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

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Message-ID: <39d52aef-d775-7e5c-95a3-52172326f5e4@axentia.se>
Date:   Wed, 30 Aug 2023 11:11:20 +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] iio: afe: rescale: Accept only offset channels

Hi!

2023-08-30 at 10:52, 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.
> 
> The code to handle channels with just offset is already there.
> 
> 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>
> ---
>  drivers/iio/afe/iio-rescale.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
> index 1f280c360701..18aafb4bdda0 100644
> --- a/drivers/iio/afe/iio-rescale.c
> +++ b/drivers/iio/afe/iio-rescale.c
> @@ -280,8 +280,9 @@ 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");

I do not think it's that simple. The rescale_read_raw function, when asked
for IIO_CHAN_INFO_OFFSET, must know the scale of the source channel in
order to rescale the offset. After this patch, trying to get the rescaled
scale/offset when the source channel has an offset but lacks a scale will
simply return -EOPNOTSUPP.

Which is not what you intended, I think?

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,