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

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Message-ID: <Z9frCUqRTD0i1Faq@smile.fi.intel.com>
Date: Mon, 17 Mar 2025 11:27:37 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Matti Vaittinen <mazziesaccount@...il.com>
Cc: Jonathan Cameron <jic23@...nel.org>,
	Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
	Lars-Peter Clausen <lars@...afoo.de>, Chen-Yu Tsai <wens@...e.org>,
	Jernej Skrabec <jernej.skrabec@...il.com>,
	Samuel Holland <samuel@...lland.org>, Nuno Sa <nuno.sa@...log.com>,
	David Lechner <dlechner@...libre.com>,
	Javier Carrasco <javier.carrasco.cruz@...il.com>,
	Olivier Moysan <olivier.moysan@...s.st.com>,
	Guillaume Stols <gstols@...libre.com>,
	Dumitru Ceclan <mitrutzceclan@...il.com>,
	Trevor Gamblin <tgamblin@...libre.com>,
	Matteo Martelli <matteomartelli3@...il.com>,
	Alisa-Dariana Roman <alisadariana@...il.com>,
	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev
Subject: Re: [PATCH v7 05/10] iio: adc: sun20i-gpadc: Use adc-helpers

On Mon, Mar 17, 2025 at 10:42:07AM +0200, Matti Vaittinen wrote:
> On 17/03/2025 09:51, Andy Shevchenko wrote:
> > On Mon, Mar 17, 2025 at 09:11:08AM +0200, Matti Vaittinen wrote:
> > > On 16/03/2025 11:41, Jonathan Cameron wrote:
> > > > On Thu, 13 Mar 2025 14:34:24 +0200
> > > > Andy Shevchenko <andriy.shevchenko@...ux.intel.com> wrote:
> > > > > On Thu, Mar 13, 2025 at 09:18:49AM +0200, Matti Vaittinen wrote:

...

> > > > > > +	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.
> > > 
> > > I agree - but I'm going to use -ENODEV instead of -ENOENT because that's
> > > what the current callers return if they find no channels. That way the
> > > drivers can return the value directly without converting -ENOENT to -ENODEV.
> > 
> > ENODEV can be easily clashed with other irrelevant cases,
> 
> Can you please explain what cases?

When it's a code path that returns the same error code for something different.

> > ENOENT is the correct
> > error code.
> 
> I kind of agree if we look this from the fwnode perspective. But, when we
> look this from the intended user's perspective, I can very well understand
> the -ENODEV. Returning -ENODEV from ADC driver's probe which can't find any
> of the channels feels correct to me.

Okay, it seems we have got yet another disagreement as I think this has to
be ENOENT. Because this is related to the firmware description and not real
hardware discovery path. If it is the latter, I may fully agree on ENODEV
choice. But AFAICS it's not the case here.

> > If drivers return this instead of another error code, nothing bad
> > happen, it's not an ABI path, correct?
> 
> I don't know if failure returned from a probe is an ABI. I still feel
> -ENODEV is correct value,

And I have the opposite opinion. I think ENODEV is _incorrect_ choice
in this case.

> and I don't want to change it for existing users -
> and I think also new ADC drivers should use -ENODEV if they find no channels
> at all.

Again, the problem is that you are trying to apply the error code for HW to the
information that comes from FW.

> Besides that I think -ENODEV to be right, changing it to -ENOENT for
> existing callers requires a buy-in from Jonathan (and/or) the driver
> maintainers.

Yeah, will wait for Jonathan to judge, but I think you can find rationale above.

-- 
With Best Regards,
Andy Shevchenko