linux-kernel - Re: [PATCH] libfdt: place new nodes & properties after the parent's ones

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Message-ID: <1cc6258c-5bf3-3d48-2d6d-1a7176af1459@samsung.com>
Date:   Wed, 12 Feb 2020 07:57:03 +0100
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Rob Herring <robh@...nel.org>,
        David Gibson <david@...son.dropbear.id.au>
Cc:     Devicetree Compiler <devicetree-compiler@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH] libfdt: place new nodes & properties after the parent's
 ones

Hi Rob,

On 11.02.2020 21:29, Rob Herring wrote:
> On Mon, Feb 10, 2020 at 5:44 PM David Gibson
> <david@...son.dropbear.id.au> wrote:
>> On Mon, Feb 10, 2020 at 12:40:19PM +0100, Marek Szyprowski wrote:
>>> Hi David,
>>>
>>> On 05.02.2020 06:45, David Gibson wrote:
>>>> On Tue, Feb 04, 2020 at 01:58:44PM +0100, Marek Szyprowski wrote:
>>>>> While applying dt-overlays using libfdt code, the order of the applied
>>>>> properties and sub-nodes is reversed. This should not be a problem in
>>>>> ideal world (mainline), but this matters for some vendor specific/custom
>>>>> dtb files. This can be easily fixed by the little change to libfdt code:
>>>>> any new properties and sub-nodes should be added after the parent's node
>>>>> properties and subnodes.
>>>>>
>>>>> Signed-off-by: Marek Szyprowski <m.szyprowski@...sung.com>
>>>> I'm not convinced this is a good idea.
>>>>
>>>> First, anything that relies on the order of properties or subnodes in
>>>> a dtb is deeply, fundamentally broken.  That can't even really be a
>>>> problem with a dtb file itself, only with the code processing it.
>>> I agree about the properties, but generally the order of nodes usually
>>> implies the order of creation of some devices or objects.
>> Huh?  From the device tree client's point of view the devices just
>> exist - the order of creation should not be visible to it.
> I'm not sure if downstream is different, but upstream this stems from
> Linux initcalls being processed in link order within a given level.
> It's much better than it used to be, but short of randomizing the
> ordering, I'm not sure we'll ever find and fix all these hidden
> dependencies.

Downstream is probably much worse, because I've seen a lots of custom 
code iterating over the nodes and doing its initialization.

>>> This sometimes
>>> has some side-effects.
>> If those side effects matter, your code is broken.  If you need to
>> apply an order to nodes, you should be looking at 'reg' or other
>> properties.
> The general preference is to sort by 'reg'. And we try to catch and
> reject any node re-ordering patches.

If one applies an dt-overlay with current libfdt, the order of the all 
nodes from that overlay is reversed.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland