Re: [PATCH v2 1/2] dt-bindings: iio: adc: adding MCP3564 ADC

[Jonathan Cameron <jic23@kernel.org>]

On Sat, 15 Jul 2023 11:28:03 +0100
Conor Dooley <conor@kernel.org> wrote:

> Hey,
> 
> On Fri, Jul 14, 2023 at 06:00:50PM +0300, marius.cristea@microchip.com wrote:
> > From: Marius Cristea <marius.cristea@microchip.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 (Microchip's
> > MCP3461, MCP3462, MCP3464, MCP3461R, MCP3462R,
> > MCP3464R, MCP3561, MCP3562, MCP3564, MCP3561R,
> > MCP3562R and MCP3564R analog to digital converters).
> > 
> > Signed-off-by: Marius Cristea <marius.cristea@microchip.com>  
> 
> This looks good to me, other than the custom property, for which I can't
> tell if a consensus was reached on last time around.
> 
> > +  microchip,hw-device-address:
> > +    $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.
> > +      Each device communication starts by a CS falling edge, followed by the
> > +      clocking of the device address (BITS[7:6] - top two bits of COMMAND BYTE
> > +      which is first one on the wire).  
> 
> On the last version, the last comment I could find on lore was
> https://lore.kernel.org/all/20230609184149.00002766@Huawei.com/
> where Jonathan and Rob were discussing whether or not a spi-mux type of
> thing could work, but it does not seem to have ended conclusively.
> 
> Rob or Jonathan, would you mind commenting on that?

Sure - as far as I'm concerned - it looks like it should be possible to do something
generic, but without a prototype it's hard to be sure how fiddly that will be.

+CC Mark Brown who might be able to give a more informed answer to whether such a
thing would work / be easy to implement.

I've no idea how common this trick is.  If it's a one off, may not be worth the bother
of a more generic mux like binding whether that is the more elegant solution or
not.

> 
> There was also a comment from Jonathan:
> > > +  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)
> > > +  
> > 
> > From a quick glance at a random datasheet, looks like there additional power supplies
> > that should be required.
> > 
> > If this is required for some devices, I'd expect to see the binding enforce
> > that with some required entries conditioned on the compatibles rather than as
> > documentation. If there are devices where it isn't even optional then the binding
> > should enforce that as well.  
> 
> The binding does now enforce the vref supply where relevant, but it
> sounds like you were looking more supplies to be documented Jonathan?
> (AVdd, DVdd etc)

Exactly.

> 
> Thanks,
> Conor.