Re: [PATCH] iio: industrialio-core: look for aliases to request device index

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Wed, 28 Feb 2024 17:11:59 +0900
Dominique Martinet <dominique.martinet@atmark-techno.com> wrote:

> Krzysztof Kozlowski wrote on Wed, Feb 28, 2024 at 08:42:46AM +0100:
> > >> Sorry, that's why you have labels and compatibles.  
> >  
> > > Setting up a fixed alias seems to be precisely what aliases are about
> > > (e.g. setting rtc0 will make a specific node become /dev/rtc0, same with
> > > ethernet0, gpio, i2c, mmc, serial...), I'm not sure I agree a new label
> > > would be more appropriate here, but perhaps I'm missing some context?  
> > 
> > Maybe I don't get your point, but your email said "sysfs", so why do you
> > refer to /dev?  
> 
> I wrote /dev/rtc0, but it also sets the name in /sys, right?
> For example /sys/class/rtc/rtc0
> 
> As far as I'm aware iio also creates character devices in /dev with the
> same name (/dev/iio/iio:deviceX), but our application doesn't use these
> at all and has to? look in /sys directly, so normal udev SYMLINK+=
> unfortunately isn't applicable or I wouldn't be bothering with all
> this..

A given IIO device driver may create multiple sysfs directories (registers
device + one or more triggers), so I'm not sure how this would work.

> 
> > > I'm not sure I understand this comment -- would you rather this doesn't
> > > use aliases but instead add a new label (e.g. `iio,index = <10>` or
> > > whatever) to the iio node itself?  
> > 
> > No, the devices already have label property.  
> 
> Thank you for pointing me at the 'label' property, looking at other
> subsystems e.g. leds I see paths in sysfs that use labels as I'd like it
> to work for iio (/sys/class/leds/<label> and
> /sys/devices/platform/<parent>/leds/<label>)
> 
> Unfortunately for iio it looks like labels isn't ignored, but instead
> create a file in the sysfs directory of the device, e.g. I now have
> /sys/bus/iio/devices/iio:device1/label which contains the label string,
> so I'm not sure that can be changed easily as that'd be a change of API
> for existing users for labels in iio devices?

Yes, don't touch that ABI.  IIO software assumes naming of
iio\:deviceX etc.

> 
> (I checked briefly and didn't find any, but there seems to be an awful
> lot of code in the iio drivers tree about labels so I'm not really
> comfortable changing that without some more background on iio first...
> Jonathan perhaps has an opinion on this?)

There are labels for channels as well as devices, but the short description
you have above is it.

I don't see why that isn't sufficient for your use case though?
What does a directory name matter when you can write a few lines of
code to retrieve the IIO device that you want.

If this was day 0 maybe we could support renaming devices like this
but we have a long history of those names not being stable and label
+ parentage of the IIO devices being used to establish stable identification.

Anything beyond a trivial single use script is going to need to deal with
not having stable names (old kernel, dt without alias etc) so this doesn't
help in general.

Jonathan

> 
> 
> Thanks,