Re: [RFC PATCH 09/15] Introduce structured tag value definition

Luca Ceresoli
2026-04-01T15:11:39Z

On Tue Feb 10, 2026 at 6:33 PM CET, Herve Codina wrote:
> The goal of structured tag values is to ease the introduction of new
> tags in future releases with the capability for an already existing
> release to ignore those structured tags. In order to do that data length
> related to the unknown tag needs to be identify.
^
identified

> Also a flag is present
"Also add a flag"

> to tell an old release if this tag can be simply skipped or must lead to
> an error.
>
> Structured tag value is defined on 32bit and is defined as follow:
>
> Bits | 31 | 30 | 29 28 | 27 0|
> ------+----+----------+-------------------+--------+
> Fields| 1 | CAN_SKIP | DATA_LNG_ENCODING | TAG_ID |
> ------+----+----------+-------------------+--------+
>
> Bit 31 is always set to 1 to identified a structured tag value.
^
identify

> Bit 30 (CAN_SKIP) is set to 1 if the tag can be safely ignore when its
^
ignored

> TAG_ID value is not a known value (unknown tag). If the CAN_SKIP bit is
> set to 0 this tag must not be ignored and an error should be reported
> when its TAG_ID value is not a known value (unknown tag).
>
> Bits 29..28 (DATA_LNG_ENCODING) indicates the length of the data related

I think "LEN" is more common than "LNG".

> to the tag. Following values are possible:
> - 0b00: No data.
> The tag is followed by the next tag value.
>
> - 0b01: 1 cell data
> The tag is followed by a 1 cell (u32) data. The next tag is
> available after this cell.
>
> - 0b10: 2 cells data
> The tag is followed by a 2 cells (2 * u32) data. The next tag
> is available after those two cells.
>
> - 0b11: Data length encoding
> The tag is followed by a cell (u32) indicating the size of the
> data. This size is given in bytes. Data are available right
> after this cell.
>
> The next tag is available after the data. Padding is present
> after the data in order to have the next tag aligned on 32bits.
> This padding is not included in the size of the data.
>
> Bits 27..0 (TAG_ID) is the tag identifier defining a specific tag.
>
> Introduce the structured tag values definition and some specific tags
> reserved for tests based on this structure definition.
>
> Signed-off-by: Herve Codina <herve.codina@bootlin.com>
> ---
> libfdt/fdt.h | 23 +++++++++++++++++++++++
> 1 file changed, 23 insertions(+)
>
> diff --git a/libfdt/fdt.h b/libfdt/fdt.h
> index a07abfc..2e07599 100644
> --- a/libfdt/fdt.h
> +++ b/libfdt/fdt.h
> @@ -49,6 +49,7 @@ struct fdt_property {
>
> #define FDT_MAGIC 0xd00dfeed /* 4: version, 4: total size */
> #define FDT_TAGSIZE sizeof(fdt32_t)
> +#define FDT_CELLSIZE sizeof(fdt32_t)
>
> #define FDT_BEGIN_NODE 0x1 /* Start node: full name */
> #define FDT_END_NODE 0x2 /* End node */
> @@ -57,6 +58,28 @@ struct fdt_property {
> #define FDT_NOP 0x4 /* nop */
> #define FDT_END 0x9
>
> +/* Tag values flags */
> +#define FDT_TAG_STRUCTURED (1<<31)
> +#define FDT_TAG_SKIP_SAFE (1<<30)

This is called CAN_SKIP in the commit message and SKIP_SAFE here. Using a
consistent name would be better IMO.

> +#define FDT_TAG_DATA_MASK (3<<28)
> +#define FDT_TAG_DATA_NONE (0<<28)
> +#define FDT_TAG_DATA_1CELL (1<<28)
> +#define FDT_TAG_DATA_2CELLS (2<<28)
> +#define FDT_TAG_DATA_LNG (3<<28)

I find _LNG (or _LEN) misleading: this is not the length, but rather an
enum value telling you the length is stored in the next cell. What about
FDT_TAG_DATA_VARLEN?

Otherwise looks good.

Luca

--
Luca Ceresoli, Bootlin
Embedded Linux and Kernel engineering