Re: [PATCH] iio: temperature: mcp9600: Fix temperature reading for negative values

[Marcelo Schmitt]

On 04/28, Jonathan Cameron wrote:
> On Sat, 27 Apr 2024 21:57:58 +0200
> Dimitri Fedrau <dima.fedrau@xxxxxxxxx> wrote:
> 
> > Am Sat, Apr 27, 2024 at 05:49:18AM -0300 schrieb Marcelo Schmitt:
> > > Hi Dimitri,
> > > 
> > > Interesting patch this one.
> > > I think this does apply, although, the cold junction register has for sign bits
> > > so I think we could also have a mask to clear those out.
> > > Some code suggestions inline.
> > >  
> > Hi Marcelo,
> > 
> > the temperature bits are in two’s complement format for hot and cold
> > junction. Equations to calculate the temperature are also the same in
> > the datasheet. There should be no difference when handling them. I don't
> > think we need to do anything more with the value except sign_extend it to
> > the appropriate data type. If the sign bits aren't right, there is a bug
> > in the chip, until then futher processing of it is unneeded. I could add
> > a comment here if it helps.

Ah yes, I missed the fact that other bits need to be set to make the value
smaller (closer to zero) when the MSB of two's complement number is set.

> 
> Agreed - by my reading the original patch is correct. Maybe it would act
> as cleaner 'documentation' to have the sign_extend32() for the cold junction be
> from bit 12 rather than 15, but I'm not sure it's worth the effort.

Yes, the original patch should be correct.
Yeah, no practical difference using bit 12 or 15 to sign extend the number so
probably not worth changing that.

> 
> Andrew, would be great if you can review this fix in case we are all missing
> something!
> 
> Jonathan
> 
> > 
> > > On 04/24, Dimitri Fedrau wrote:  
> > > > Temperature is stored as 16bit value in two's complement format. Current
> > > > implementation ignores the sign bit. Make it aware of the sign bit by
> > > > using sign_extend32.
> > > > 

Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@xxxxxxxxx>

> > > > Fixes: 3f6b9598b6df ("iio: temperature: Add MCP9600 thermocouple EMF converter")
> > > > Signed-off-by: Dimitri Fedrau <dima.fedrau@xxxxxxxxx>
> > > > ---
> > > >  drivers/iio/temperature/mcp9600.c | 3 ++-
> > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/iio/temperature/mcp9600.c b/drivers/iio/temperature/mcp9600.c
> > > > index 46845804292b..7a3eef5d5e75 100644
> > > > --- a/drivers/iio/temperature/mcp9600.c
> > > > +++ b/drivers/iio/temperature/mcp9600.c  
> > > 
> > > #define MCP9600_COLD_JUNCTION_SIGN_MSK GENMASK(15,12)
> > > ...
> > >   
> > > > @@ -52,7 +52,8 @@ static int mcp9600_read(struct mcp9600_data *data,
> > > >  
> > > >  	if (ret < 0)
> > > >  		return ret;
> > > > -	*val = ret;
> > > > +
> > > > +	*val = sign_extend32(ret, 15);  
> > > 	if (chan->address == MCP9600_COLD_JUNCTION)
> > > 		*val &= ~MCP9600_COLD_JUNCTION_SIGN_MSK;
> > >  
> > This won't work. Assuming int is 32-bit ret = 0xfffe and *val = -2 after
> > sign_extends32, this would result in *val = -61442 which is wrong.
> > > >  
> > > >  	return 0;
> > > >  }
> > > > -- 
> > > > 2.39.2
> > > > 
> > > >   
> > Best regards,
> > Dimitri Fedrau
>