Re: [PATCH v4 03/10] iio: adc: add helpers for parsing ADC nodes

David Lechner
2025-02-26T00:26:45Z

On 2/24/25 12:33 PM, Matti Vaittinen wrote:
> There are ADC ICs which may have some of the AIN pins usable for other
> functions. These ICs may have some of the AIN pins wired so that they
> should not be used for ADC.
>
> (Preferred?) way for marking pins which can be used as ADC inputs is to
> add corresponding channels@N nodes in the device tree as described in
> the ADC binding yaml.

I think "preferred?" is the key question here. Currently, it is assumed
that basically all IIO bindings have channels implicitly even if the
binding doesn't call them out. It just means that there is nothing
special about the channel that needs to be documented, but the channel
is still there.

Similarly, on several drivers we added recently that make use of adc.yaml
(adi,ad7380, adi,ad4695) we wrote the bindings with the intention that
if a channel was wired in the default configuration, then you would just
omit the channel node for that input pin. Therefore, this helper couldn't
be used by these drivers since we always have a fixed number of channels
used in the driver regardless of if there are explicit channel nodes in
the devicetree or not.

In my experience, the only time we don't populate all available channels
on an ADC, even if not used, is in cases like differential chips where
any two inputs can be mixed and matched to form a channel. Some of these,
like adi,ad7173-8 would have 100s or 1000s of channels if we tried to
include all possible channels. In those cases, we make an exception and
use a dynamic number of channels based on the devicetree. But for chips
that have less than 20 total possible channels or so we've always
provided all possible channels to userspace. It makes writing userspace
software for a specific chip easier if we can always assume that chip
has the same number of channels.

>
> Add couple of helper functions which can be used to retrieve the channel
> information from the device node.
>
> Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
>