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

["Nuno Sá" <noname.nuno@gmail.com>]

On Sat, 2026-10-17 at 21:28 +0100, Jonathan Cameron wrote:
> On Fri, 16 Oct 2026 15:07:02 +0100
> Nuno Sá <nuno.sa@analog.com> wrote:
> 
> > 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"
> 
> Not sure mandatory or not is relevant.  Two optional backends where the
> driver can run with either or both of them would hit the same problem of
> present property but particular backend not being there.
> 

True.

> > 
> > 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.
> 
> I'm in two minds over this. It's unused complexity, but it also makes the
> function behave how a user might expect.  Definitely needs some
> good justifying comments in the code though as it will look weird!
> 

Agreed! Yes, already asked for it as well special because of the non common -ENODATA
check.

- Nuno Sá

> Jonathan
> 
> 
> > 
> > - Nuno Sá
> > 
> > > 
> > > -- 
> > > With Best Regards,
> > > Andy Shevchenko
> > > 
> > >