iio: afe: rescale: fix accuracy for small fractional scales

The approximation caused by integer divisions can be costly on smaller
scale values since the decimal part is significant compared to the
integer part. Switch to an IIO_VAL_INT_PLUS_NANO scale type in such
cases to maintain accuracy.

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/20220213025739.2561834-5-liambeguin@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
index 8eaf766e28cc12a1026be84ddf4c7cb2c7188810..5d78f0cf47d28109463d760b37d1cd577f4d27db 100644 (file)
--- a/drivers/iio/afe/iio-rescale.c
+++ b/drivers/iio/afe/iio-rescale.c
@@ -24,7 +24,7 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
                          int *val, int *val2)
 {
        s64 tmp;
-       s32 rem;
+       s32 rem, rem2;
        u32 mult;
        u32 neg;
 
@@ -43,9 +43,23 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
                tmp = (s64)*val * 1000000000LL;
                tmp = div_s64(tmp, rescale->denominator);
                tmp *= rescale->numerator;
-               tmp = div_s64(tmp, 1000000000LL);
+
+               tmp = div_s64_rem(tmp, 1000000000LL, &rem);
                *val = tmp;
-               return scale_type;
+
+               if (!rem)
+                       return scale_type;
+
+               tmp = 1 << *val2;
+
+               rem2 = *val % (int)tmp;
+               *val = *val / (int)tmp;
+
+               *val2 = rem / (int)tmp;
+               if (rem2)
+                       *val2 += div_s64((s64)rem2 * 1000000000LL, tmp);
+
+               return IIO_VAL_INT_PLUS_NANO;
        case IIO_VAL_INT_PLUS_NANO:
        case IIO_VAL_INT_PLUS_MICRO:
                mult = scale_type == IIO_VAL_INT_PLUS_NANO ? 1000000000L : 1000000L;