On Tue, Sep 06, 2022 at 08:33:40AM +0000, Eliav Farber wrote: > 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. This is really downside of C... ... > - *val = (PVT_N_CONST * n - PVT_R_CONST) >> PVT_CONV_BITS; > + *val = (PVT_N_CONST * (long)n - PVT_R_CONST) / (1 << PVT_CONV_BITS); Wondering if we can use BIT(PVT_CONV_BITS) for two (quite unlikely to happen, I hope) purposes: 1) Somebody copies such code where PVT_CONV_BITS analogue can be 31, which is according to C standard is UB (undefined behaviour). 2) It makes shorter the line and also drops the pattern where some dumb robot may propose a patch to basically revert the division change. -- With Best Regards, Andy Shevchenko
