linux-kernel - Re: [PATCH 1/2] dt-bindings: iio: adc: adding MCP3564 ADC

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Date:   Fri, 9 Jun 2023 18:41:49 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Rob Herring <robh@...nel.org>
CC:     Jonathan Cameron <jic23@...nel.org>,
        Conor Dooley <conor@...nel.org>,
        <marius.cristea@...rochip.com>, <lars@...afoo.de>,
        <linux-iio@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Chris Packham <chris.packham@...iedtelesis.co.nz>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH 1/2] dt-bindings: iio: adc: adding MCP3564 ADC

On Thu, 8 Jun 2023 13:34:43 -0600
Rob Herring <robh@...nel.org> wrote:

> On Sat, May 20, 2023 at 04:17:53PM +0100, Jonathan Cameron wrote:
> > On Fri, 19 May 2023 19:29:15 +0100
> > Conor Dooley <conor@...nel.org> wrote:
> >   
> > > Hey Marius,
> > > 
> > > On Fri, May 19, 2023 at 07:01:44PM +0300, marius.cristea@...rochip.com wrote:  
> > > > From: Marius Cristea <marius.cristea@...rochip.com>
> > > > 
> > > > This is the device tree schema for iio driver for
> > > > Microchip family of 153.6 ksps, Low-Noise 16/24-Bit
> > > > Delta-Sigma ADCs with an SPI interface.    
> > > 
> > > Just one quick process bit, please try to CC all of the maintainers
> > > listed by get_maintainer.pl - you unfortunately managed to miss 2 of the
> > > 3 dt-binding maintainers :/ Perhaps you ran get_maintainer.pl using our
> > > vendor tree?
> > >   
> > > > Signed-off-by: Marius Cristea <marius.cristea@...rochip.com>
> > > > ---    
> > >   
> > > > +  vref-supply:
> > > > +    description:
> > > > +      Some devices have a specific reference voltage supplied on a different
> > > > +      pin to the other supplies. Needed to be able to establish channel scaling
> > > > +      unless there is also an internal reference available (e.g. mcp3564r)    
> > > 
> > > Should this be marked as a required property for the non-r devices that
> > > do not have an internal reference?
> > >   
> > > > +  microchip,hw-device-address:    
> > > 
> > > Hopefully Rob or Jonathan etc can chime in as to whether a common
> > > property exists for this type of thing...
> > >   
> > Nope. This is a new one for me - there are devices that work on a daisy chain
> > principle but I think this one works by encoding stuff in the actual message
> > which is unusual for SPI.  
> 
> Not something I've seen either.
> 
> >   
> > > > +    $ref: /schemas/types.yaml#/definitions/uint32
> > > > +    minimum: 0
> > > > +    maximum: 3
> > > > +    description:
> > > > +      The address is set on a per-device basis by fuses in the factory,
> > > > +      configured on request. If not requested, the fuses are set for 0x1.
> > > > +      The device address is part of the device markings to avoid
> > > > +      potential confusion. This address is coded on two bits, so four possible
> > > > +      addresses are available when multiple devices are present on the same
> > > > +      SPI bus with only one Chip Select line for all devices.    
> 
> What's this going to look like with more than one device? It would need 
> to be incorporated into 'reg' and the unit-address to work. Something 
> like this is 
> 
> spi {
>   device0@0 {
>     reg = <0>;
>     microchip,hw-device-address = <0>;
>   };
> 
>   device1@0 {
>     reg = <0>;
>     microchip,hw-device-address = <1>;
>   };
> };
> 
> That should throw warnings because you have 2 nodes at the same address 
> which is not good practice.
> 
> I think you need a spi mux in here with the mux addresses being the 
> microchip,hw-device-address values.

Something that looked like an spi-mux would be cute I'm not sure how
easy it would be to make it work given need to modify the messages
(rather sending extra ones before and after with a different chip select).

It would be nice if it were somewhat generic - so binding included which
bits would be replaced with the 'chip select' for the spi controller that
represents the mux.

Jonathan


> 
> Rob
>