linux-kernel - Re: [PATCH v2 2/4] iio: backend: add devm_iio_backend_get_by

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Message-ID: <c25fced275d861f89f6885e83b9f9d3abfdc7810.camel@gmail.com>
Date: Mon, 09 Feb 2026 17:48:12 +0000
From: Nuno Sá <noname.nuno@...il.com>
To: David Lechner <dlechner@...libre.com>, Antoniu Miclaus	
 <antoniu.miclaus@...log.com>, Lars-Peter Clausen <lars@...afoo.de>, Michael
 Hennerich <Michael.Hennerich@...log.com>, Jonathan Cameron
 <jic23@...nel.org>, Nuno Sá	 <nuno.sa@...log.com>, Andy
 Shevchenko <andy@...nel.org>, Rob Herring	 <robh@...nel.org>, Krzysztof
 Kozlowski <krzk+dt@...nel.org>, Conor Dooley	 <conor+dt@...nel.org>,
 Olivier Moysan <olivier.moysan@...s.st.com>, Mark Brown	
 <broonie@...nel.org>, linux-iio@...r.kernel.org,
 devicetree@...r.kernel.org, 	linux-kernel@...r.kernel.org,
 linux-spi@...r.kernel.org
Subject: Re: [PATCH v2 2/4] iio: backend: add devm_iio_backend_get_by_index()

On Mon, 2026-02-09 at 16:47 +0000, Nuno Sá wrote:
> On Mon, 2026-02-09 at 09:28 -0600, David Lechner wrote:
> > On 2/8/26 3:24 AM, Nuno Sá wrote:
> > > On Fri, 2026-02-06 at 18:07 +0200, Antoniu Miclaus wrote:
> > > > Add a new function to get an IIO backend by its index in the
> > > > io-backends device tree property. This is useful for multi-channel
> > > > devices that have multiple backends, where looking up by index is
> > > > more straightforward than using named backends.
> > > > 
> > > > The new function directly uses the index to find the backend reference
> > > > in the io-backends property, avoiding the need for io-backend-names.
> > > > 
> > > > Signed-off-by: Antoniu Miclaus <antoniu.miclaus@...log.com>
> > > > ---
> > > >  drivers/iio/industrialio-backend.c | 51 ++++++++++++++++++++++++++++++
> > > >  include/linux/iio/backend.h        |  2 ++
> > > >  2 files changed, 53 insertions(+)
> > > > 
> > > > diff --git a/drivers/iio/industrialio-backend.c b/drivers/iio/industrialio-
> > > > backend.c
> > > > index 447b694d6d5f..3b692d48481e 100644
> > > > --- a/drivers/iio/industrialio-backend.c
> > > > +++ b/drivers/iio/industrialio-backend.c
> > > > @@ -1008,6 +1008,57 @@ struct iio_backend *devm_iio_backend_get(struct device *dev,
> > > > const char *name)
> > > >  }
> > > >  EXPORT_SYMBOL_NS_GPL(devm_iio_backend_get, "IIO_BACKEND");
> > > >  
> > > > +static struct iio_backend *
> > > > +__devm_iio_backend_fwnode_get_by_index(struct device *dev,
> > > > +				       struct fwnode_handle *fwnode,
> > > > +				       unsigned int index)
> > > > +{
> > > > +	struct fwnode_handle *fwnode_back;
> > > > +	struct iio_backend *back;
> > > > +	int ret;
> > > > +
> > > > +	fwnode_back = fwnode_find_reference(fwnode, "", index);
> > > > +	if (IS_ERR(fwnode_back))
> > > > +		return dev_err_cast_probe(dev, fwnode_back,
> > > > +					  "Cannot get Firmware reference\n");
> > > > +
> > > > +	guard(mutex)(&iio_back_lock);
> > > > +	list_for_each_entry(back, &iio_back_list, entry) {
> > > > +		if (!device_match_fwnode(back->dev, fwnode_back))
> > > > +			continue;
> > > > +
> > > > +		fwnode_handle_put(fwnode_back);
> > > > +		ret = __devm_iio_backend_get(dev, back);
> > > > +		if (ret)
> > > > +			return ERR_PTR(ret);
> > > > +
> > > > +		back->idx = index;
> > > > +
> > > > +		return back;
> > > > +	}
> > > > +
> > > > +	fwnode_handle_put(fwnode_back);
> > > > +	return ERR_PTR(-EPROBE_DEFER);
> > > > +}
> > > 
> > > I believe we don't necessarily need this. Why can't we use io-backend-names? I get
> > > that in here we just want something matching the number of channels we have so giving
> > > names is probably does not add much added value. But still, I would prefer t have
> > > more simplicity in the API and it should be fairly easy for the frontend to use the
> > > names argument.
> > > 
> > > _ Nuno Sá
> > > 
> > 
> > IMHO, using names in this case would just be annoying because we would have to
> > sprintf the string to add the index to the string. And also have to spend time
> > coming up with more complex DT bindings. Using the index seems much simpler.
> > 
> > If you really feel strongly about it though, maybe we could make a
> > devm_iio_backend_fwnode_get_fmt() function instead that handles the
> > sprintf() part so that we only have to write that once?
> > 
> 
> uHu? Maybe I'm completely missing your point but what I had in mind was just something like: 
> 
> 
> // from the frontend:
> 
> static const char * const names[] = { "adc1", "adc2" }
> 
> for (c = 0; c < ARRAY_SIZE(names); c++) {
> 	back = devm_iio_backend_get(dev, names[c]);
> }
> 
> So yes, I agree we would have a bit more complex bindings and more complexity in the
> frontend. But on the bright side, no need to change backend code at all. And the
> -names property is already used like the above fairly often If I'm not mistaken.
> 
> But again, I can agree that for this usecase getting things by index makes sense. Given
> that we just want n backends for n channels, the name does not add much.
> 

Given Krzysztof's feedback, let's not do the above. So, Antoniu, disregard my comments about using
io-backend-name.

- Nuno Sá