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SubjectRe: [PATCH v3 06/10] iio: afe: rescale: add offset support
On Wed, 30 Jun 2021 21:00:30 -0400
Liam Beguin <liambeguin@gmail.com> wrote:

> From: Liam Beguin <lvb@xiphos.com>
>
> This is a preparatory change required for the addition of temperature
> sensing front ends.
>
> Signed-off-by: Liam Beguin <lvb@xiphos.com>
Hi Liam.

A few remaining things inline.

Looking good in general.

Jonathan

> ---
> 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;


> + */
> + 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...


> + 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?

> + *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)

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