On Fri, 16 Jul 2021 15:18:33 -0400
"Liam Beguin" [off-list ref] wrote:

On Thu Jul 15, 2021 at 5:48 AM EDT, Peter Rosin wrote:

quoted

On 2021-07-15 05:12, Liam Beguin wrote:  

quoted

From: Liam Beguin <redacted>

Some ADCs use IIO_VAL_INT_PLUS_{NANO,MICRO} scale types.
Add support for these to allow using the iio-rescaler with them.

Signed-off-by: Liam Beguin <redacted>
---
 drivers/iio/afe/iio-rescale.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
index 4c3cfd4d5181..a2b220b5ba86 100644
--- a/drivers/iio/afe/iio-rescale.c
+++ b/drivers/iio/afe/iio-rescale.c

@@ -92,7 +92,22 @@ static int rescale_read_raw(struct iio_dev *indio_dev,
 			do_div(tmp, 1000000000LL);
 			*val = tmp;
 			return ret;
+		case IIO_VAL_INT_PLUS_NANO:
+			tmp = ((s64)*val * 1000000000LL + *val2) * rescale->numerator;
+			do_div(tmp, rescale->denominator);
+
+			*val = div_s64(tmp, 1000000000LL);
+			*val2 = tmp - *val * 1000000000LL;
+			return ret;  

This is too simplistic and prone to overflow. We need something like
this
(untested)

tmp = (s64)*val * rescale->numerator;
rem = do_div(tmp, rescale->denominator);
*val = tmp;
tmp = ((s64)rem * 1000000000LL + (s64)*val2) * rescale->numerator;
do_div(tmp, rescale->denominator);
*val2 = tmp;

Still not very safe with numerator and denominator both "large", but
much
better. And then we need normalizing the fraction part after the above,
of
course.
 

Understood, I'll test that.

quoted

And, of course, I'm not sure what *val == -1 and *val2 == 500000000
really
means. Is that -1.5 or -0.5? The above may very well need adjusting for
negative values...
 

I would've assumed the correct answer is -1 + 500000000e-9 = -0.5
but adding a test case to iio-test-format.c seems to return -1.5...

No. -1.5 is as intended, though the IIO_VAL_PLUS_MICRO is rather confusing
naming :( We should perhaps add more documentation for that.  Signs were
always a bit of a pain with this two integer scheme for fixed point.

The intent is to have moderately readable look up tables with the problem that
we don't have a signed 0 available.  Meh, maybe this decision a long time
back wasn't a the right one, but it may be a pain to change now as too many
drivers to check!

1, 0000000  == 1
0, 5000000  == 0.5
0, 0000000  == 0
0, -5000000 == -0.5
-1, 5000000 == -1.5

I believe that's a bug but we can work around if for now by moving the
integer part of *val2 to *val.

Yup.  Fiddly corner cases..

Jonathan

Liam

quoted

Cheers,
Peter
 

quoted

+		case IIO_VAL_INT_PLUS_MICRO:
+			tmp = ((s64)*val * 1000000LL + *val2) * rescale->numerator;
+			do_div(tmp, rescale->denominator);
+
+			*val = div_s64(tmp, 1000000LL);
+			*val2 = tmp - *val * 1000000LL;
+			return ret;
 		default:
+			dev_err(&indio_dev->dev, "unsupported type %d\n", ret);
 			return -EOPNOTSUPP;
 		}
 	default: