Re: [PATCH 02/10] iio: backend: add devm_iio_backend_get_optional()

["Nuno Sá" <nuno.sa@analog.com>]

On Fri, Oct 16, 2026 at 04:19:33PM +0300, Andy Shevchenko wrote:
> On Fri, Oct 16, 2026 at 08:18:08AM -0300, Rodrigo Alencar wrote:
> > On 16/10/26 09:45, Nuno Sá wrote:
> > > On Thu, Oct 15, 2026 at 09:41:40PM +0300, Andy Shevchenko wrote:
> > > > On Thu, Oct 15, 2026 at 01:49:42PM -0300, Rodrigo Alencar via B4 Relay wrote:
> > > > 
> > > > > Frontend devices might not need a backend driver to rely on. An IIO device
> > > > > that supports backends can behave differently when they are not available
> > > > > and that should be acceptable. Such parts may provide basic functionality
> > > > > solely using the control interface (SPI for instance), being able to have
> > > > > extended features when a backend is present.
> > > > > 
> > > > > Add devm_iio_backend_get_optional(), which returns NULL when the backend
> > > > > is not described in firmware, i.e., when the io-backends property is not
> > > > > present or, for a named lookup, when the name is not listed in the
> > > > > io-backend-names property. Other errors, such as probe deferral, are
> > > > > still returned.
> > > > > 
> > > > > To make that possible, a named lookup that does not match any entry of
> > > > > io-backend-names now fails with -ENOENT, the same error returned when the
> > > > > io-backends reference is missing. Also, the missing reference is no longer
> > > > > logged as an error, leaving that to the caller.
> 
> ...
> 
> > > > >  		ret = device_property_match_string(dev, "io-backend-names", name);
> > > > > +		if (ret == -EINVAL || ret == -ENODATA)
> > > > > +			return ERR_PTR(-ENOENT);
> > > > 
> > > > No. Please, use device_property_present() instead.
> > > 
> > > Oh right! Totally forgot about that internally :). And it fits into my
> > > internal point where I don't agree much in treating -ENODATA the same
> > > way as property does not exist.
> 
> +1
> 
> > Say a frontend consumes multiple backends, but some of them are optional.
> > If we care only about the presence of the property we would fail when
> > trying to acquire the optional ones when the required ones are defined.
> > am I thinking correctly?
> 
> If there is an empty property for names, shouldn't we simply fail? I rather
> would consider the firmware table to be broken in such a case.

What Rodrigo is saying is that an IIO frontend driver can have multiple
backends. So in practice:

io-backend-names = "foo", "bar";

But we might want to have foo as optional while bar is mandatory so that

io-backend-names = "bar"

So if we treat all errors (after checking for presence) the same we will fail
in the optional one. Note -ENODATA also means match_string() failed.

As of now there's no such usecase in tree or oeven out of tree (that I'm
aware of) but to support it is also fairly simple that I don't mind to
have it now.

- Nuno Sá

> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
>