linux-kernel - Re: [PATCH v2 4/4] hwmon: iio: add label for channels read by iio

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Date:	Tue, 19 Jul 2016 08:55:17 +0200
From:	Quentin Schulz <quentin.schulz@...e-electrons.com>
To:	Jonathan Cameron <jic23@...nel.org>, jdelvare@...e.com,
	linux@...ck-us.net, knaack.h@....de, lars@...afoo.de,
	pmeerw@...erw.net, maxime.ripard@...e-electrons.com, wens@...e.org,
	lee.jones@...aro.org
Cc:	linux-kernel@...r.kernel.org, linux-hwmon@...r.kernel.org,
	linux-iio@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	thomas.petazzoni@...e-electrons.com,
	antoine.tenart@...e-electrons.com
Subject: Re: [PATCH v2 4/4] hwmon: iio: add label for channels read by
 iio_hwmon

On 18/07/2016 14:24, Jonathan Cameron wrote:
> On 15/07/16 10:59, Quentin Schulz wrote:
>> Currently, iio_hwmon only exposes values of the IIO channels it can read
>> but no label by channel is exposed.
>>
>> This adds exposition of sysfs files containing label for IIO channels it
>> can read based on extended_name field of the iio_chan_spec of the channel.
>> If the extended_name field is empty, the sysfs file is not created by
>> iio_hwmon.
> Hmm. This is not the intent of extended name at all.  That exists to add
> a small amount of information to an constructed IIO channel name.
> Typically it's used to indicate physically wired stuff like:
> 
> in_voltage0_vdd_raw for cases where that channel of the ADC is hard wired
> to the vdd.  In this particular case the use might actually work as the
> vdd makes it clear it's a voltage - in general that's not the case.
> 
> Use of extended_name at all in IIO is only done after extensive review.
> It adds nasty custom ABI to a device, so the gain has to be considerable
> to use it.
> 
> When I read your original suggestion of adding labels, I was expecting the
> use of datasheet_name.  That has the advantage of being well defined by
> the datasheet (if not it should not be provided) + not being used in
> the construction of the IIO channel related attributes.  However, that
> may still not correspond well to the expected labelling in hwmon.

Good to know for extend_name use cases. While doing further testing, I
noticed the extend_name is also appended to the sysfs filename.. which
is definitely not what we want.

I've checked for datasheet_name and it is only used to be compared to
adc_channel_label from iio_map structure. Same for adc_channel_label
(which has to be the same as datasheet_name of the iio_chan_spec it is
linked to). So I could use this instead of extend_name to put a label on
a channel.
However, I thought of it to be more a way to identify the hardware in
the datasheet more than giving users a hint on what it is. That's what
"git grep adc_channel_label" told me. It's definitely better to use
datasheet_name over extend_name for channel labeling but I don't know if
it's really the good variable to use for labeling? In my understanding
of datasheet_name, in my case it would be more "temp_gpadc" than "SoC
temperature", that's what I mean.

> Thinking more on this, the label is going to often be a function of how
> the board is wired up...  Perhaps it should be a characteristic of the
> channel_map (hence from DT or similar) rather than part of the IIO driver
> itself?

Hmm.. I would not put a property in the DT only for labeling.

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