linux-kernel - Re: [PATCH 4/4] iio: afe/rescale: Implement write

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Message-ID: <b8539e31-b20a-6eb5-4b86-59a17d7c6939@axentia.se>
Date:   Fri, 22 Jul 2022 00:16:36 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Paul Cercueil <paul@...pouillou.net>,
        Jonathan Cameron <jic23@...nel.org>
Cc:     Lars-Peter Clausen <lars@...afoo.de>, list@...ndingux.net,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/4] iio: afe/rescale: Implement write_raw

Hi!

2022-07-21 at 21:15, Paul Cercueil wrote:
> Implement write_raw by converting the value if writing the scale, or
> just calling the managed channel driver's write_raw otherwise.
> 
> Signed-off-by: Paul Cercueil <paul@...pouillou.net>
> ---
>  drivers/iio/afe/iio-rescale.c | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> index 5c9970b93384..0edb62ee4508 100644
> --- a/drivers/iio/afe/iio-rescale.c
> +++ b/drivers/iio/afe/iio-rescale.c
> @@ -141,6 +141,27 @@ int rescale_process_offset(struct rescale *rescale, int scale_type,
>  	}
>  }
>  
> +static int rescale_write_raw(struct iio_dev *indio_dev,
> +			     struct iio_chan_spec const *chan,
> +			     int val, int val2, long mask)
> +{
> +	struct rescale *rescale = iio_priv(indio_dev);
> +	unsigned long long tmp;
> +
> +	switch (mask) {
> +	case IIO_CHAN_INFO_SCALE:
> +		tmp = val * 1000000000LL;
> +		do_div(tmp, rescale->numerator);
> +		tmp *= rescale->denominator;
> +		do_div(tmp, 1000000000LL);

do_div is for unsigned operands. Can val never ever be negative?
What about the numerator and denominator, can those be negative? I
think this code should live in a new rescale_process_inverse_scale
function, or something like that (and a few tests could be added to
drivers/iio/test/iio-test-rescale.c)

> +		return iio_write_channel_attribute(rescale->source, tmp, 0,
> +						   IIO_CHAN_INFO_SCALE);
> +	default:

What if the source driver has a .write_raw_get_fmt callback? That bit
of info is silently dropped (with no comment that a shortcut has been
taken). How does inverse rescaling mix with a .write_raw_get_fmt that
returns e.g. IIO_VAL_INT_PLUS_MICRO_DB anyway? I think all cases might
get a bit hairy to support, so I think you need to do some filtering
and somehow fail the .write_raw call if the .write_raw_get_fmt of the
source returns something that gets too difficult to support.

Cheers,
Peter

> +		return iio_write_channel_attribute(rescale->source,
> +						   val, val2, mask);
> +	}
> +}
> +
>  static int rescale_read_raw(struct iio_dev *indio_dev,
>  			    struct iio_chan_spec const *chan,
>  			    int *val, int *val2, long mask)
> @@ -250,6 +271,7 @@ static int rescale_read_avail(struct iio_dev *indio_dev,
>  }
>  
>  static const struct iio_info rescale_info = {
> +	.write_raw = rescale_write_raw,
>  	.read_raw = rescale_read_raw,
>  	.read_avail = rescale_read_avail,
>  };