On Sun Jul 4, 2021 at 12:53 PM EDT, Jonathan Cameron wrote:

On Wed, 30 Jun 2021 21:00:30 -0400
Liam Beguin [off-list ref] wrote:

quoted

From: Liam Beguin <redacted>

This is a preparatory change required for the addition of temperature
sensing front ends.

Signed-off-by: Liam Beguin <redacted>

Hi Liam.

A few remaining things inline.

Looking good in general.

Jonathan

quoted

---
 drivers/iio/afe/iio-rescale.c | 63 +++++++++++++++++++++++++++++++++++
 1 file changed, 63 insertions(+)

diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
index 8f79c582519c..c8750286c308 100644
--- a/drivers/iio/afe/iio-rescale.c
+++ b/drivers/iio/afe/iio-rescale.c

@@ -32,6 +32,7 @@ struct rescale {
 	bool chan_processed;
 	s32 numerator;
 	s32 denominator;
+	s32 offset;
 };
 
 static int rescale_read_raw(struct iio_dev *indio_dev,

@@ -39,6 +40,8 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
 			    int *val, int *val2, long mask)
 {
 	struct rescale *rescale = iio_priv(indio_dev);
+	int scale, scale2;
+	int schan_off = 0;
 	s64 tmp, tmp2;
 	u32 factor;
 	int ret;

@@ -99,6 +102,62 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
 			dev_err(&indio_dev->dev, "unsupported type %d\n", ret);
 			return -EOPNOTSUPP;
 		}
+	case IIO_CHAN_INFO_OFFSET:
+		/*
+		 * Processed channels are scaled 1-to-1 and source offset is
+		 * already taken into account.
+		 *
+		 * In other cases, the final offset value is defined by:
+		 *	offset = schan_offset + rescaler_offset / schan_scale

Maths is right, but perhaps useful to express how this is derived as I
had
to scribble it down to check.

Want to express real world measurement as:
scale * (raw + offset)
Given we applying scale and offset recursively we have.
rescaler_scale * (schan_scale * (raw + schan_offset) + rescaler_offset)
which can be rearrange into the correct form at.
rescaler_scale *schan_scale* (raw + (schan_offset +
rescaler_offset/schan_scale))
Thus the scale and offset we expose to userspace should be.
scale = rescaler_scale * schan_scale
offset = schan_offset + rescaler_offset/schan_scale;

Understood, I'll update the comment with more details.

quoted

+		 */
+		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 < 0)
+				return ret;

if you've returned in the first branch of the if, no need to worry about
the
else for checking the second condition.

if (ret != IIO_VAL_INT)
return...

Right, I'll combine both statements.

quoted

+			else if (ret != IIO_VAL_INT)
+				return -EOPNOTSUPP;
+		}
+
+		ret = iio_read_channel_scale(rescale->source, &scale, &scale2);
+		switch (ret) {
+		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 * 1000000000UL;
+			tmp2 = ((s64)scale * 1000000000UL) + scale2;
+			factor = gcd(tmp, tmp2);
+			tmp /= factor;
+			tmp2 /= factor;

What is the benefit to doing gcd division before div_s64?

You're right the GCD division is unnecessary here, will drop.

quoted

+			*val = div_s64(tmp, tmp2) + schan_off;
+			return IIO_VAL_INT;
+		case IIO_VAL_INT_PLUS_MICRO:
+			tmp = (s64)rescale->offset * 1000000UL;
+			tmp2 = ((s64)scale * 1000000UL) + scale2;
+			factor = gcd(tmp, tmp2);
+			tmp /= factor;
+			tmp2 /= factor;
+			*val = div_s64(tmp, tmp2) + schan_off;
+			return IIO_VAL_INT;
+		default:
+			dev_err(&indio_dev->dev, "unsupported type %d\n", ret);
+			return -EOPNOTSUPP;
+		}
 	default:
 		return -EINVAL;
 	}

@@ -175,6 +234,9 @@ 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

@@ -339,6 +401,7 @@ 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)