));
> -	if (ret < 0) {
> -		dev_err(&spi_dev->dev, "SPI data read error\n");
> -		return ret;
> -	}
> +	if (ret < 0)
> +		goto unlock;
>  
>  	*data = be16_to_cpu(buf);
>  
> +unlock:
> +	mutex_unlock(&chip->io_lock);
>  	return ret;
>  }
>  
> @@ -107,12 +108,13 @@ static int ad7816_spi_write(struct ad7816_chip_info *chip, u8 data)
>  	struct spi_device *spi_dev = chip->spi_dev;
>  	int ret;
>  
> +	mutex_lock(&chip->io_lock);
> +
>  	gpiod_set_value(chip->rdwr_pin, 1);
>  	gpiod_set_value(chip->rdwr_pin, 0);
>  	ret = spi_write(spi_dev, &data, sizeof(data));
> -	if (ret < 0)
> -		dev_err(&spi_dev->dev, "SPI oti data write error\n");
>  
> +	mutex_unlock(&chip->io_lock);
>  	return ret;
>  }
>  

...

> @@ -363,6 +371,7 @@ static int ad7816_probe(struct spi_device *spi_dev)
>  	dev_set_drvdata(&spi_dev->dev, indio_dev);
>  
>  	chip->spi_dev = spi_dev;
> +	mutex_init(&chip->io_lock);

Prefer devm_mutex_init() instead.

>  	for (i = 0; i <= AD7816_CS_MAX; i++)
>  		chip->oti_data[i] = 203;
>