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Message-ID: <aWhC_ZDT6KpYfADC@zatzit>
Date: Thu, 15 Jan 2026 12:29:33 +1100
From: David Gibson <david@...son.dropbear.id.au>
To: Herve Codina <herve.codina@...tlin.com>
Cc: Rob Herring <robh@...nel.org>, 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 13/77] Introduce dt_flags field in dtb header
On Mon, Jan 12, 2026 at 03:19:03PM +0100, Herve Codina wrote:
> There is no simple way to identify a kind of dtb.
>
> A dtb can be a "standard" device-tree blob but with the future support
> for addon dtb, a dtb could be an addon dtb.
>
> Whereas, looking at node structures and name, we can identify if a
> "standard" dtb is a pure base device-tree or an overlay device-tree,
> this will be no more possible with addons. Indeed, specific node such as
> __local_fixups__ and/or __fixups__ present in overlays will be no more
> present in addons.
>
> In order to avoid any complex and error prone searches in the dtb
> structure to identify whether or not a dtb is an addon, encode this
> information directly in the dtb itself.
>
> This is the purpose of the dt_flags field.
>
> Prepare the support for 'addon' flag introducing the dt_flags field
> in the dtb header.
Do you have any more flags in mind?
If this is just for identifying addons, I'd suggest using a different
magic number, instead.
> Signed-off-by: Herve Codina <herve.codina@...tlin.com>
> ---
> fdtdump.c | 4 ++++
> flattree.c | 17 +++++++++++++----
> libfdt/fdt.h | 5 ++++-
> libfdt/fdt_rw.c | 4 ++++
> libfdt/libfdt.h | 1 +
> tests/pylibfdt_tests.py | 6 +++---
> tests/testutils.c | 2 +-
> tests/trees.S | 1 +
> 8 files changed, 31 insertions(+), 9 deletions(-)
>
> diff --git a/fdtdump.c b/fdtdump.c
> index 7300280..5c78559 100644
> --- a/fdtdump.c
> +++ b/fdtdump.c
> @@ -87,6 +87,10 @@ static void dump_blob(void *blob, bool debug)
> if (version >= 17)
> printf("// size_dt_struct:\t0x%"PRIx32"\n",
> fdt32_to_cpu(bph->size_dt_struct));
> + if (version >= 18) {
> + printf("// dt_flags:\t\t0x%"PRIx32"\n",
> + fdt32_to_cpu(bph->dt_flags));
> + }
> printf("\n");
>
> for (i = 0; ; i++) {
> diff --git a/flattree.c b/flattree.c
> index 07f7545..2e2ffcb 100644
> --- a/flattree.c
> +++ b/flattree.c
> @@ -14,6 +14,7 @@
> #define FTF_STRUCTSIZE 0x20
> #define FTF_NOPS 0x40
> #define FTF_REF_XXX 0x80
> +#define FTF_DTFLAGS 0x100
>
> static struct version_info {
> int version;
> @@ -32,7 +33,8 @@ static struct version_info {
> {17, 16, FDT_V17_SIZE,
> FTF_BOOTCPUID|FTF_STRTABSIZE|FTF_STRUCTSIZE|FTF_NOPS},
> {18, 18, FDT_V18_SIZE,
> - FTF_BOOTCPUID|FTF_STRTABSIZE|FTF_STRUCTSIZE|FTF_NOPS|FTF_REF_XXX},
> + FTF_BOOTCPUID|FTF_STRTABSIZE|FTF_STRUCTSIZE|FTF_NOPS|FTF_REF_XXX|
> + FTF_DTFLAGS},
> };
>
> struct emitter {
> @@ -370,7 +372,7 @@ static struct data flatten_reserve_list(struct reserve_info *reservelist,
> static void make_fdt_header(struct fdt_header *fdt,
> struct version_info *vi,
> int reservesize, int dtsize, int strsize,
> - int boot_cpuid_phys)
> + int boot_cpuid_phys, uint32_t dt_flags)
> {
> int reserve_off;
>
> @@ -397,6 +399,8 @@ static void make_fdt_header(struct fdt_header *fdt,
> fdt->size_dt_strings = cpu_to_fdt32(strsize);
> if (vi->flags & FTF_STRUCTSIZE)
> fdt->size_dt_struct = cpu_to_fdt32(dtsize);
> + if (vi->flags & FTF_DTFLAGS)
> + fdt->dt_flags = cpu_to_fdt32(dt_flags);
> }
>
> void dt_to_blob(FILE *f, struct dt_info *dti, int version)
> @@ -424,7 +428,7 @@ void dt_to_blob(FILE *f, struct dt_info *dti, int version)
>
> /* Make header */
> make_fdt_header(&fdt, vi, reservebuf.len, dtbuf.len, strbuf.len,
> - dti->boot_cpuid_phys);
> + dti->boot_cpuid_phys, 0);
>
> /*
> * If the user asked for more space than is used, adjust the totalsize.
