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Date:   Thu, 10 Jun 2021 15:49:58 +0100
From:   Paul Cercueil <paul@crapouillou.net>
Subject: Re: [PATCH 2/2] iio: core: Add "extended_name" attribute to all
 channels
To:     Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Cc:     Jonathan Cameron <jic23@kernel.org>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Michael Hennerich <michael.hennerich@analog.com>,
        linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org
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In-Reply-To: <20210610153425.000029b6@Huawei.com>
References: <20210610124556.34507-1-paul@crapouillou.net>
        <20210610124556.34507-3-paul@crapouillou.net>
        <20210610153425.000029b6@Huawei.com>
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Le jeu., juin 10 2021 at 15:34:25 +0100, Jonathan Cameron 
<Jonathan.Cameron@Huawei.com> a ?crit :
> On Thu, 10 Jun 2021 13:45:56 +0100
> Paul Cercueil <paul@crapouillou.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).

Are there cases where extended_name and label are both used?

If they are exclusive, then it would be fine to put it in the label 
attribute. Parsing would be a bit more awkward because of the extended 
name but still possible (e.g. libiio would read 'in_voltage0_foo_label' 
and check that it returns 'foo').

-Paul

> 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@crapouillou.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++) {
>