Re: [PATCH v3 7/8] dt-bindings: mfd: Add img,boston-platform-regs

["Jiaxun Yang" <jiaxun.yang@flygoat.com>]

在2024年6月22日六月 下午7:12，Krzysztof Kozlowski写道：
> On 21/06/2024 17:51, Jiaxun Yang wrote:
>> 
>> 
>> 在2024年6月20日六月 上午7:40，Krzysztof Kozlowski写道：
>> [...]
>>>>
>>>> Hi Krzysztof,
>>>>
>>>> I believe U-Boot's implementation is correct. As per simple-mfd binding:
>>>>
>>>> ```
>>>> simple-mfd" - this signifies that the operating system should
>>>>   consider all subnodes of the MFD device as separate devices akin to how
>>>>   "simple-bus" indicates when to see subnodes as children for a simple
>>>>   memory-mapped bus.
>>>> ```
>>>>
>>>> This reads to me as "if you want sub nodes to be populated as devices
>>>> you need this."
>>>>
>>>> In our case there are "clock" and "reset" node sub nodes which should be
>>>> probed as regular device, so it's true for us.
>>>
>>> No, you already got comment from Rob.
>>>
>>> Your children depend on parent to provide IO address, so this is not
>>> simple-mfd. Rule for simple-mfd is that children do not rely on parent
>>> at all.
>>>
>> Hi Krzysztof,
>> 
>> Sorry but can I ask for clarification on "depend on parent to provide IO
>> address", do you mind explaining it a little bit? Does it mean children
>> should get regmap node from a phandle property, not the parent node? Or there
>> should be a reg property for child node to tell register offset etc?
>> 
>> There are way too much usage that children "depends" on parents somehow
>> in tree, so I want to confirm my understanding.
>
>
> Your driver relies on parent IO address to be provided - what's more to
> explain here? If parent does not provide syscon, does the child work?
> No. Therefore it is not suited for simple-mfd.
>
I can name too much "simple-mfd" devices that depending on parent to get
it's syscon, in fact it's true for almost all "simple-mfd" users now.

I greped RISC-V's DTS, and all two users have child nodes depends on parent
node to get "IO address". For "canaan,k210-sysctl" it's both "canaan,k210-clk"
and "canaan,k210-rst", for "starfive,jh7110-sys-syscon" that's "starfive,jh7110-pll".

If that's something prohibited, then we may need a generic driver in kernel to
catch all those syscon devices lost eligibility to "simple-mfd" to get their childs
populated.

We also need to think about how to handle "syscon-reboot-mode" and "syscon-reboot".
"syscon-reboot-mode" is explicitly relying on parent IO address, for "syscon-reboot"
the ability of not relying on parent node (regmap property) is deprecated as well.

I think we need to make those rules explicit, I'm happy to write a document or update
Devicetree specification about that, but I need to make it crystal clear to myself first.

Thanks
[...]
>
> Best regards,
> Krzysztof

-- 
- Jiaxun