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
