linux-kernel - Re: [PATCH v3 2/6] iio: improve doc for available_scan

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Date:   Sun, 24 Sep 2023 16:59:08 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Matti Vaittinen <mazziesaccount@...il.com>
Cc:     Matti Vaittinen <matti.vaittinen@...rohmeurope.com>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Angel Iglesias <ang.iglesiasg@...il.com>,
        Andreas Klinger <ak@...klinger.de>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Benjamin Bara <bbara93@...il.com>, linux-iio@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/6] iio: improve doc for available_scan_mask

On Fri, 22 Sep 2023 14:16:57 +0300
Matti Vaittinen <mazziesaccount@...il.com> wrote:

> The available_scan_mask is an array of bitmaps representing the channels
> which can be simultaneously enabled by the driver. In many cases the
> hardware can offer more channels than what the user is interested in
> obtaining. In such cases it may be preferred that only subset of
> channels are enabled, and driver reads only a subset of the channels from
> the hardware.
> 
> Some devices can't support all channel combinations. For example the
> BM1390 pressure sensor must always read the pressure data in order to
> acknowledge the watermark IRQ, while reading temperature can be omitted.
> So, the available scan mask would be 'pressure and temperature' and
> 'pressure only'.
> 
> When IIO seatchs for the scan mask it asks the driver to use, it will

Spell check description.  searches

> pick the first suitable one from the 'available_scan_mask' array. Hence,
> ordering the masks in the array makes difference. We should 'prefer'
> reading just the pressure from the hardware (as it is cheaper operation
> than reading both pressure and temperature) over reading both pressure
> and temperature. Hence, we should set the 'only pressure' as first scan
> mask in available_scan_mask array. If we set the 'pressure and
> temperature' as first in array, then the 'only temperature' will never
> get used as 'pressure and temperature' can always serve the user's
> needs.
> 
> Add (minimal) kerneldoc to the 'available_scan_mask' to hint the user
> that ordering of masks matters.
> 
> Signed-off-by: Matti Vaittinen <mazziesaccount@...il.com>
> ---
>  include/linux/iio/iio.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
> index 202e55b0a28b..7bfa1b9bc8a2 100644
> --- a/include/linux/iio/iio.h
> +++ b/include/linux/iio/iio.h
> @@ -556,7 +556,9 @@ struct iio_buffer_setup_ops {
>   *			and owner
>   * @buffer:		[DRIVER] any buffer present
>   * @scan_bytes:		[INTERN] num bytes captured to be fed to buffer demux
> - * @available_scan_masks: [DRIVER] optional array of allowed bitmasks
> + * @available_scan_masks: [DRIVER] optional array of allowed bitmasks. Sort the
> + *			   array in order of preference, the most preferred
> + *			   masks first.

LGTM
>   * @masklength:		[INTERN] the length of the mask established from
>   *			channels
>   * @active_scan_mask:	[INTERN] union of all scan masks requested by buffers