Re: [PATCH v4 3/5] dt-bindings: iio: dac: Add adi,ltc2664.yaml

[David Lechner <dlechner@baylibre.com>]

>>> +  adi,manual-span-operation-config:
>>> +    description:
>>> +      This property must mimic the MSPAN pin configurations. By tying the
>> MSPAN
>>> +      pins (MSP2, MSP1 and MSP0) to GND and/or VCC, any output range can
>> be
>>> +      hardware-configured with different mid-scale or zero-scale reset options.
>>> +      The hardware configuration is latched during power on reset for proper
>>> +      operation.
>>> +        0 - MPS2=GND, MPS1=GND, MSP0=GND (+-10V, reset to 0V)
>>> +        1 - MPS2=GND, MPS1=GND, MSP0=VCC (+-5V, reset to 0V)
>>> +        2 - MPS2=GND, MPS1=VCC, MSP0=GND (+-2.5V, reset to 0V)
>>> +        3 - MPS2=GND, MPS1=VCC, MSP0=VCC (0V to 10, reset to 0V)
>>> +        4 - MPS2=VCC, MPS1=GND, MSP0=GND (0V to 10V, reset to 5V)
>>> +        5 - MPS2=VCC, MPS1=GND, MSP0=VCC (0V to 5V, reset to 0V)
>>> +        6 - MPS2=VCC, MPS1=VCC, MSP0=GND (0V to 5V, reset to 2.5V)
>>> +        7 - MPS2=VCC, MPS1=VCC, MSP0=VCC (0V to 5V, reset to 0V, enables
>> SoftSpan)
>>> +    $ref: /schemas/types.yaml#/definitions/uint32
>>> +    enum: [0, 1, 2, 3, 4, 5, 6, 7]
>>
>> Can you explain why this property is required, when below there's one that sets
>> the ranges in microvolts? Isn't the only new information that this provides the
>> reset values (in a few cases that it is not 0).
>> What am I missing?
> 
> For specifying output range and reset options without relying on software initialization
> routines, and also for enabling the softspan feature, I think this property seems essential.

So in other words, this describes how the MSP pins are hardwired and
the per-channel adi,output-range-microvolt is only permissible if

	 adi,manual-span-operation-config = <7>;

(or omitted since 7 is the default)

because in that case each individual pin could have a different
required range based on what is wire up to it?

But if adi,manual-span-operation-config is anything other than 7,
then adi,output-range-microvolt should be not allowed since all
channels will have the same range because of the hard-wired pins.

correct?

The description could probably just be simplified to say that
this describes how the 3 pins are hardwired and to see Table 4
in the datasheet to understand the actual implications rather
than reproducing that table here.

But I do agree that we need both properties. I think we are
just missing:

- if:
    properties:
      adi,manual-span-operation-config:
        const: 7
  then:
    patternProperties:
      "^channel@[0-3]$":
       adi,output-range-microvolt: false

(not tested - may need two ifs, one with

- if:
    required:
      - adi,manual-span-operation-config
    properties:
      adi,manual-span-operation-config:
        const: 7

and one with

- if:
    not:
      required:
        - adi,manual-span-operation-config

to make it work properly)

> 
>>> +    default: 7
>>> +
>>> +  io-channels:
>>> +    description:
>>> +      ADC channel to monitor voltages and temperature at the MUXOUT pin.
>>> +    maxItems: 1
>>> +
>>> +  '#address-cells':
>>> +    const: 1
>>> +
>>> +  '#size-cells':
>>> +    const: 0
>>> +
>>> +patternProperties:
>>> +  "^channel@[0-3]$":
>>> +    type: object
>>> +    additionalProperties: false
>>> +
>>> +    properties:
>>> +      reg:
>>> +        description: The channel number representing the DAC output channel.
>>> +        maximum: 3
>>> +
>>> +      adi,toggle-mode:
>>> +        description:
>>> +          Set the channel as a toggle enabled channel. Toggle operation enables
>>> +          fast switching of a DAC output between two different DAC codes
>> without
>>> +          any SPI transaction.
>>> +        type: boolean
>>> +
>>> +      adi,output-range-microvolt:
>>> +        description: Specify the channel output full scale range.
>>> +        oneOf:
>>> +          - items:
>>> +              - const: 0
>>> +              - enum: [5000000, 10000000]
>>> +          - items:
>>> +              - const: -5000000
>>> +              - const: 5000000
>>> +          - items:
>>> +              - const: -10000000
>>> +              - const: 10000000
>>> +          - items:
>>> +              - const: -2500000
>>> +              - const: 2500000
>>> +
>>> +    required:
>>> +      - reg
>>> +      - adi,output-range-microvolt

And adi,output-range-microvolt should not be required. When SoftSpan
is not available (because MSP != 0x7), then the range is determined
by adi,manual-span-operation-config.

And even when adi,manual-span-operation-config = <7>, there is still
a default range, so adi,output-range-microvolt should still not be
required in that case.