Re: [PATCH v7 05/10] iio: adc: sun20i-gpadc: Use adc-helpers

[Jonathan Cameron <jic23@kernel.org>]

On Thu, 13 Mar 2025 14:34:24 +0200
Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote:

> On Thu, Mar 13, 2025 at 09:18:49AM +0200, Matti Vaittinen wrote:
> > The new devm_iio_adc_device_alloc_chaninfo_se() -helper is intended to
> > help drivers avoid open-coding the for_each_node -loop for getting the
> > channel IDs. The helper provides standard way to detect the ADC channel
> > nodes (by the node name), and a standard way to convert the "reg"
> > -properties to channel identification numbers, used in the struct
> > iio_chan_spec. Furthermore, the helper can optionally check the found
> > channel IDs are smaller than given maximum. This is useful for callers
> > which later use the IDs for example for indexing a channel data array.
> > 
> > The original driver treated all found child nodes as channel nodes. The
> > new helper requires channel nodes to be named channel[@N]. This should
> > help avoid problems with devices which may contain also other but ADC
> > child nodes. Quick grep from arch/* with the sun20i-gpadc's compatible
> > string didn't reveal any in-tree .dts with channel nodes named
> > otherwise. Also, same grep shows all the in-tree .dts seem to have
> > channel IDs between 0..num of channels.
> > 
> > Use the new helper.  
> 
> ...
> 
> > +	num_channels = devm_iio_adc_device_alloc_chaninfo_se(dev,
> > +				&sun20i_gpadc_chan_template, -1, &channels);
> > +	if (num_channels < 0)
> > +		return num_channels;
> > +
> >  	if (num_channels == 0)
> >  		return dev_err_probe(dev, -ENODEV, "no channel children\n");  
> 
> Note, this what I would expected in your helper to see, i.e. separated cases
> for < 0 (error code) and == 0, no channels.
> 
> Also, are all users going to have this check? Usually in other similar APIs
> we return -ENOENT. And user won't need to have an additional check in case of
> 0 being considered as an error case too.
In a few cases we'll need to do the dance the other way in the caller.
So specifically check for -ENOENT and not treat it as an error.

That stems from channel nodes being optionally added to drivers after
they have been around a while (usually to add more specific configuration)
and needing to maintain old behaviour of presenting all channels with default
settings.

I agree that returning -ENOENT is a reasonable way to handle this.

Jonathan

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