linux-kernel - Re: fwnode_for_each_child_node() and OF backend discrepancy

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Date:   Wed, 29 Jun 2022 14:01:00 +0100
From:   Grant Likely <grant.likely@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Michael Walle <michael@...le.cc>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Saravana Kannan <saravanak@...gle.com>
Subject: Re: fwnode_for_each_child_node() and OF backend discrepancy



> On 29 Jun 2022, at 11:50, Rafael J. Wysocki <rafael@...nel.org> wrote:
> 
> On Tue, Jun 28, 2022 at 12:32 PM Krzysztof Kozlowski
> <krzysztof.kozlowski@...aro.org> wrote:
>> 
>> On 27/06/2022 15:33, Rafael J. Wysocki wrote:
>>> On Mon, Jun 27, 2022 at 3:08 PM Krzysztof Kozlowski
>>> <krzysztof.kozlowski@...aro.org> wrote:
>>>> 
>>>> On 27/06/2022 14:49, Michael Walle wrote:
>>>>> Hi,
>>>>> 
>>>>> I tired to iterate over all child nodes, regardless if they are
>>>>> available
>>>>> or not. Now there is that handy fwnode_for_each_child_node() (and the
>>>>> fwnode_for_each_available_child_node()). The only thing is the OF
>>>>> backend
>>>>> already skips disabled nodes [1], making fwnode_for_each_child_node()
>>>>> and
>>>>> fwnode_for_each_available_child_node() behave the same with the OF
>>>>> backend.
>>>>> 
>>>>> Doesn't seem to be noticed by anyone for now. I'm not sure how to fix
>>>>> that
>>>>> one. fwnode_for_each_child_node() and also fwnode_get_next_child_node()
>>>>> are
>>>>> used by a handful of drivers. I've looked at some, but couldn't decide
>>>>> whether they really want to iterate over all child nodes or just the
>>>>> enabled
>>>>> ones.
>>>> 
>>>> If I get it correctly, this was introduced  by 8a0662d9ed29 ("Driver
>>>> core: Unified interface for firmware node properties")
>>>> .
>>> 
>>> Originally it was, but then it has been reworked a few times.
>>> 
>>> The backend callbacks were introduced by Sakari, in particular.
>> 
>> I see you as an author of 8a0662d9ed29 which adds
>> device_get_next_child_node() and uses of_get_next_available_child()
>> instead of of_get_next_child(). Although it was back in 2014, so maybe
>> it will be tricky to get original intention. :)
> 
> The OF part of this was based on Grant's suggestions.  My
> understanding at that time was that this was the right thing to do for
> OF and nobody told me otherwise.
> 
>> Which commit do you mean when you refer to Sakari's work?
> 
> 3708184afc77 device property: Move FW type specific functionality to
> FW specific files
> 
> However, it didn't change the "available" vs "any" behavior for OF.

Back in the mists of time indeed. I don’t remember anything specific about all/available variants of the fwnode_ helpers. Auditing the existing users is probably needed to decide whether or not it can be changed.

g.

> 
>>> 
>>>> The question to Rafael - what was your intention when you added
>>>> device_get_next_child_node() looking only for available nodes?
>>> 
>>> That depends on the backend.
>> 
>> We talk about OF backend. In your commit device_get_next_child_node for
>> OF uses explicitly available node, not any node.
> 
> Yes, it does.
> 
> If that doesn't match the cases in which it is used, I guess it can be adjusted.
> 
>>> fwnode_for_each_available_child_node() is more specific and IIRC it
>>> was introduced for fw_devlink (CC Saravana).
>>> 
>>>> My understanding is that this implementation should be consistent with
>>>> OF implementation, so fwnode_get_next_child_node=get any child.
>>> 
>>> IIUC, the OF implementation is not consistent with the
>>> fwnode_get_next_child_node=get any child thing.
>>> 
>>>> However maybe ACPI treats it somehow differently?
>>> 
>>> acpi_get_next_subnode() simply returns the next subnode it can find.
> 
> I guess that the confusion is related to what "available" means for ACPI and OF.
> 
> In the ACPI case it means "this a device object corresponding to a
> device that is present".  In OF it is related to the "status" property
> AFAICS.