Re: [PATCH v2 1/2] iio: temperature: mcp9600: Provide index for both channels

[Jonathan Cameron <Jonathan.Cameron@Huawei.com>]

On Sun, 19 May 2024 22:32:50 +0200
Dimitri Fedrau <dima.fedrau@gmail.com> wrote:

> Am Sun, May 19, 2024 at 05:14:38PM +0100 schrieb Jonathan Cameron:
> > On Fri, 17 May 2024 10:10:49 +0200
> > Dimitri Fedrau <dima.fedrau@gmail.com> wrote:
> >   
> > > The mapping from cold junction to ambient temperature is inaccurate. We
> > > provide an index for hot and cold junction temperatures.
> > > 
> > > Suggested-by: Jonathan Cameron <jic23@kernel.org>
> > > Signed-off-by: Dimitri Fedrau <dima.fedrau@gmail.com>  
> > Hi Dmitri,
> >  
> Hi Jonathan,
> 
> > I'm not sure you replied to the question in previous review of what
> > sysfs files exist for this device.  Whilst I am at least a little
> > open to changing the ABI, I'd like to fully understand what
> > is currently presented and why iio_info is having trouble with it.
> >   
> I did, see: https://lore.kernel.org/linux-iio/20240509193125.GA3614@debian/T/#u

Ah thanks!  Oddly seem to have missed that on my other email account, but
have it here.  Thanks for the link.

> 
> But maybe not to the point. Sysfs is working correct and iio_info
> probably not. There is only one channel found, the temp_ambient. I would
> have expected two channels. Instead there are four attributes, I would
> have expected two. It seems to me that they are just duplicated. I also
> added the output when setting channel2 member of channel 0 to
> IIO_MOD_TEMP_OBJECT. This time iio_info works fine.

I'd be tempted to look at whether iio_info can be easily 'fixed' as a starting point.
This corner case may well occur for other devices in future.
It is 'stretching' the ABI definition but I don't think the current documentation
rules this case out.

> 
> > I also want an ack from Andrew on this one given might break it existing
> > usage.
> > 
> > The current interface is perhaps less than ideal, but I don't think it
> > is wrong as such. Whilst I wasn't particularly keen on the cold junction
> > == ambient I'm not sure moving to just indexed is an improvement.
> > Hence looking for input from Andrew. +CC Nuno as someone who is both
> > active in IIO and has written thermocouple front end drivers in
> > the past.
> >   
> I just thought the setting of channel2 member to IIO_MOD_TEMP_OBJECT was
> missing. But it turned out that it is not set for a reason. I'm fine
> with the existing mapping, but would be still interesting to know how
> others think about the mapping.

Agreed - I'm not sure on the right thing to do here.
I suspect leaving the modifier and add the index may be the best we can do
but let us wait to see what others think.

Jonathan

> 
> Dimitri
> 
> >   
> > > ---
> > >  drivers/iio/temperature/mcp9600.c | 9 +++++++--
> > >  1 file changed, 7 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/iio/temperature/mcp9600.c b/drivers/iio/temperature/mcp9600.c
> > > index 46845804292b..22451d1d9e1f 100644
> > > --- a/drivers/iio/temperature/mcp9600.c
> > > +++ b/drivers/iio/temperature/mcp9600.c
> > > @@ -14,6 +14,9 @@
> > >  
> > >  #include <linux/iio/iio.h>
> > >  
> > > +#define MCP9600_CHAN_HOT_JUNCTION	0
> > > +#define MCP9600_CHAN_COLD_JUNCTION	1
> > > +
> > >  /* MCP9600 registers */
> > >  #define MCP9600_HOT_JUNCTION 0x0
> > >  #define MCP9600_COLD_JUNCTION 0x2
> > > @@ -25,17 +28,19 @@
> > >  static const struct iio_chan_spec mcp9600_channels[] = {
> > >  	{
> > >  		.type = IIO_TEMP,
> > > +		.channel = MCP9600_CHAN_HOT_JUNCTION,
> > >  		.address = MCP9600_HOT_JUNCTION,
> > >  		.info_mask_separate =
> > >  			BIT(IIO_CHAN_INFO_RAW) | BIT(IIO_CHAN_INFO_SCALE),
> > > +		.indexed = 1,
> > >  	},
> > >  	{
> > >  		.type = IIO_TEMP,
> > > +		.channel = MCP9600_CHAN_COLD_JUNCTION,
> > >  		.address = MCP9600_COLD_JUNCTION,
> > > -		.channel2 = IIO_MOD_TEMP_AMBIENT,
> > > -		.modified = 1,
> > >  		.info_mask_separate =
> > >  			BIT(IIO_CHAN_INFO_RAW) | BIT(IIO_CHAN_INFO_SCALE),
> > > +		.indexed = 1,
> > >  	},
> > >  };
> > >    
> >   
>