Message-ID: <aXq6fKUytYzpMQAT@zatzit>
Date: Thu, 29 Jan 2026 12:40:12 +1100
From: David Gibson <david@...son.dropbear.id.au>
To: Rob Herring <robh@...nel.org>
Cc: Herve Codina <herve.codina@...tlin.com>,
	Krzysztof Kozlowski <krzk@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Ayush Singh <ayush@...gleboard.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	devicetree-compiler@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree-spec@...r.kernel.org,
	Hui Pu <hui.pu@...ealthcare.com>,
	Ian Ray <ian.ray@...ealthcare.com>,
	Luca Ceresoli <luca.ceresoli@...tlin.com>,
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [RFC PATCH 02/77] Introduce v18 dtb version

On Tue, Jan 20, 2026 at 02:38:45PM -0600, Rob Herring wrote:
> On Sun, Jan 18, 2026 at 11:18 PM David Gibson
> <david@...son.dropbear.id.au> wrote:
> >
> > On Fri, Jan 16, 2026 at 10:09:34AM +0100, Herve Codina wrote:
> > > Hi David,
> > >
> > > On Thu, 15 Jan 2026 11:12:49 +1100
> > > David Gibson <david@...son.dropbear.id.au> wrote:
> > >
> > > > On Mon, Jan 12, 2026 at 03:18:52PM +0100, Herve Codina wrote:
> > > > > This v18 version will add support for
> > > > >  - metadata in device-tree blobs in order to have a better handling of
> > > > >    phandles and unresolved references.
> > > > >  - Addon device-tree blob (successor of device-tree overlay)
> > > > >  - Import and export symbols feature
> > > > >  - multiple trees in a addon device-tree blob (i.e. root device tree and
> > > > >    orphan node tree)
> > > >
> > > > So, once this patch is applied, the rest of the series pretty much has
> > > > to be applied "atomically" - otherwise a version built in the interim
> > > > will be lying in saying that it supports v18.
> > > >
> > > > I therefore suggest moving any changes that *can* be moved before this
> > > > patch, should be moved before this patch.  That will assist in
> > > > reviewing and merging the series piecemeal, rather than as a single
> > > > giant blob.
> > > >
> > > >
> > > > Regarding the content itself.  It seems like this is a pretty major
> > > > change to the dtb format - maybe that would suggest bumping the
> > > > version by more than one (e.g. like we went from v3 to v16 in the
> > > > past).
> > >
> > > I see your point.
> > >
> > > Maybe the Rob's idea related to 'unknown tag' and the suggestion I did [1]
> > > related to the generic tag value definition to support those 'unknown tag'
> > > could help here.
> >
> > Having a standard encoding of tag length so unknown tags can be
> > skipped is a reasonable idea.  I think you do need provision to mark a
> > tag as "safe to ignore" or not - e.g. something like FDT_BEGIN_NODE
> > could never be safely ignored.
> >
> > > As a reminder here, this generic tag value definition consist in:
> > > --- 8< ---
> > > A tag value is on 32bits. We can define the structure of this value.
> > >   - bit 31 (msb):
> > >      - 0: This is not a new kind to tag and so it doesn't follow this definition.
> > >           All existing tags are in this category
> > >      - 1: New kind of tag adopting this definition
> > >
> > >   - bits 30..28:
> > >      tag data length encoding
> > >      0b000: No data related to the tag
> > >      0b001: 1 data cell (u32) directly follows the tag
> > >      0b010: 2 data cells (2 u32) directly follow the tag
> > >      ...
> > >      0b110: 6 data cells (6 u32) directly follow the tag
> > >      0b111: Tag is followed by a cell (u32) indicating the size (in bytes)
> > >             of data available just after this cell (including any padding
> > >             if needed).
> >
> > I'd suggesting giving a byte length not including alignment padding.
> > That way if you wanted to encode a bytestring in there, you wouldn't
> > need a way of encoding the unpadded length in adddition to the
> > standard way encoding the padded length.
> >
> > >           Because this size include some possible padding, its value is a
> > >             multiple of 4 bytes.
> > >             The offset of the tag + 4 + size points to the next tag.
> > >
> > >
> > >   - bit 27..0
> > >      tag specific identifier
> > > --- 8< ---
> > >
> > > I mean dtb version v20 could be:
> > >
> > >  - New header size with dt_flags added in the header (if this new field is
> > >    kept).
> > >
> > >  - Support for the generic tag values and so the notion of 'unknown tag'
> > >
> > > With that done, everything else added afterward will have no impact on the
> > > dtb format itself.
> >
> > Well... maybe.  It's not entirely clear to me whether all the new tags
> > can be safely ignored by something that doesn't understand them.
> > e.g. a consumer can't safely ignore the tags which give unresolved
> > phandle references if it then expects the phandle values in the actual
> > property values to be correct.
> 
> I think we'd want some higher level "this is an addon or base DT" than
> presence of tags. Maybe that's just the version. Perhaps a new header
> field to say this is a base or addon DT. Or both?

I think Herve's flags proposed flags field does that.  I tend to
prefer the idea of using new and different magic numbers for the
variant forms though - makes it really clear that they're a different
thing from a "normal" fdt.

> 
> Everything in this series intended for the base DT should be safe to
> ignore just as __symbols__ (and __local_fixups__ if you add /plugin/)
> is safe to ignore.

That kind of depends what you're doing with the DT.  If you need to do
phandle lookups, you can't safely ignore the fixups.

> It's only software that understands and wants to
> use the new "addons" that needs to understand.
> 
> Rob
> 

-- 
David Gibson (he or they)	| 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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