[PATCH v2 0/1] hwmon: (isl28022) Fix current reading calculation

[PATCH v2 0/1] hwmon: (isl28022) Fix current reading calculation

[Yikai Tsai]

Fix the driver for Renesas ISL28022 power monitor chip.
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.

v2: Add the missing variable declaration
v1: fix current reading calculation

Yikai Tsai (1):
  hwmon: (isl28022) Fix current reading calculation

 drivers/hwmon/isl28022.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

-- 
2.25.1

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

[Yikai Tsai]

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@gmail.com>
---
 drivers/hwmon/isl28022.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/hwmon/isl28022.c b/drivers/hwmon/isl28022.c
index 1fb9864635db..1b4fb0824d6c 100644
--- 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;
 		break;
 	default:
 		return -EOPNOTSUPP;
-- 
2.25.1

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

[Guenter Roeck]

On Mon, May 19, 2025 at 04:40:51PM +0800, Yikai Tsai 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@gmail.com>

Applied.

Guenter

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

[Geert Uytterhoeven]

Hi Yikai,

On Mon, 19 May 2025 at 10:48, Yikai Tsai <yikai.tsai.wiwynn@gmail.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@gmail.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@linux-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

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

[Guenter Roeck]

On 6/10/25 05:27, Geert Uytterhoeven wrote:
> Hi Yikai,
> 
> On Mon, 19 May 2025 at 10:48, Yikai Tsai <yikai.tsai.wiwynn@gmail.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@gmail.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
write unit test code and use it to simplify the driver.

Guenter

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

[Guenter Roeck]

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@gmail.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@gmail.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
> 

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@roeck-us.net> wrote:
> On 6/10/25 05:27, Geert Uytterhoeven wrote:
> > On Mon, 19 May 2025 at 10:48, Yikai Tsai <yikai.tsai.wiwynn@gmail.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@gmail.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
> 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@linux-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