linux-kernel - Re: [PATCH v4 05/21] hwmon: (mr75203) fix voltage equation for negative source input

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Date:   Tue, 6 Sep 2022 15:03:37 +0300
From:   Andy Shevchenko <andriy.shevchenko@...el.com>
To:     Eliav Farber <farbere@...zon.com>
Cc:     jdelvare@...e.com, linux@...ck-us.net, robh+dt@...nel.org,
        p.zabel@...gutronix.de, rtanwar@...linear.com,
        linux-hwmon@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, hhhawa@...zon.com, jonnyc@...zon.com
Subject: Re: [PATCH v4 05/21] hwmon: (mr75203) fix voltage equation for
 negative source input

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