Re: [RFC PATCH 02/77] Introduce v18 dtb version

On Sun, Jan 18, 2026 at 11:18=E2=80=AFPM David Gibson
<[email protected]> 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 <[email protected]> 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 tre=
e and
> > > >    orphan node tree)
> > >
> > > So, once this patch is applied, the rest of the series pretty much ha=
s
> > > 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 thi=
s
> > > 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 t=
ag'
> > 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 d=
efinition.
> >           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 byt=
es)
> >             of data available just after this cell (including any paddi=
ng
> >             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?

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

Rob