Re: [PATCH] iio: dac: ad7303: replace mlock with own lock

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 19 Sep 2019 16:13:28 +0300
Alexandru Ardelean <alexandru.ardelean@analog.com> wrote:

> This change replaces indio_dev's mlock with the driver's own lock. The lock
> is mostly needed to protect state when changing the `config` & `dac_cache`
> fields. As such, the lock has been extended to covert the read of these
> fields.
This last bit would be needed if we had anywhere that needed to see a consistent
state across those two fields. I can't immediately see where that happens.
> 
> Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
See below.

> ---
>  drivers/iio/dac/ad7303.c | 21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/iio/dac/ad7303.c b/drivers/iio/dac/ad7303.c
> index 8de9f40226e6..f45352e80db7 100644
> --- a/drivers/iio/dac/ad7303.c
> +++ b/drivers/iio/dac/ad7303.c
> @@ -41,6 +41,7 @@ struct ad7303_state {
>  	struct regulator *vdd_reg;
>  	struct regulator *vref_reg;
>  
> +	struct mutex lock;
>  	/*
>  	 * DMA (thus cache coherency maintenance) requires the
>  	 * transfer buffers to live in their own cache lines.
> @@ -62,9 +63,13 @@ static ssize_t ad7303_read_dac_powerdown(struct iio_dev *indio_dev,
>  	uintptr_t private, const struct iio_chan_spec *chan, char *buf)
>  {
>  	struct ad7303_state *st = iio_priv(indio_dev);
> +	int val;
>  
> -	return sprintf(buf, "%d\n", (bool)(st->config &
> -		AD7303_CFG_POWER_DOWN(chan->channel)));
> +	mutex_lock(&st->lock);
> +	val = (bool)(st->config & AD7303_CFG_POWER_DOWN(chan->channel));
> +	mutex_unlock(&st->lock);

I think the read of st->config should be atomic anyway so you'll either
get the value before or after some concurrent write. That's true
even with the locks, so I don't htink they are needed here.

> +
> +	return sprintf(buf, "%d\n", val);
>  }
>  
>  static ssize_t ad7303_write_dac_powerdown(struct iio_dev *indio_dev,
> @@ -79,7 +84,7 @@ static ssize_t ad7303_write_dac_powerdown(struct iio_dev *indio_dev,
>  	if (ret)
>  		return ret;
>  
> -	mutex_lock(&indio_dev->mlock);
> +	mutex_lock(&st->lock);
>  
>  	if (pwr_down)
>  		st->config |= AD7303_CFG_POWER_DOWN(chan->channel);
> @@ -90,7 +95,7 @@ static ssize_t ad7303_write_dac_powerdown(struct iio_dev *indio_dev,
>  	 * mode, so just write one of the DAC channels again */
>  	ad7303_write(st, chan->channel, st->dac_cache[chan->channel]);
>  
> -	mutex_unlock(&indio_dev->mlock);
> +	mutex_unlock(&st->lock);
>  	return len;
>  }
>  
> @@ -116,7 +121,9 @@ static int ad7303_read_raw(struct iio_dev *indio_dev,
>  
>  	switch (info) {
>  	case IIO_CHAN_INFO_RAW:
> +		mutex_lock(&st->lock);
>  		*val = st->dac_cache[chan->channel];
> +		mutex_unlock(&st->lock);
>  		return IIO_VAL_INT;
>  	case IIO_CHAN_INFO_SCALE:
>  		vref_uv = ad7303_get_vref(st, chan);
> @@ -144,11 +151,11 @@ static int ad7303_write_raw(struct iio_dev *indio_dev,
>  		if (val >= (1 << chan->scan_type.realbits) || val < 0)
>  			return -EINVAL;
>  
> -		mutex_lock(&indio_dev->mlock);
> +		mutex_lock(&st->lock);
>  		ret = ad7303_write(st, chan->address, val);
>  		if (ret == 0)
>  			st->dac_cache[chan->channel] = val;
> -		mutex_unlock(&indio_dev->mlock);
> +		mutex_unlock(&st->lock);
>  		break;
>  	default:
>  		ret = -EINVAL;
> @@ -211,6 +218,8 @@ static int ad7303_probe(struct spi_device *spi)
>  
>  	st->spi = spi;
>  
> +	mutex_init(&st->lock);
> +
>  	st->vdd_reg = devm_regulator_get(&spi->dev, "Vdd");
>  	if (IS_ERR(st->vdd_reg))
>  		return PTR_ERR(st->vdd_reg);