iio: afe: rescale: add offset support

This is a preparatory change required for the addition of temperature
sensing front ends.
Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Reviewed-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220213025739.2561834-4-liambeguin@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/afe/iio-rescale.c

@@ -3,6 +3,7 @@
  * IIO rescale driver
  *
  * Copyright (C) 2018 Axentia Technologies AB
+ * Copyright (C) 2022 Liam Beguin <liambeguin@gmail.com>
  *
  * Author: Peter Rosin <peda@axentia.se>
  */
@@ -81,11 +82,46 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
 	}
 }
 
+int rescale_process_offset(struct rescale *rescale, int scale_type,
+			   int scale, int scale2, int schan_off,
+			   int *val, int *val2)
+{
+	s64 tmp, tmp2;
+
+	switch (scale_type) {
+	case IIO_VAL_FRACTIONAL:
+		tmp = (s64)rescale->offset * scale2;
+		*val = div_s64(tmp, scale) + schan_off;
+		return IIO_VAL_INT;
+	case IIO_VAL_INT:
+		*val = div_s64(rescale->offset, scale) + schan_off;
+		return IIO_VAL_INT;
+	case IIO_VAL_FRACTIONAL_LOG2:
+		tmp = (s64)rescale->offset * (1 << scale2);
+		*val = div_s64(tmp, scale) + schan_off;
+		return IIO_VAL_INT;
+	case IIO_VAL_INT_PLUS_NANO:
+		tmp = (s64)rescale->offset * 1000000000LL;
+		tmp2 = ((s64)scale * 1000000000LL) + scale2;
+		*val = div64_s64(tmp, tmp2) + schan_off;
+		return IIO_VAL_INT;
+	case IIO_VAL_INT_PLUS_MICRO:
+		tmp = (s64)rescale->offset * 1000000LL;
+		tmp2 = ((s64)scale * 1000000LL) + scale2;
+		*val = div64_s64(tmp, tmp2) + schan_off;
+		return IIO_VAL_INT;
+	default:
+		return -EOPNOTSUPP;
+	}
+}
+
 static int rescale_read_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);
+	int scale, scale2;
+	int schan_off = 0;
 	int ret;
 
 	switch (mask) {
@@ -112,6 +148,47 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
 			ret = iio_read_channel_scale(rescale->source, val, val2);
 		}
 		return rescale_process_scale(rescale, ret, val, val2);
+	case IIO_CHAN_INFO_OFFSET:
+		/*
+		 * Processed channels are scaled 1-to-1 and source offset is
+		 * already taken into account.
+		 *
+		 * In other cases, real world measurement are expressed as:
+		 *
+		 *	schan_scale * (raw + schan_offset)
+		 *
+		 * Given that the rescaler parameters are applied recursively:
+		 *
+		 *	rescaler_scale * (schan_scale * (raw + schan_offset) +
+		 *		rescaler_offset)
+		 *
+		 * Or,
+		 *
+		 *	(rescaler_scale * schan_scale) * (raw +
+		 *		(schan_offset +	rescaler_offset / schan_scale)
+		 *
+		 * Thus, reusing the original expression the parameters exposed
+		 * to userspace are:
+		 *
+		 *	scale = schan_scale * rescaler_scale
+		 *	offset = schan_offset + rescaler_offset / schan_scale
+		 */
+		if (rescale->chan_processed) {
+			*val = rescale->offset;
+			return IIO_VAL_INT;
+		}
+
+		if (iio_channel_has_info(rescale->source->channel,
+					 IIO_CHAN_INFO_OFFSET)) {
+			ret = iio_read_channel_offset(rescale->source,
+						      &schan_off, NULL);
+			if (ret != IIO_VAL_INT)
+				return ret < 0 ? ret : -EOPNOTSUPP;
+		}
+
+		ret = iio_read_channel_scale(rescale->source, &scale, &scale2);
+		return rescale_process_offset(rescale, ret, scale, scale2,
+					      schan_off, val, val2);
 	default:
 		return -EINVAL;
 	}
@@ -188,6 +265,9 @@ static int rescale_configure_channel(struct device *dev,
 	chan->info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |
 		BIT(IIO_CHAN_INFO_SCALE);
 
+	if (rescale->offset)
+		chan->info_mask_separate |= BIT(IIO_CHAN_INFO_OFFSET);
+
 	/*
 	 * Using .read_avail() is fringe to begin with and makes no sense
 	 * whatsoever for processed channels, so we make sure that this cannot
@@ -352,6 +432,7 @@ static int rescale_probe(struct platform_device *pdev)
 	rescale->cfg = of_device_get_match_data(dev);
 	rescale->numerator = 1;
 	rescale->denominator = 1;
+	rescale->offset = 0;
 
 	ret = rescale->cfg->props(dev, rescale);
 	if (ret)

include/linux/iio/afe/rescale.h

@@ -25,8 +25,12 @@ struct rescale {
 	bool chan_processed;
 	s32 numerator;
 	s32 denominator;
+	s32 offset;
 };
 
 int rescale_process_scale(struct rescale *rescale, int scale_type,
 			  int *val, int *val2);
+int rescale_process_offset(struct rescale *rescale, int scale_type,
+			   int scale, int scale2, int schan_off,
+			   int *val, int *val2);
 #endif /* __IIO_RESCALE_H__ */