Message-ID: <Z4WtbRG3gWQCwTmE@JSANTO12-L01.ad.analog.com>
Date: Mon, 13 Jan 2025 21:18:53 -0300
From: Jonathan Santos <jonath4nns@...il.com>
To: Jonathan Cameron <jic23@...nel.org>
Cc: dc7f6461-6fce-4dbd-9be4-f7814053e7dc@...libre.com,
	David Lechner <dlechner@...libre.com>,
	Jonathan Santos <Jonathan.Santos@...log.com>,
	linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, lars@...afoo.de,
	Michael.Hennerich@...log.com, robh@...nel.org, krzk+dt@...nel.org,
	conor+dt@...nel.org, marcelo.schmitt1@...il.com
Subject: Re: [PATCH v1 01/15] dt-bindings: iio: adc: ad7768-1: add
 synchronization over SPI property

On 01/12, Jonathan Cameron wrote:
> On Sat, 11 Jan 2025 19:34:14 -0300
> Jonathan Santos <jonath4nns@...il.com> wrote:
> 
> > On 01/07, David Lechner wrote:
> > > On 1/7/25 9:24 AM, Jonathan Santos wrote:  
> > > > Add adi,sync-in-spi property to enable synchronization over SPI.
> > > > This should be used in the case when the GPIO cannot provide a
> > > > pulse synchronous with the base MCLK signal.
> > > > 
> > > > User can choose between SPI, GPIO synchronization or neither of them,
> > > > but only if a external pulse can be provided, for example, by another
> > > > device in a multidevice setup.
> > > >   
> > > 
> > > While we are fixing up these bindings, we could add some more trivial things,
> > > like power supplies.
> > > 
> > > Also, the interrupt property could use a description since the chip has multiple
> > > output pins. I assume it means the /DRDY pin?
> > >   
> > 
> > Right! Yes, the interrupt pin refers to the /DRDY.
> > 
> > > > Signed-off-by: Jonathan Santos <Jonathan.Santos@...log.com>
> > > > ---
> > > >  .../bindings/iio/adc/adi,ad7768-1.yaml        | 24 ++++++++++++++++++-
> > > >  1 file changed, 23 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/Documentation/devicetree/bindings/iio/adc/adi,ad7768-1.yaml b/Documentation/devicetree/bindings/iio/adc/adi,ad7768-1.yaml
> > > > index 3ce59d4d065f..55cec27bfe60 100644
> > > > --- a/Documentation/devicetree/bindings/iio/adc/adi,ad7768-1.yaml
> > > > +++ b/Documentation/devicetree/bindings/iio/adc/adi,ad7768-1.yaml
> > > > @@ -47,6 +47,15 @@ properties:
> > > >        in any way, for example if the filter decimation rate changes.
> > > >        As the line is active low, it should be marked GPIO_ACTIVE_LOW.
> > > >  
> > > > +  adi,sync-in-spi:  
> > > 
> > > If this is saying that SYNC_OUT is connected to SYNC_IN, then I think the name
> > > should be something like adi,sync-in-sync-out. SPI seems irrelevant here since
> > > we should just be describing how things are wired up, not how it is being used.
> > > 
> > > But if we also need to consider the case where SYNC_OUT of one chip is connected
> > > to SYNC_IN of another chip, we might want to consider using trigger-source
> > > bindings instead (recently standardized in dtschema).
> > >   
> > 
> > Do you mean the trigger-sources used for LEDs? I can try to see if it works, but would it
> > handle the non-GPIO case? While testing a multidevice setup, I found it simpler to 
> > have a single device to manage everything. It lets us toggle the GPIO or /SYNC_OUT
> > without referencing another device and makes simultaneous buffered reads easier.
> 
> Daisy-chain mode (figure 131)?  In that case we normally end up with a single presented device
> with a 'lot' of channels. (See the electric car style battery charging chips, those can
> be chained in very large numbers!)
>

Actually, it is more like Figure 133 , but the premise is similar. We
have here a Quad setup.

> Probably similar for figure 133 (which is a dual SPI setup) as the SPI clock must
> be shared so we still see it over a single interface.
> 

We could view them as a single device with multiple channels, and since
the goal is to read them simultaneously with buffered reads, some parameters
such as sampling frequency should be equal to all devices.

However, there are some implications: If we do the above, we have
limitations in the customization of the "channels", they would have
the same filter, frequency modulator and scale (we plan to add support
for ADAQ776x-1 series, which include PGA and AAF gain).

To customize them separetely, we would need to assert only the
corresponding chip select, which is only possible with different
instances, as far as I know.

> If those are the only two options then keeping this within the driver is fine.
> For daisy chain there are examples in tree and it normally means we have to
> have a DT parameter that says how long the chain is, though we maybe can
> do that with per channel nodes as well if those make sense here.
> 
> Jonathan
> 

Those are the options in the datasheet and in hardware so far. I was 
considering other scenarios in case the user combine them differently.
I believe keping within the driver covers the main cases. 

> 
> > 
> > Maybe we could stick to synchronization within the chip for now.
> > 
> > > > +    description:
> > > > +      Enables synchronization of multiple devices over SPI. This property is
> > > > +      used when a signal synchronous to the base MCLK signal cannot be provided
> > > > +      via GPIO. It requires the SYNC_OUT pin to be connected to the SYNC_IN pin
> > > > +      on the ADC. In the case of multiple devices, the SYNC_OUT pin of one device
> > > > +      should be routed to the SYNC_IN pins of the other devices.
> > > > +    type: boolean
> > > > +
> > > >    reset-gpios:
> > > >      maxItems: 1
> > > >  
> > > > @@ -65,7 +74,6 @@ required:
> > > >    - vref-supply
> > > >    - spi-cpol
> > > >    - spi-cpha
> > > > -  - adi,sync-in-gpios
> > > >  
> > > >  patternProperties:
> > > >    "^channel@([0-9]|1[0-5])$":
> > > > @@ -89,6 +97,20 @@ patternProperties:
> > > >  allOf:
> > > >    - $ref: /schemas/spi/spi-peripheral-props.yaml#
> > > >  
> > > > +  # adi,sync-in-gpios and adi,sync-in-spi are mutually exclusive (neither is also valid)
> > > > +  - if:
> > > > +      required:
> > > > +        - adi,sync-in-gpios
> > > > +    then:
> > > > +      properties:
> > > > +        adi,sync-in-spi: false
> > > > +  - if:
> > > > +      required:
> > > > +        - adi,sync-in-spi
> > > > +    then:
> > > > +      properties:
> > > > +        adi,sync-in-gpios: false  
> > > 
> > > I think this can be simplified to using oneOf: to provide XOR validation
> > >   
> > 
> > Right!
> > 
> > > > +
> > > >  unevaluatedProperties: false
> > > >  
> > > >  examples:  
> > >   
> 

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