Re: [PATCH] iio: temperature: max30208: fix wrong scale value

[Jonathan Cameron]

On Tue, 28 Apr 2026 14:36:00 +0100
Salah Triki <salah.triki@xxxxxxxxx> wrote:

> On Tue, Apr 28, 2026 at 02:00:48PM +0300, Andy Shevchenko wrote:
> > On Tue, Apr 28, 2026 at 09:54:15AM +0100, Salah Triki wrote:  
> > > The driver currently returns a scale of 5 for IIO_CHAN_INFO_SCALE, which
> > > leads to incorrect temperature readings.
> > > 
> > > According to the MAX30208 datasheet, the temperature resolution is 0.005°C
> > > per LSB. Using IIO_VAL_FRACTIONAL with 5/1000 correctly represents this
> > > 16-bit resolution.
> > > 
> > > Fix the scale value to ensure user space tools report the temperature
> > > correctly in Celsius.  
> >   
> > > Fixes: 9ee95ae4cffd ("iio: temperature: Add driver support for Maxim
> > > MAX30208")  
> > 
> > The tags should go with 1 tag per one (single) line. Do not wrap them.  
> 
> Sorry about that, I will fix the tag wrapping in v2.
> 
> >   
> > > Signed-off-by: Salah Triki <salah.triki@xxxxxxxxx>  
> > 
> > ...
> >   
> > >  	case IIO_CHAN_INFO_SCALE:
> > >  		*val = 5;
> > > -		return IIO_VAL_INT;
> > > +		*val2 = 1000;
> > > +		return IIO_VAL_FRACTIONAL;  
> > 
> > Isn't it an ABI change?
> >   
> 
> Yes, it technically changes the value exposed to user space. However, the
> current scale (5) is objectively incorrect according to the datasheet 
> (0.005°C per LSB).
> 
> With the current scale, a raw reading of 5000 (25°C) is reported as 
> 25000°C by user space tools. This makes the driver practically unusable 
> for standard IIO consumers without custom workarounds. Since the driver is
> relatively recent, I believe fixing it now to match the hardware 
> specification is preferable to keeping a broken ABI.
> 

Yes.  It's fine to fix completely wrong ABI like this.  It can get messier
in more subtle corners but out by a factor of 1000 is an easy one!

Jonathan

> What do you think?
> 
> Best regards,
> --
> Salah Triki
>