linux-kernel - Re: [PATCH 1/3] dt-bindings: iio: adc: add AD4695 and similar ADCs

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Message-ID: <d802086d-5f88-49c3-996c-ada251043187@baylibre.com>
Date: Thu, 13 Jun 2024 15:09:40 -0500
From: David Lechner <dlechner@...libre.com>
To: Rob Herring <robh@...nel.org>, Nuno Sá
 <noname.nuno@...il.com>
Cc: Krzysztof Kozlowski <krzk@...nel.org>, Jonathan Cameron
 <jic23@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>,
 Michael Hennerich <michael.hennerich@...log.com>,
 Nuno Sá <nuno.sa@...log.com>,
 Jonathan Corbet <corbet@....net>, linux-iio@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org
Subject: Re: [PATCH 1/3] dt-bindings: iio: adc: add AD4695 and similar ADCs

On 6/13/24 2:43 PM, Rob Herring wrote:
> On Thu, Jun 13, 2024 at 05:11:48PM +0200, Nuno Sá wrote:
>> On Thu, 2024-06-13 at 09:39 -0500, David Lechner wrote:
>>> On 6/13/24 9:18 AM, Krzysztof Kozlowski wrote:
>>>> On 13/06/2024 15:57, David Lechner wrote:
>>>>>
>>>>>>
>>>>>>> +          - const: adi,ad4695
>>>>>>> +      - items:
>>>>>>> +          - const: adi,ad4697-wlcsp
>>>>>>> +          - const: adi,ad4697
>>>>>>> +      # same chips with higher max sample rate
>>>>>
>>>>> I suppose one could make the argument that the programming model is
>>>>> the same on these too, but the maximum sampling frequency does seem
>>>>> like an important bit of information so that you don't try to set
>>>>> the conversion trigger rate too high.
>>>>>
>>>>
>>>> which property is that? I don't see differences in the driver, so I
>>>> don't get how these wlcsp compatibles allow you to control value of
>>>> conversion trigger.
>>>
>>> This comment is unrelated to the package type (WLCSP or LFCSP).
>>>
>>> What I mean is that e.g. AD4695 and AD4696 are virtually identical
>>> other than the maximum allowable sample rate (500 kSPS or 1 MSPS).
>>>
>>> So my thinking was that it would make sense to have:
>>>
>>> 	compatible = "ad4695";
>>>
>>> for the lower sample rate chip and
>>>
>>> 	compatible = "ad4696", "ad4695";
>>>
>>> for the higher sample rate chip since ad4696 can do everything
>>> that ad4695 does plus a bit more.
>>>
>>
>> IMO, that would make sense yes. If the higher sample rate chip fallsback, it will
>> still work but not at full speed. The other way around is the one that we can't allow
>> naturally.
>>
>> But possibly dumb question now... since both devices will be supported at the same
>> time, do we actually care about having the fallback compatible? My understanding of
>> the fallback story is that we may load a DTS in an older kernel where chip A is
>> supported but chip B is not and it is ok for chip B to fallback to chip A. Since
>> these devices will be supported at the same time, do we need to care? Unless out of
>> tree stuff enters the equation?
> 
> Yeah, it doesn't really matter much in that case.
> 
>> Or is there another usecase that I'm not aware about (or maybe it just makes sense to
>> document properly...)?
> 
> Somewhat I guess. Perhaps if there's a 3rd chip with higher rate, then 
> it will be more obvious what to do and we don't have to have this 
> discussion again for it. :)
> 
> Rob

It sounds like maybe the best thing to do here then is just keep it simple?

Like this:

  compatible:
    enum:
      - adi,ad4695
      - adi,ad4696
      - adi,ad4697
      - adi,ad4698