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Date:   Sat, 30 Sep 2023 17:54:04 +0100
From:   Jonathan Cameron <jic23@kernel.org>
To:     Matti Vaittinen <mazziesaccount@gmail.com>
Cc:     Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Conor Dooley <conor+dt@kernel.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Angel Iglesias <ang.iglesiasg@gmail.com>,
        Andreas Klinger <ak@it-klinger.de>,
        Benjamin Bara <bbara93@gmail.com>,
        Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
        linux-iio@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4 2/5] iio: improve doc for available_scan_mask
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On Wed, 27 Sep 2023 11:27:41 +0300
Matti Vaittinen <mazziesaccount@gmail.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 a subset of
> channels are enabled, and the 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 masks would be 'pressure and temperature' and
> 'pressure only'.
> 
> When IIO searches for the scan mask it asks the driver to use, it will
> pick the first suitable one from the 'available_scan_mask' array. Hence,
> ordering the masks in the array makes a difference. We should 'prefer'
> reading just the pressure from the hardware (as it is a cheaper operation
> than reading both pressure and temperature) over reading both pressure
> and temperature. Hence, we should set the 'only pressure' as the first
> scan mask in available_scan_mask array. If we set the 'pressure and
> temperature' as first in the 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 the ordering of masks matters.
> 
> Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>

Applied this patch to the togreg branch of iio.git
If you fancy turning the patch description into some more formal
documentation under Documentation/... that might be a good thing in the
long run. I always mean to spend some time working on that, but too manyt
things to do :(

Thanks,

Jonathan

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