Re: [PATCH v3 3/3] hwmon: Driver for Texas Instruments INA238

[Guenter Roeck]

On 10/31/21 10:55 PM, Nathan Rossi wrote:
> On Mon, 1 Nov 2021 at 13:48, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> > On 10/31/21 7:20 PM, Nathan Rossi wrote:
> > [ ... ]
> > > > > +
> > > > > +     if (attr != hwmon_in_max && attr != hwmon_in_min)
> > > > > +             return -EOPNOTSUPP;
> > > > > +
> > > > > +     /* convert decimal to register value */
> > > > > +     switch (channel) {
> > > > > +     case 0:
> > > > > +             /* signed value, clamp to max range +/-163 mV */
> > > > > +             regval = clamp_val(val, -163, 163);
> > > > > +             regval = (regval * 1000L * (4 - (int)data->gain + 1)) /
> > > >
> > > > nit: The typecast "(int)" is not needed here.
> > >
> > > Due to the unsigned type of gain, it causes promotion of regval (and
> > > the rest of the numerator) to unsigned long which causes issues with
> > > negative numbers on the divide. It makes more sense for gain to be an
> > > int to begin with, I will change it to int to avoid the need for type
> > > casting.
> >
> > Are you sure ? I initially thought that as well and wrote a little test
> > program with that expression, but it didn't do the promotion to unsigned.
>
> It definitely calculates incorrectly at run time (on an arm 32-bit
> platform), looking at the gcc output from -fdump-tree-original reveals
> some more insight. Which is that the promotion to long overrides the
> unsigned (from the 1000L) on long=64 but not on long=32.
>
> Where regval is int, and gain is unsigned int (u32).
>
> regval = (regval * 1000L * (4 - gain + 1)) / 5;
> -> armv7-a (invalid)
> regval = (int) ((((long unsigned int) regval * (long unsigned int) (5
> - gain)) * 1000) / 5);
> -> x86-64 (valid result)
> regval = (int) ((unsigned int) (gain * 4294967096 + 1000) * (unsigned
> int) regval);
>
> note: 4294967096 is -800, 1000 * (4 - gain + 1) => (-800 * gain) + 1000
>
> Slight variation without the 1000 being long.
>
> regval = (regval * 1000 * (4 - gain + 1)) / 5;
> -> armv7-a (invalid)
> regval = (int) ((((unsigned int) regval * (5 - gain)) * 1000) / 5);
> -> x86-64 (invalid)
> regval = (int) ((((unsigned int) regval * (5 - gain)) * 1000) / 5);
>
> regval = (regval * 1000LL * (4 - gain + 1)) / 5;
> -> armv7-a (valid)
> regval = (int) ((unsigned int) (gain * 4294967096 + 1000) * (unsigned
> int) regval);
> -> x86-64 (valid)
> regval = (int) ((unsigned int) (gain * 4294967096 + 1000) * (unsigned
> int) regval);
>
> I think it still makes sense to change gain to be int, and avoid the
> unsigned type issues.

Thanks for the details. I agree, changing gain to int makes sense.

Thanks,
Guenter