Re: [RFC PATCH 06/77] Add support for FDT_REF_LOCAL dtb tag

[Rob Herring]

On Mon, Jan 12, 2026 at 8:20 AM Herve Codina <herve.codina@xxxxxxxxxxx> wrote:
>
> FDT_REF_LOCAL dtb tag is a meta-data tag attached to a property.
>
> It indicates that the property defined before this tag (FDT_PROP) uses a
> phandle value and the node related to this phandle value is local (i.e.
> the node is present in the device-tree blob).
>
> It is followed by one value:
>  - offset (32bit):
>      Offset in the property data where the phandle is available.
>
> Example:
>   FDT_PROP 0x00000008 xxxxxxxx 0xca 0xfe 0xde 0xca 0x01 0x02 0x03 0x04
>   FDT_REF_LOCAL 0x00000004
>
>   This means that at the offset 4 of the property data, the value
>   (0x01020304) is a phandle and the related node is available in the
>   dtb.
>
>   This is what is encoded in the dtb when the related dts has a property
>   with the value set to <0xcafedeca &foo> with 'foo' a reference to an
>   existing node where the phandle value is 0x01020304.
>
> If several local phandles are used in the property data, several
> FDT_REF_LOCAL are present after the FDT_PROP tag. Each of them points
> with its offset value to the position of one phandle.
>
> For instance, if a first property with 8 bytes of data has a phandle
> value at offset 4 and a second property with 16 bytes of data has
> phandle values at offset 0 and 8, the following tags sequence is
> present:
>   FDT_PROP 0x00000008 xxxxxxxx <data bytes>
>   FDT_REF_LOCAL 0x00000004
>   FDT_PROP 0x00000010 xxxxxxxx <data bytes>
>   FDT_REF_LOCAL 0x00000000
>   FDT_REF_LOCAL 0x00000008

To follow up on my desire to both be easily extended and have more
type info, I have something like this in mind:

FDT_TYPE_INFO 0x10 FDT_REF_LOCAL 0x0 FDT_TYPE_U32 0x4 FDT_REF_LOCAL
0x8 FDT_TYPE_U32 0xc

Length is 16 because I would do u16 for the types and lengths. I'm not
sure what the max property length is. In theory 2^32, but I suspect
we'd run into other issues (e.g. libfdt offsets are 32-bit on 32-bit
systems). We could also do 8 bits for type and 24 bits for offset.
Offset could also be relative to the prior offset.

Rob