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

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Date:   Tue, 11 Feb 2020 14:29:22 -0600
From:   Rob Herring <robh@...nel.org>
To:     David Gibson <david@...son.dropbear.id.au>
Cc:     Marek Szyprowski <m.szyprowski@...sung.com>,
        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

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.

> > 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.

Rob