iio: afe: rescale: Accept only offset channels

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: 53ebee9 ("iio: afe: iio-rescale: Support processed channels")
Fixes: 9decacd ("iio: afe: rescale: Fix boolean logic bug")
Reported-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Peter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/20230902-iio-rescale-only-offset-v2-1-988b807754c8@linaro.org
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: linusw

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");
 	} else if (iio_channel_has_info(schan, IIO_CHAN_INFO_PROCESSED)) {
 		dev_info(dev, "using processed channel\n");
 		rescale->chan_processed = true;