Re: [PATCH v2 2/2] iio:temperature:max31856:Add device tree bind info

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 16 Oct 2018 17:16:15 -0500
Paresh Chaudhary <paresh.chaudhary@rockwellcollins.com> wrote:

> Hi Jonathan,
> 
> Please find my comment inline.
> 
> Thanks,
> Paresh
> 
> On Sat, Oct 13, 2018 at 8:54 AM Jonathan Cameron <jic23@kernel.org> wrote:
> >
> > On Thu, 11 Oct 2018 11:38:10 -0500
> > Matt Weber <matthew.weber@rockwellcollins.com> wrote:
> >  
> > > From: Paresh Chaudhary <paresh.chaudhary@rockwellcollins.com>
> > >
> > > This patch added device tree binding info for MAX31856 driver.
> > >
> > > Signed-off-by: Paresh Chaudhary <paresh.chaudhary@rockwellcollins.com>
> > > Signed-off-by: Matt Weber <matthew.weber@rockwellcollins.com>  
> > Device tree bindings should be cc'd to the devicetree list and maintainers.
> >
> > Thanks,
> >
> > Jonathan
> >  
> > > ---
> > > Changes v1 -> v2
> > > [Matt
> > >  - Removed comment block and added possibilities of
> > >    thermocouple type in device tree binding doc.
> > >
> > > ---
> > >  .../bindings/iio/temperature/max31856.txt          | 32 ++++++++++++++++++++++
> > >  MAINTAINERS                                        |  1 +
> > >  2 files changed, 33 insertions(+)
> > >  create mode 100644 Documentation/devicetree/bindings/iio/temperature/max31856.txt
> > >
> > > diff --git a/Documentation/devicetree/bindings/iio/temperature/max31856.txt b/Documentation/devicetree/bindings/iio/temperature/max31856.txt
> > > new file mode 100644
> > > index 0000000..e1def9f
> > > --- /dev/null
> > > +++ b/Documentation/devicetree/bindings/iio/temperature/max31856.txt
> > > @@ -0,0 +1,32 @@
> > > +Maxim MAX31856 thermocouple support
> > > +
> > > +https://datasheets.maximintegrated.com/en/ds/MAX31856.pdf
> > > +
> > > +Required properties:
> > > +     - compatible: must be "max31856"
> > > +     - reg: SPI chip select number for the device
> > > +     - spi-max-frequency: As per datasheet max. supported freq is 5000000
> > > +     - spi-cpha: must be defined for max31856 to enable SPI mode 1
> > > +     - type: Type of thermocouple (By default is K-Type)
> > > +             0x00 : TYPE_B
> > > +             0x01 : TYPE_E
> > > +             0x02 : TYPE_J
> > > +             0x03 : TYPE_K (default)
> > > +             0x04 : TYPE_N
> > > +             0x05 : TYPE_R
> > > +             0x06 : TYPE_S
> > > +             0x07 : TYPE_T
> > > +
> > > +     Refer to spi/spi-bus.txt for generic SPI slave bindings.
> > > +
> > > +Optional properties:
> > > +     - one-shot: Enable one-shot Conversion mode (By default mode is auto)  
> >
> > Why is this something that should be in devicetree rather than controlled
> > in the driver in response to some userspace interaction?
> >
> >  
> There is no specific reason. This is just providing info to the driver
> so the driver can configure chip while probe for specified mode.
> I understand your concern but as of now, the driver does not have
> support to decide mode based on userspace interaction. We will work on
> that when we reopen for enhancement (e.g threshold for temp. range)

Hmm. The problem is that this should not be a devicetree decision and
if we allow it to be so, then we end up stuck with having to support
the interface here going forward. It also sets a precedence that
will get picked up by other drivers.

So, sorry but I really don't think this is a good idea.  For now you
may just have pick a default and go with it until you have the time
to add some automated switching between them at some later date.


Jonathan
> > > +
> > > + Example:
> > > +     max31856@0 {
> > > +             compatible = "max31856";
> > > +             reg = <0>;
> > > +             spi-max-frequency = <5000000>;
> > > +             spi-cpha;
> > > +             type = <0x03>;
> > > +     };
> > > diff --git a/MAINTAINERS b/MAINTAINERS
> > > index dd9a83d..6482ae8 100644
> > > --- a/MAINTAINERS
> > > +++ b/MAINTAINERS
> > > @@ -7161,6 +7161,7 @@ MAX31856 IIO DRIVER
> > >  M:   Matthew Weber <matthew.weber@rockwellcollins.com>
> > >  S:   Maintained
> > >  F:   drivers/iio/temperature/max31856.c
> > > +F:   Documentation/devicetree/bindings/iio/temperature/max31856.txt
> > >
> > >  IIO UNIT CONVERTER
> > >  M:   Peter Rosin <peda@axentia.se>  
> >