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

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Message-ID: <20200210234406.GH22584@umbus.fritz.box>
Date:   Tue, 11 Feb 2020 10:44:06 +1100
From:   David Gibson <david@...son.dropbear.id.au>
To:     Marek Szyprowski <m.szyprowski@...sung.com>
Cc:     devicetree-compiler@...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 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.

> 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.

> For comparison, the other lib used for fdt manipulation (libufdt) 
> applies overlays in a such way, that the order of properties and nodes 
> is not reversed.
> 
> > I'm also concerned this could have a negative performance impact,
> > since it has to skip over a bunch of existing things before adding the
> > new one.  On the other hand, that may be offset by the fact that it
> > will reduce the amount of stuff that needs to be memmove()ed later on.
> 
> This code is already slow (especially in the way the uboot's use it for 
> 'fdt apply' command), but in practice I've didn't observe negative 
> impact on the performance of applying large overlays at all.

I'm going to need numbers, not just "I didn't see anything".

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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