Messages in this thread Patch in this message | | | From | Eliav Farber <> | Subject | [PATCH v4 05/21] hwmon: (mr75203) fix voltage equation for negative source input | Date | Tue, 6 Sep 2022 08:33:40 +0000 |
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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 it results in -1 (0xf...fffff).
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> --- 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 | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/hwmon/mr75203.c b/drivers/hwmon/mr75203.c index a209f5d95f4b..78dc471e843c 100644 --- a/drivers/hwmon/mr75203.c +++ b/drivers/hwmon/mr75203.c @@ -202,7 +202,7 @@ static int pvt_read_in(struct device *dev, u32 attr, int channel, long *val) n &= SAMPLE_DATA_MSK; /* Convert the N bitstream count into voltage */ - *val = (PVT_N_CONST * n - PVT_R_CONST) >> PVT_CONV_BITS; + *val = (PVT_N_CONST * (long)n - PVT_R_CONST) / (1 << PVT_CONV_BITS); return 0; default: -- 2.37.1
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