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

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Message-ID: <CAMuHMdVX0risng0cgUp7uOycv2wUNsZ3SgCX8sq=14j6NXjNBw@mail.gmail.com>
Date: Tue, 10 Jun 2025 14:27:36 +0200
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Yikai Tsai <yikai.tsai.wiwynn@...il.com>
Cc: patrick@...cx.xyz, Carsten Spieß <mail@...sten-spiess.de>, 
	Jean Delvare <jdelvare@...e.com>, Guenter Roeck <linux@...ck-us.net>, linux-hwmon@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] hwmon: (isl28022) Fix current reading calculation

Hi Yikai,

On Mon, 19 May 2025 at 10:48, Yikai Tsai <yikai.tsai.wiwynn@...il.com> 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@...il.com>


> --- 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


>                 break;
>         default:
>                 return -EOPNOTSUPP;

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

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