[PATCH v5 04/21] hwmon: (mr75203) fix voltage equation for negative source input

[Eliav Farber]

According to Moortec Embedded Voltage Monitor (MEVM) series 3 data
sheet, the minimum input signal is -100mv and maximum input signal
is +1000mv.

The equation used to convert the digital word to voltage uses mixed
types (*val signed and n unsigned), and on 64 bit machines also has
different size, since sizeof(u32) = 4 and sizeof(long) = 8.

So when measuring a negative input, n will be small enough, such that
PVT_N_CONST * n < PVT_R_CONST, and the result of
(PVT_N_CONST * n - PVT_R_CONST) will overflow to a very big positive
32 bit number. Then when storing the result in *val it will be the same
value just in 64 bit (instead of it representing a negative number which
will what happen when sizeof(long) = 4).

When -1023 <= (PVT_N_CONST * n - PVT_R_CONST) <= -1
dividing the number by 1024 should result of in 0, but because ">> 10"
is used, and the sign bit is used to fill the vacated bit positions, it
results in -1 (0xf...fffff) which is wrong.

This change fixes the sign problem and supports negative values by
casting n to long and replacing the shift right with div operation.

Fixes: 9d823351a337 ("hwmon: Add hardware monitoring driver for Moortec MR75203 PVT controller")
Signed-off-by: Eliav Farber <farbere@xxxxxxxxxx>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
---
V5 -> V4:
- Add detailed comment in code explaining the cast and the div instead of right
  shift or use of BIT().

V4 -> V3:
- Remove unrelated change (add of empty line).

V3 -> V2:
- Fix equation to support negative values instead of limiting value to zero.

 drivers/hwmon/mr75203.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/hwmon/mr75203.c b/drivers/hwmon/mr75203.c
index 62df0c9498f3..8211d463495d 100644
--- a/drivers/hwmon/mr75203.c
+++ b/drivers/hwmon/mr75203.c
@@ -201,8 +201,18 @@ static int pvt_read_in(struct device *dev, u32 attr, int channel, long *val)
 			return ret;
 
 		n &= SAMPLE_DATA_MSK;
-		/* Convert the N bitstream count into voltage */
-		*val = (PVT_N_CONST * n - PVT_R_CONST) >> PVT_CONV_BITS;
+		/*
+		 * Convert the N bitstream count into voltage.
+		 * To support negative voltage calculation for 64bit machines
+		 * n must be cast to long, since n and *val differ both in
+		 * signedness and in size.
+		 * Division is used instead of right shift, because for signed
+		 * numbers, the sign bit is used to fill the vacated bit
+		 * positions, and if the number is negative, 1 is used.
+		 * BIT(x) may not be used instead of (1 << x) because it's
+		 * unsigned.
+		 */
+		*val = (PVT_N_CONST * (long)n - PVT_R_CONST) / (1 << PVT_CONV_BITS);
 
 		return 0;
 	default:
-- 
2.37.1