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

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Date: Sun, 19 May 2024 22:32:50 +0200
From: Dimitri Fedrau <dima.fedrau@...il.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: Lars-Peter Clausen <lars@...afoo.de>,
	Andrew Hepp <andrew.hepp@...pp.dev>,
	Marcelo Schmitt <marcelo.schmitt1@...il.com>,
	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org,
	Nuno Sá <nuno.sa@...log.com>
Subject: Re: [PATCH v2 1/2] iio: temperature: mcp9600: Provide index for both
 channels

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@...il.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@...nel.org>
> > Signed-off-by: Dimitri Fedrau <dima.fedrau@...il.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

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 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.

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,
> >  	},
> >  };
> >  
>