staging: iio: cdc: ad7746: Switch from _offset to _zeropoint for differential channels.

As this offset is applied equally to both lines of the differential
pair, _ofset should not be used. Use the new ABI _zeropoint instead
to avoid userspace software applying this value when calculating
real value = (_raw + _offset) * _scale

Also add a comment to explain why an offset of 0x800000 is applied
within the driver rather than exposed to userspace.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220626122938.582107-13-jic23@kernel.org

diff --git a/drivers/staging/iio/cdc/ad7746.c b/drivers/staging/iio/cdc/ad7746.c
index 3e8e7836a5831ba96cfdf96fa285c2b237376d62..d323ae4d99921633a406ee7d02f9209e1ab10eca 100644 (file)
--- a/drivers/staging/iio/cdc/ad7746.c
+++ b/drivers/staging/iio/cdc/ad7746.c
@@ -207,7 +207,7 @@ static const struct iio_chan_spec ad7746_channels[] = {
                .channel = 0,
                .channel2 = 2,
                .info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |
-               BIT(IIO_CHAN_INFO_CALIBSCALE) | BIT(IIO_CHAN_INFO_OFFSET),
+               BIT(IIO_CHAN_INFO_CALIBSCALE) | BIT(IIO_CHAN_INFO_ZEROPOINT),
                .info_mask_shared_by_type = BIT(IIO_CHAN_INFO_CALIBBIAS) |
                BIT(IIO_CHAN_INFO_SCALE) | BIT(IIO_CHAN_INFO_SAMP_FREQ),
                .address = CIN1_DIFF,
@@ -229,7 +229,7 @@ static const struct iio_chan_spec ad7746_channels[] = {
                .channel = 1,
                .channel2 = 3,
                .info_mask_separate = BIT(IIO_CHAN_INFO_RAW) |
-               BIT(IIO_CHAN_INFO_CALIBSCALE) | BIT(IIO_CHAN_INFO_OFFSET),
+               BIT(IIO_CHAN_INFO_CALIBSCALE) | BIT(IIO_CHAN_INFO_ZEROPOINT),
                .info_mask_shared_by_type = BIT(IIO_CHAN_INFO_CALIBBIAS) |
                BIT(IIO_CHAN_INFO_SCALE) | BIT(IIO_CHAN_INFO_SAMP_FREQ),
                .address = CIN2_DIFF,
@@ -506,6 +506,7 @@ static int ad7746_write_raw(struct iio_dev *indio_dev,
 
                return 0;
        case IIO_CHAN_INFO_OFFSET:
+       case IIO_CHAN_INFO_ZEROPOINT:
                if (val < 0 || val > 43008000) /* 21pF */
                        return -EINVAL;
 
@@ -581,6 +582,10 @@ static int ad7746_read_channel(struct iio_dev *indio_dev,
        if (ret < 0)
                return ret;
 
+       /*
+        * Offset applied internally becaue the _offset userspace interface is
+        * needed for the CAP DACs which apply a controllable offset.
+        */
        *val = get_unaligned_be24(data) - 0x800000;
 
        return 0;
@@ -637,6 +642,7 @@ static int ad7746_read_raw(struct iio_dev *indio_dev,
 
                return IIO_VAL_INT;
        case IIO_CHAN_INFO_OFFSET:
+       case IIO_CHAN_INFO_ZEROPOINT:
                *val = FIELD_GET(AD7746_CAPDAC_DACP_MASK,
                                 chip->capdac[chan->channel][chan->differential]) * 338646;