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

["Nuno Sá" <nuno.sa@analog.com>]

On Thu, Oct 15, 2026 at 01:49:42PM -0300, Rodrigo Alencar via B4 Relay wrote:
> From: Rodrigo Alencar <rodrigo.alencar@analog.com>
> 
> 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.
> 
> Signed-off-by: Rodrigo Alencar <rodrigo.alencar@analog.com>
> ---

With device_property_present() (not strong feelings about the other
comment from Andy):

Reviewed-by: Nuno Sá <nuno.sa@analog.com>

>  drivers/iio/industrialio-backend.c | 34 +++++++++++++++++++++++++++++++++-
>  include/linux/iio/backend.h        |  2 ++
>  2 files changed, 35 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/industrialio-backend.c b/drivers/iio/industrialio-backend.c
> index fbf5c39d7655..7af41ba50a65 100644
> --- a/drivers/iio/industrialio-backend.c
> +++ b/drivers/iio/industrialio-backend.c
> @@ -1006,9 +1006,14 @@ static struct iio_backend *__devm_iio_backend_fwnode_get_by_index(struct device
>  
>  	struct fwnode_handle *fwnode_back __free(fwnode_handle) =
>  		fwnode_find_reference(fwnode, "io-backends", index);
> -	if (IS_ERR(fwnode_back))
> +	if (IS_ERR(fwnode_back)) {
> +		/* not described in firmware, let the caller decide what to do */
> +		if (PTR_ERR(fwnode_back) == -ENOENT)
> +			return ERR_CAST(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) {
> @@ -1035,6 +1040,8 @@ static struct iio_backend *__devm_iio_backend_fwnode_get(struct device *dev, con
>  
>  	if (name) {
>  		ret = device_property_match_string(dev, "io-backend-names", name);
> +		if (ret == -EINVAL || ret == -ENODATA)
> +			return ERR_PTR(-ENOENT);
>  		if (ret < 0)
>  			return ERR_PTR(ret);
>  		index = ret;
> @@ -1061,6 +1068,31 @@ struct iio_backend *devm_iio_backend_get(struct device *dev, const char *name)
>  }
>  EXPORT_SYMBOL_NS_GPL(devm_iio_backend_get, "IIO_BACKEND");
>  
> +/**
> + * devm_iio_backend_get_optional - Optional device managed backend device get
> + * @dev: Consumer device for the backend
> + * @name: Backend name
> + *
> + * Gets the backend associated with @dev. A backend not described in firmware,
> + * i.e., the io-backends property is not present or @name is not listed in
> + * the io-backend-names property, is not treated as an error.
> + *
> + * RETURNS:
> + * A backend pointer, NULL if the backend is not described in firmware,
> + * negative error pointer otherwise.
> + */
> +struct iio_backend *devm_iio_backend_get_optional(struct device *dev,
> +						  const char *name)
> +{
> +	struct iio_backend *back = devm_iio_backend_get(dev, name);
> +
> +	if (IS_ERR(back) && PTR_ERR(back) == -ENOENT)
> +		return NULL;
> +
> +	return back;
> +}
> +EXPORT_SYMBOL_NS_GPL(devm_iio_backend_get_optional, "IIO_BACKEND");
> +
>  /**
>   * devm_iio_backend_get_by_index - Device managed backend device get by index
>   * @dev: Consumer device for the backend
> diff --git a/include/linux/iio/backend.h b/include/linux/iio/backend.h
> index 9c16b695a602..93d5f2073cc3 100644
> --- a/include/linux/iio/backend.h
> +++ b/include/linux/iio/backend.h
> @@ -267,6 +267,8 @@ int iio_backend_extend_chan_spec(struct iio_backend *back,
>  bool iio_backend_has_caps(struct iio_backend *back, u32 caps);
>  void *iio_backend_get_priv(const struct iio_backend *conv);
>  struct iio_backend *devm_iio_backend_get(struct device *dev, const char *name);
> +struct iio_backend *devm_iio_backend_get_optional(struct device *dev,
> +						  const char *name);
>  struct iio_backend *devm_iio_backend_get_by_index(struct device *dev, unsigned int index);
>  struct iio_backend *devm_iio_backend_fwnode_get(struct device *dev,
>  						const char *name,
> 
> -- 
> 2.43.0
> 
>