Re: [PATCH v2 1/1] hwmon: (isl28022) Fix current reading calculation

[Geert Uytterhoeven]

Hi Guenter,

On Mon, 16 Jun 2025 at 14:59, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> On 6/10/25 05:27, Geert Uytterhoeven wrote:
> > On Mon, 19 May 2025 at 10:48, Yikai Tsai <yikai.tsai.wiwynn@xxxxxxxxx> wrote:
> >> According to the ISL28022 datasheet, bit15 of the current register is
> >> representing -32768. Fix the calculation to properly handle this bit,
> >> ensuring correct measurements for negative values.
> >>
> >> Signed-off-by: Yikai Tsai <yikai.tsai.wiwynn@xxxxxxxxx>
> >
> >
> >> --- a/drivers/hwmon/isl28022.c
> >> +++ b/drivers/hwmon/isl28022.c
> >> @@ -154,6 +154,7 @@ static int isl28022_read_current(struct device *dev, u32 attr, long *val)
> >>          struct isl28022_data *data = dev_get_drvdata(dev);
> >>          unsigned int regval;
> >>          int err;
> >> +       u16 sign_bit;
> >>
> >>          switch (attr) {
> >>          case hwmon_curr_input:
> >> @@ -161,8 +162,9 @@ static int isl28022_read_current(struct device *dev, u32 attr, long *val)
> >>                                    ISL28022_REG_CURRENT, &regval);
> >>                  if (err < 0)
> >>                          return err;
> >> -               *val = ((long)regval * 1250L * (long)data->gain) /
> >> -                       (long)data->shunt;
> >> +               sign_bit = (regval >> 15) & 0x01;
> >> +               *val = (((long)(((u16)regval) & 0x7FFF) - (sign_bit * 32768)) *
> >> +                       1250L * (long)data->gain) / (long)data->shunt;
> >
> > Isn't this complex operation to convert the 16-bit register value to
> > a two-complement signed number equivalent to a simple cast?
> >
> >      (s16)regval
> >
> > isl28022_read_in() has similar code, but as the sign bit is not always
> > the MSB, it needs two additional shifts:
> >
> >      ((s16)(regval << shift)) >> shift
> >
>
> Yes, I know. This could all be simplified using sign_extend32(), but I really
> wanted to be able to apply it. If someone sends me a register map, maybe I can
> write unit test code and use it to simplify the driver.

Cool, I didn't know we had a helper for that... Thanks ;-)

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds