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

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Message-ID: <454245c8-ce59-4c87-9d92-4a924f83aa1f@roeck-us.net>
Date: Mon, 16 Jun 2025 06:00:24 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Geert Uytterhoeven <geert@...ux-m68k.org>,
 Yikai Tsai <yikai.tsai.wiwynn@...il.com>
Cc: patrick@...cx.xyz, Carsten Spieß
 <mail@...sten-spiess.de>, Jean Delvare <jdelvare@...e.com>,
 linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] hwmon: (isl28022) Fix current reading calculation

On 6/16/25 05:59, Guenter Roeck wrote:
> On 6/10/25 05:27, Geert Uytterhoeven wrote:
>> 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
>>
> 
> 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

s/map/dump/

> write unit test code and use it to simplify the driver.
> 
> Guenter
>