diff options
| author | Eliav Farber <farbere@amazon.com> | 2022-09-08 18:24:32 +0300 |
|---|---|---|
| committer | Guenter Roeck <linux@roeck-us.net> | 2022-09-08 21:55:24 +0300 |
| commit | 227a3a2fc31d8e4bb9c88d4804e19530af245b1b (patch) | |
| tree | 1044a6b6d4f56abc4874a1f3ee6edb60cc822c7b /drivers | |
| parent | bb9195bd6664d94d71647631593e09f705ff5edd (diff) | |
| download | linux-227a3a2fc31d8e4bb9c88d4804e19530af245b1b.tar.xz | |
hwmon: (mr75203) fix voltage equation for negative source input
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@amazon.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220908152449.35457-5-farbere@amazon.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Diffstat (limited to 'drivers')
| -rw-r--r-- | drivers/hwmon/mr75203.c | 14 |
1 files changed, 12 insertions, 2 deletions
diff --git a/drivers/hwmon/mr75203.c b/drivers/hwmon/mr75203.c index be02f32bf143..6d3b3c499ed8 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: |