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

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Date: Sun, 28 Apr 2024 14:46:06 +0100
From: Jonathan Cameron <jic23@...nel.org>
To: Dimitri Fedrau <dima.fedrau@...il.com>
Cc: Marcelo Schmitt <marcelo.schmitt1@...il.com>, Lars-Peter Clausen
 <lars@...afoo.de>, Andrew Hepp <andrew.hepp@...pp.dev>,
 linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iio: temperature: mcp9600: Fix temperature reading for
 negative values

On Sat, 27 Apr 2024 21:57:58 +0200
Dimitri Fedrau <dima.fedrau@...il.com> 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.

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.

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.
> > > 
> > > Fixes: 3f6b9598b6df ("iio: temperature: Add MCP9600 thermocouple EMF converter")
> > > Signed-off-by: Dimitri Fedrau <dima.fedrau@...il.com>
> > > ---
> > >  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