Messages in this thread | | | Subject | Re: [PATCH v4 05/21] hwmon: (mr75203) fix voltage equation for negative source input | Date | Tue, 6 Sep 2022 16:27:13 +0300 | From | "Farber, Eliav" <> |
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On 9/6/2022 3:03 PM, Andy Shevchenko wrote: > 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. I originally tried to use BIT(PVT_CONV_BITS) but it gave a different result. e.g. If n = 2720 *val = (PVT_N_CONST * (long)n - PVT_R_CONST) / (1 << PVT_CONV_BITS) = 0 *val = (PVT_N_CONST * (long)n - PVT_R_CONST) / BIT(PVT_CONV_BITS) = 18014398509481983
I can try fitting it in one line, either by adding a define for (1 << PVT_CONV_BITS) or exceeding 80 characters, but keep in mind that in a later patch (#15) it gets even longer (and I must use more than one line) since it is multiplied by a pre-scaler factor.
-- Regards, Eliav
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