Re: [PATCH 2/4] iio: inkern: Add a helper to query an available minimum raw value

[Jonathan Cameron <jic23@kernel.org>]

On Fri, 21 Apr 2023 14:41:20 +0200
Herve Codina <herve.codina@bootlin.com> wrote:

> A helper, iio_read_max_channel_raw() exists to read the available
> maximum raw value of a channel but nothing similar exists to read the
> available minimum raw value.
> 
> This new helper, iio_read_min_channel_raw(), fills the hole and can be
> used for reading the available minimum raw value of a channel.
> It is fully based on the existing iio_read_max_channel_raw().
> 
> Signed-off-by: Herve Codina <herve.codina@bootlin.com>

Hi Herve,

All the comments on this are really comments on the existing code.
If you don't mind fixing the first one about checking the error code whilst
you are here that would be great.  Don't worry about the docs comment.
There are lots of instances of that and the point is rather subtle and probably
post dates this code being written.  In a few cases raw doesn't mean ADC counts
but rather something slightly modified... Long story for why!

Jonathan

> ---
>  drivers/iio/inkern.c         | 67 ++++++++++++++++++++++++++++++++++++
>  include/linux/iio/consumer.h | 11 ++++++
>  2 files changed, 78 insertions(+)
> 
> diff --git a/drivers/iio/inkern.c b/drivers/iio/inkern.c
> index 872fd5c24147..914fc69c718a 100644
> --- a/drivers/iio/inkern.c
> +++ b/drivers/iio/inkern.c
> @@ -912,6 +912,73 @@ int iio_read_max_channel_raw(struct iio_channel *chan, int *val)
>  }
>  EXPORT_SYMBOL_GPL(iio_read_max_channel_raw);
>  
> +static int iio_channel_read_min(struct iio_channel *chan,
> +				int *val, int *val2, int *type,
> +				enum iio_chan_info_enum info)
> +{
> +	int unused;
> +	const int *vals;
> +	int length;
> +	int ret;
> +
> +	if (!val2)
> +		val2 = &unused;
> +
> +	ret = iio_channel_read_avail(chan, &vals, type, &length, info);
Obviously this is copied from *_read_max() but look at it here...

We should check for an error first with
if (ret < 0)
	return ret;
then the switch.

Currently a different positive ret would result in that value
being returned which would be odd. Not a problem today, but if we add other
iio_avail_type enum entries in future and don't keep up with all the
utility functions then a mess may result.

If you agree with change and wouldn't mind adding another patch to this series
tidying that up for the _max case that would be great! Otherwise I'll get to
fixing that at some point but not anytime soon.

> +	switch (ret) {
> +	case IIO_AVAIL_RANGE:
> +		switch (*type) {
> +		case IIO_VAL_INT:
> +			*val = vals[0];
> +			break;
> +		default:
> +			*val = vals[0];
> +			*val2 = vals[1];
> +		}
> +		return 0;
> +
> +	case IIO_AVAIL_LIST:
> +		if (length <= 0)
> +			return -EINVAL;
> +		switch (*type) {
> +		case IIO_VAL_INT:
> +			*val = vals[--length];
> +			while (length) {
> +				if (vals[--length] < *val)
> +					*val = vals[length];
> +			}
> +			break;
> +		default:
> +			/* FIXME: learn about min for other iio values */
> +			return -EINVAL;
> +		}
> +		return 0;
> +
> +	default:
> +		return ret;
> +	}
> +}
> +
> +int iio_read_min_channel_raw(struct iio_channel *chan, int *val)
> +{
> +	struct iio_dev_opaque *iio_dev_opaque = to_iio_dev_opaque(chan->indio_dev);
> +	int ret;
> +	int type;
> +
> +	mutex_lock(&iio_dev_opaque->info_exist_lock);
> +	if (!chan->indio_dev->info) {
> +		ret = -ENODEV;
> +		goto err_unlock;
> +	}
> +
> +	ret = iio_channel_read_min(chan, val, NULL, &type, IIO_CHAN_INFO_RAW);
> +err_unlock:
> +	mutex_unlock(&iio_dev_opaque->info_exist_lock);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(iio_read_min_channel_raw);
> +
>  int iio_get_channel_type(struct iio_channel *chan, enum iio_chan_type *type)
>  {
>  	struct iio_dev_opaque *iio_dev_opaque = to_iio_dev_opaque(chan->indio_dev);
> diff --git a/include/linux/iio/consumer.h b/include/linux/iio/consumer.h
> index 6802596b017c..956120d8b5a3 100644
> --- a/include/linux/iio/consumer.h
> +++ b/include/linux/iio/consumer.h
> @@ -297,6 +297,17 @@ int iio_write_channel_raw(struct iio_channel *chan, int val);
>   */
>  int iio_read_max_channel_raw(struct iio_channel *chan, int *val);
>  
> +/**
> + * iio_read_min_channel_raw() - read minimum available raw value from a given
> + *				channel, i.e. the minimum possible value.
> + * @chan:		The channel being queried.
> + * @val:		Value read back.
> + *
> + * Note raw reads from iio channels are in adc counts and hence
> + * scale will need to be applied if standard units are required.

Hmm. That comment is almost always true, but not quite.  Not related to
your patch but some cleanup of this documentation and pushing it down next
to implementations should be done at some point.  If anyone is really
bored and wants to take this on that's fine. If not, another one for the
todo list ;)

> + */
> +int iio_read_min_channel_raw(struct iio_channel *chan, int *val);
> +
>  /**
>   * iio_read_avail_channel_raw() - read available raw values from a given channel
>   * @chan:		The channel being queried.