linux-kernel - Re: [PATCH v5 1/4] iio: industrialio-backend: support backend capabilities

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Message-ID: <688cbfb1-0944-492a-929d-91ebb9ab22fb@baylibre.com>
Date: Sat, 31 Jan 2026 14:30:41 -0600
From: David Lechner <dlechner@...libre.com>
To: Tomas Melin <tomas.melin@...sala.com>,
 Michael Hennerich <Michael.Hennerich@...log.com>,
 Nuno Sa <nuno.sa@...log.com>, Lars-Peter Clausen <lars@...afoo.de>,
 Jonathan Cameron <jic23@...nel.org>, Andy Shevchenko <andy@...nel.org>,
 Olivier Moysan <olivier.moysan@...s.st.com>
Cc: linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/4] iio: industrialio-backend: support backend
 capabilities

On 1/30/26 3:16 AM, Tomas Melin wrote:
> Not all backends support the full set of capabilities provided by the
> industrialio-backend framework. Capability bits can be used in frontends
> and backends for checking for a certain feature set, or if using
> related functions can be expected to fail.
> 
> Capability bits should be set by a compatible backend and provided when
> registering the backend.
> 

...

> +/**
> + * enum iio_backend_capabilities - Backend capabilities
> + * Backend capabilities can be used by frontends to check if a given
> + * functionality is supported by the backend. This is useful for frontend
> + * devices which are expected to work with alternative backend
> + * implementations. Capabilities are loosely coupled with operations,
> + * meaning that a capability requires certain operations to be implemented
> + * by the backend. A capability might be mapped to a single operation or
> + * multiple operations.

It would be helpful to list these operations explicitly for each
enum member.

> + *
> + * @IIO_BACKEND_CAP_CALIBRATION: Backend supports digital interface
> + * calibration. Calibration procedure is device specific.
> + * @IIO_BACKEND_CAP_BUFFERING: Backend supports buffering.

I assume this means it supports devm_iio_backend_request_buffer()?

In IIO, we usually say "buffer" and rarely "buffering" so this name and
description is a bit confusing to me.

> + * @IIO_BACKEND_CAP_ALWAYS_ON: Backend does not need to be explicitly
> + * enabled/disabled. It is always on.

Do we actually need this one? Alternative could be, for example:

int iio_backend_enable(struct iio_backend *back)
{
	int ret;

	ret = iio_backend_op_call(back, enable);
	
	return ret == -EOPNOTSUPP ? 0 : ret;
}

Or make a iio_backend_optional_op_call() macro.

Or if there is some other nuance where this capability makes a
difference, it should be explained in more detail.


> + */
> +enum iio_backend_capabilities {

Documenting members like this works too (saves a bit of cross-referencing).

	/** @IIO_BACKEND_CAP_CALIBRATION: ... */
> +	IIO_BACKEND_CAP_CALIBRATION = BIT(0),
> +	IIO_BACKEND_CAP_BUFFERING = BIT(1),
> +	IIO_BACKEND_CAP_ALWAYS_ON = BIT(2),
> +};