linux-kernel - Re: [PATCH 2/2] iio: core: Add "extended

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Date:   Thu, 10 Jun 2021 15:34:25 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Paul Cercueil <paul@...pouillou.net>
CC:     Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Michael Hennerich <michael.hennerich@...log.com>,
        <linux-iio@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] iio: core: Add "extended_name" attribute to all
 channels

On Thu, 10 Jun 2021 13:45:56 +0100
Paul Cercueil <paul@...pouillou.net> wrote:

> The point of this new attribute is to make the IIO tree actually
> parsable.
> 
> Before, given this attribute as a filename:
> in_voltage0_aux_sample_rate
> 
> Userspace had no way to know if the attribute name was
> "aux_sample_rate" with no extended name, or "sample_rate" with "aux" as
> the extended name, or just "rate" with "aux_sample" as the extended
> name.
> 
> This was somewhat possible to deduce when there was more than one
> attribute present for a given channel, e.g:
> in_voltage0_aux_sample_rate
> in_voltage0_aux_frequency
> 
> There, it was possible to deduce that "aux" was the extended name. But
> even with more than one attribute, this wasn't very robust, as two
> attributes starting with the same prefix (e.g. "sample_rate" and
> "sample_size") would result in the first part of the prefix being
> interpreted as being part of the extended name.
> 
> To address this issue, add an "extended_name" attribute to all channels
> that actually do have an extended name. 

Change the patch title to make it clear that it only applies to those
that have extended_name set.

> For this attribute, the extended
> name is not present in the filename; so in this example, the file name
> would be "in_voltage0_extended_name", and reading it would return "aux".

Ah. Now I see the slightly issue with my immediate thought that we should
just put this in the label attribute (and not allow both extended_name
and label to be provided). 

Hmm. It's a bit ugly but given it hopefully doesn't effect that many drivers
I could probably live with it.

However, needs a patch to Documentation/ABI/testing/sysfs-bus-iio
and a clear statement that this is for backwards compatibility reasons.
I don't want to see extended_name getting added to new drivers!

Jonathan

> 
> Signed-off-by: Paul Cercueil <paul@...pouillou.net>
> ---
>  drivers/iio/industrialio-core.c | 41 +++++++++++++++++++++++++++++++++
>  1 file changed, 41 insertions(+)
> 
> diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
> index ec34d930920c..4cdf9f092d73 100644
> --- a/drivers/iio/industrialio-core.c
> +++ b/drivers/iio/industrialio-core.c
> @@ -723,6 +723,16 @@ static ssize_t iio_read_channel_label(struct device *dev,
>  	return indio_dev->info->read_label(indio_dev, this_attr->c, buf);
>  }
>  
> +static ssize_t iio_read_channel_extended_name(struct device *dev,
> +					      struct device_attribute *attr,
> +					      char *buf)
> +{
> +	const struct iio_dev_attr *this_attr = to_iio_dev_attr(attr);
> +	const struct iio_chan_spec *chan = this_attr->c;
> +
> +	return sprintf(buf, "%s\n", chan->extend_name);
> +}
> +
>  static ssize_t iio_read_channel_info(struct device *dev,
>  				     struct device_attribute *attr,
>  				     char *buf)
> @@ -1185,6 +1195,32 @@ static int iio_device_add_channel_label(struct iio_dev *indio_dev,
>  	return 1;
>  }
>  
> +static int
> +iio_device_add_channel_extended_name(struct iio_dev *indio_dev,
> +				     struct iio_chan_spec const *chan)
> +{
> +	struct iio_dev_opaque *iio_dev_opaque = to_iio_dev_opaque(indio_dev);
> +	int ret;
> +
> +	if (!chan->extend_name)
> +		return 0;
> +
> +	ret = __iio_add_chan_devattr("extended_name",
> +				     chan,
> +				     &iio_read_channel_extended_name,
> +				     NULL,
> +				     0,
> +				     IIO_SEPARATE,
> +				     &indio_dev->dev,
> +				     NULL,
> +				     &iio_dev_opaque->channel_attr_list,
> +				     false);
> +	if (ret < 0)
> +		return ret;
> +
> +	return 1;
> +}
> +
>  static int iio_device_add_info_mask_type(struct iio_dev *indio_dev,
>  					 struct iio_chan_spec const *chan,
>  					 enum iio_shared_by shared_by,
> @@ -1327,6 +1363,11 @@ static int iio_device_add_channel_sysfs(struct iio_dev *indio_dev,
>  		return ret;
>  	attrcount += ret;
>  
> +	ret = iio_device_add_channel_extended_name(indio_dev, chan);
> +	if (ret < 0)
> +		return ret;
> +	attrcount += ret;
> +
>  	if (chan->ext_info) {
>  		unsigned int i = 0;
>  		for (ext_info = chan->ext_info; ext_info->name; ext_info++) {