linux-kernel - Re: [PATCH v2] iio: afe: rescale: Accept only offset channels

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Message-ID: <a0a55276-297b-e2b7-79b3-e4aadd39b400@axentia.se>
Date:   Sun, 15 Oct 2023 00:38:50 +0200
From:   Peter Rosin <peda@...ntia.se>
To:     Linus Walleij <linus.walleij@...aro.org>,
        Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Liam Beguin <liambeguin@...il.com>
Cc:     Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] iio: afe: rescale: Accept only offset channels

Hi!

Sorry for the delay, and thank you Jonathan for the reminder.

2023-09-02 at 21:46, Linus Walleij wrote:
> As noted by Jonathan Cameron: it is perfectly legal for a channel
> to have an offset but no scale in addition to the raw interface.
> The conversion will imply that scale is 1:1.
> 
> Make rescale_configure_channel() accept just scale, or just offset
> to process a channel.
> 
> When a user asks for IIO_CHAN_INFO_OFFSET in rescale_read_raw()
> we now have to deal with the fact that OFFSET could be present
> but SCALE missing. Add code to simply scale 1:1 in this case.
> 
> Link: https://lore.kernel.org/linux-iio/CACRpkdZXBjHU4t-GVOCFxRO-AHGxKnxMeHD2s4Y4PuC29gBq6g@mail.gmail.com/
> Fixes: 53ebee949980 ("iio: afe: iio-rescale: Support processed channels")
> Fixes: 9decacd8b3a4 ("iio: afe: rescale: Fix boolean logic bug")
> Reported-by: Jonathan Cameron <jic23@...nel.org>
> Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
> ---
> Changes in v2:
> - Fix rescale_read_raw() handle channels with offset but no scale.
> - Link to v1: https://lore.kernel.org/r/20230830-iio-rescale-only-offset-v1-1-40ab9f4436c7@linaro.org
> ---
>  drivers/iio/afe/iio-rescale.c | 19 +++++++++++++++----
>  1 file changed, 15 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> index 1f280c360701..56e5913ab82d 100644
> --- a/drivers/iio/afe/iio-rescale.c
> +++ b/drivers/iio/afe/iio-rescale.c
> @@ -214,8 +214,18 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
>  				return ret < 0 ? ret : -EOPNOTSUPP;
>  		}
>  
> -		ret = iio_read_channel_scale(rescale->source, &scale, &scale2);
> -		return rescale_process_offset(rescale, ret, scale, scale2,
> +		if (iio_channel_has_info(rescale->source->channel,
> +					 IIO_CHAN_INFO_SCALE)) {
> +			ret = iio_read_channel_scale(rescale->source, &scale, &scale2);
> +			return rescale_process_offset(rescale, ret, scale, scale2,
> +						      schan_off, val, val2);
> +		}
> +
> +		/*
> +		 * If we get here we have no scale so scale 1:1 but apply
> +		 * rescaler and offset, if any.
> +		 */
> +		return rescale_process_offset(rescale, IIO_VAL_FRACTIONAL, 1, 1,
>  					      schan_off, val, val2);
>  	default:
>  		return -EINVAL;
> @@ -280,8 +290,9 @@ static int rescale_configure_channel(struct device *dev,
>  	chan->type = rescale->cfg->type;
>  
>  	if (iio_channel_has_info(schan, IIO_CHAN_INFO_RAW) &&
> -	    iio_channel_has_info(schan, IIO_CHAN_INFO_SCALE)) {
> -		dev_info(dev, "using raw+scale source channel\n");
> +	    (iio_channel_has_info(schan, IIO_CHAN_INFO_SCALE) ||
> +	     iio_channel_has_info(schan, IIO_CHAN_INFO_OFFSET))) {
> +		dev_info(dev, "using raw+scale/offset source channel\n");

If the rules really are that when not provided scale is 1 and offset 0
(reasonable of course) then the above still looks suspect to me. Should
this part not simply be

  	if (iio_channel_has_info(schan, IIO_CHAN_INFO_RAW)) {
		dev_info(dev, "using raw source channel\n");

in that case?

Or was "raw + processed" some kind of special case that we want to handle
as processed? If that's the case then we need to have more complex logic.

Cheers,
Peter

>  	} else if (iio_channel_has_info(schan, IIO_CHAN_INFO_PROCESSED)) {
>  		dev_info(dev, "using processed channel\n");
>  		rescale->chan_processed = true;
> 
> ---
> base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
> change-id: 20230830-iio-rescale-only-offset-f28e05bd2deb
> 
> Best regards,