linux-kernel - Re: [PATCH v16 1/3] dt-bindings: i2c: aspeed: support for AST2600-i2cv2

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Message-ID: <52943e49aaea7bb6def5bc51dfd57392b6ae66e4.camel@ozlabs.org>
Date: Wed, 10 Sep 2025 16:31:12 +0800
From: Jeremy Kerr <jk@...abs.org>
To: Krzysztof Kozlowski <krzk@...nel.org>, Ryan Chen
 <ryan_chen@...eedtech.com>
Cc: "robh@...nel.org" <robh@...nel.org>, "conor+dt@...nel.org"
	 <conor+dt@...nel.org>, "andriy.shevchenko@...ux.intel.com"
	 <andriy.shevchenko@...ux.intel.com>, "andi.shyti@...nel.org"
	 <andi.shyti@...nel.org>, "linux-aspeed@...ts.ozlabs.org"
	 <linux-aspeed@...ts.ozlabs.org>, "devicetree@...r.kernel.org"
	 <devicetree@...r.kernel.org>, "openbmc@...ts.ozlabs.org"
	 <openbmc@...ts.ozlabs.org>, Mo Elbadry <elbadrym@...gle.com>, 
	"linux-kernel@...r.kernel.org"
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	"p.zabel@...gutronix.de"
	 <p.zabel@...gutronix.de>, "krzk+dt@...nel.org" <krzk+dt@...nel.org>, 
	"linux-arm-kernel@...ts.infradead.org"
	 <linux-arm-kernel@...ts.infradead.org>, "linux-i2c@...r.kernel.org"
	 <linux-i2c@...r.kernel.org>
Subject: Re: [PATCH v16 1/3] dt-bindings: i2c: aspeed: support for
 AST2600-i2cv2

Hi Krzysztof,

> You trimmed response and brought some very old thread which does not
> exist in my inbox.
> 
> I have absolutely no clue what this refers to.

OK, reconstructing the relevant parts of that thread - Ryan providing
background on the old/new register interfaces (trimmed a little for
brevity; original context at [1] if you need):

>On 24/03/2025 09:30, Ryan Chen wrote:
>>> Subject: Re: [PATCH v16 1/3] dt-bindings: i2c: aspeed: support for
>>> AST2600-i2cv2
>>>
>>> On 19/03/2025 12:12, Ryan Chen wrote:
>>>>> Subject: Re: [PATCH v16 1/3] dt-bindings: i2c: aspeed: support for
>>>>> AST2600-i2cv2
>>>>>
>>>>> On 17/03/2025 10:21, Ryan Chen wrote:
>>>>>>> Neither this.
>>>>>>>
>>>>>>> So it seems you describe already existing and documented I2C, but
>>>>>>> for some reason you want second compatible. The problem is that you
>>>>>>> do not provide reason from the point of view of bindings.
>>>>>>>
>>>>>>> To summarize: what your users want - don't care. Start properly
>>>>>>> describing hardware and your SoC.
>>>>>>
>>>>>> OK, for ast2600 i2c controller have two register mode setting.
>>>>>> One, I call it is old register setting, that is right now
>>>>>> i2c-aspeed.c .compatible = "aspeed,ast2600-i2c-bus", And there
>>>>>> have a global register that can set i2c controller as new mode
>>>>>> register set. That I am going to drive. That I post is all
>>>>>> register in new an old register list.
>>>>>>
>>>>>> For example,
>>>>>> Global register [2] = 0 => i2c present as old register set Global
>>>>>> register [2] = 1 => i2c present as new register set
>>>>> It's the same device though, so the same compatible.
>>>>
>>>> Sorry, it is different design, and it share the same register
>>>> space. So that the reason add new compatible "aspeed,ast2600-i2cv2"
>>>> for this driver. It is different register layout.
>>>
>>> Which device is described by the existing "aspeed,ast2600-i2c-bus"
>>> compatible? And which device is described by new compatible?
>>>
>> On the AST2600 SoC, there are up to 16 I2C controller instances (I2C1 ~ I2C16).
>
> So you have 16 same devices.
>
>> Each of these controllers is hardwired at the SoC level to use either
>> the legacy register layout or the new v2 register layout. The mode is
>> selected by a bit in the global register, these represent two
>> different hardware blocks: "aspeed,ast2600-i2c-bus" describes
>> controllers using the legacy register layout. "aspeed,ast2600-i2cv2"
>> describes controllers using the new register layout
> 
> Which part of "same device" is not clear? You have one device, one
> compatible. Whatever you do with register layout, is already defined by
> that compatible. It does not matter that you forgot to implement it in
> the Linux kernel.

So, I'm trying to pick up (from Ryan) on whether we're actually dealing
with separate devices here; that was ambiguous in his responses.

To me, it seems like we do have separate IP cores, just multiplexed to
the same MMIO space. And if so, what the preference on binding
implementation is, particularly with different SoCs having either only
the "old", only the "new", or a switchable set of both.

Hence my query:

> Given there are actual behavioural differences between the two
> peripherals - beyond just the register set - that would seem to indicate
> separate binding types (+ a syscon mux control) to me, but I'm keen to
> hear any other options.
>
> Krzysztof, if that is the case, any thoughts on the representation of
> separate bindings?

- given we may not be dealing with "the same device" in actual hardware,
in reference to Ryan's proposed compatible split between the two.

Cheers,


Jeremy

[1]: https://lore.kernel.org/all/20250224055936.1804279-2-ryan_chen@aspeedtech.com/t/#u