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authorEliav Farber <farbere@amazon.com>2022-09-08 18:24:32 +0300
committerGuenter Roeck <linux@roeck-us.net>2022-09-08 21:55:24 +0300
commit227a3a2fc31d8e4bb9c88d4804e19530af245b1b (patch)
tree1044a6b6d4f56abc4874a1f3ee6edb60cc822c7b /drivers/hwmon/mr75203.c
parentbb9195bd6664d94d71647631593e09f705ff5edd (diff)
downloadlinux-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/hwmon/mr75203.c')
-rw-r--r--drivers/hwmon/mr75203.c14
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: