iio: afe: rescale: add INT_PLUS_{MICRO,NANO} support

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 <liambeguin@gmail.com>
Reviewed-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220213025739.2561834-3-liambeguin@gmail.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/afe/iio-rescale.c

@@ -23,6 +23,9 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
 			  int *val, int *val2)
 {
 	s64 tmp;
+	s32 rem;
+	u32 mult;
+	u32 neg;
 
 	switch (scale_type) {
 	case IIO_VAL_FRACTIONAL:
@@ -41,6 +44,37 @@ int rescale_process_scale(struct rescale *rescale, int scale_type,
 		tmp *= rescale->numerator;
 		tmp = div_s64(tmp, 1000000000LL);
 		*val = tmp;
+		return scale_type;
+	case IIO_VAL_INT_PLUS_NANO:
+	case IIO_VAL_INT_PLUS_MICRO:
+		mult = scale_type == IIO_VAL_INT_PLUS_NANO ? 1000000000L : 1000000L;
+
+		/*
+		 * For IIO_VAL_INT_PLUS_{MICRO,NANO} scale types if either *val
+		 * OR *val2 is negative the schan scale is negative, i.e.
+		 * *val = 1 and *val2 = -0.5 yields -1.5 not -0.5.
+		 */
+		neg = *val < 0 || *val2 < 0;
+
+		tmp = (s64)abs(*val) * abs(rescale->numerator);
+		*val = div_s64_rem(tmp, abs(rescale->denominator), &rem);
+
+		tmp = (s64)rem * mult + (s64)abs(*val2) * abs(rescale->numerator);
+		tmp = div_s64(tmp, abs(rescale->denominator));
+
+		*val += div_s64_rem(tmp, mult, val2);
+
+		/*
+		 * If only one of the rescaler elements or the schan scale is
+		 * negative, the combined scale is negative.
+		 */
+		if (neg ^ ((rescale->numerator < 0) ^ (rescale->denominator < 0))) {
+			if (*val)
+				*val = -*val;
+			else
+				*val2 = -*val2;
+		}
+
 		return scale_type;
 	default:
 		return -EOPNOTSUPP;