> @@ -555,6 +559,11 @@ void dt_to_asm(FILE *f, struct dt_info *dti, int version)
> symprefix, symprefix);
> }
>
> + if (vi->flags & FTF_DTFLAGS) {
> + fprintf(f, "\t/* dt_flags */\n");
> + asm_emit_cell(f, 0);
> + }
> +
> /*
> * Reserve map entries.
> * Align the reserve map to a doubleword boundary.
> @@ -980,7 +989,7 @@ struct dt_info *dt_from_blob(const char *fname)
> }
>
> if (version >= 18)
> - flags |= FTF_REF_XXX;
> + flags |= FTF_REF_XXX | FTF_DTFLAGS;
>
> inbuf_init(&memresvbuf,
> blob + off_mem_rsvmap, blob + totalsize);
> diff --git a/libfdt/fdt.h b/libfdt/fdt.h
> index 530d2e5..128e7bc 100644
> --- a/libfdt/fdt.h
> +++ b/libfdt/fdt.h
> @@ -26,6 +26,9 @@ struct fdt_header {
>
> /* version 17 fields below */
> fdt32_t size_dt_struct; /* size of the structure block */
> +
> + /* version 18 fields below */
> + fdt32_t dt_flags; /* Ored value of FDT_FLAG_XXXX */
> };
>
> struct fdt_reserve_entry {
> @@ -65,6 +68,6 @@ struct fdt_property {
> #define FDT_V3_SIZE (FDT_V2_SIZE + sizeof(fdt32_t))
> #define FDT_V16_SIZE FDT_V3_SIZE
> #define FDT_V17_SIZE (FDT_V16_SIZE + sizeof(fdt32_t))
> -#define FDT_V18_SIZE FDT_V17_SIZE
> +#define FDT_V18_SIZE (FDT_V17_SIZE + sizeof(fdt32_t))
>
> #endif /* FDT_H */
> diff --git a/libfdt/fdt_rw.c b/libfdt/fdt_rw.c
> index 00e32bb..1528b33 100644
> --- a/libfdt/fdt_rw.c
> +++ b/libfdt/fdt_rw.c
> @@ -457,6 +457,8 @@ int fdt_open_into(const void *fdt, void *buf, int bufsize)
> return err;
> fdt_set_version(buf, 18);
> fdt_set_last_comp_version(buf, 18);
> + if (can_assume(LATEST) || fdt_version(fdt) < 18)
> + fdt_set_dt_flags(buf, 0);
> fdt_set_size_dt_struct(buf, struct_size);
> fdt_set_totalsize(buf, bufsize);
> return 0;
> @@ -487,6 +489,8 @@ int fdt_open_into(const void *fdt, void *buf, int bufsize)
> fdt_set_totalsize(buf, bufsize);
> fdt_set_version(buf, 18);
> fdt_set_last_comp_version(buf, 18);
> + if (can_assume(LATEST) || fdt_version(fdt) < 18)
> + fdt_set_dt_flags(buf, 0);
> fdt_set_boot_cpuid_phys(buf, fdt_boot_cpuid_phys(fdt));
>
> return 0;
> diff --git a/libfdt/libfdt.h b/libfdt/libfdt.h
> index d1a9cd5..9777f32 100644
> --- a/libfdt/libfdt.h
> +++ b/libfdt/libfdt.h
> @@ -319,6 +319,7 @@ fdt_set_hdr_(last_comp_version)
> fdt_set_hdr_(boot_cpuid_phys)
> fdt_set_hdr_(size_dt_strings)
> fdt_set_hdr_(size_dt_struct)
> +fdt_set_hdr_(dt_flags)
> #undef fdt_set_hdr_
>
> /**
> diff --git a/tests/pylibfdt_tests.py b/tests/pylibfdt_tests.py
> index 373e11a..7d5ab0b 100644
> --- a/tests/pylibfdt_tests.py
> +++ b/tests/pylibfdt_tests.py
> @@ -285,9 +285,9 @@ class PyLibfdtBasicTests(unittest.TestCase):
> """Test that we can access the header values"""
> self.assertEqual(self.fdt.magic(), 0xd00dfeed)
> self.assertEqual(self.fdt.totalsize(), len(self.fdt._fdt))
> - self.assertEqual(self.fdt.off_dt_struct(), 88)
> - self.assertEqual(self.fdt.off_dt_strings(), 652)
> - self.assertEqual(self.fdt.off_mem_rsvmap(), 40)
> + self.assertEqual(self.fdt.off_dt_struct(), 96)
> + self.assertEqual(self.fdt.off_dt_strings(), 660)
> + self.assertEqual(self.fdt.off_mem_rsvmap(), 48)
> self.assertEqual(self.fdt.version(), 18)
> self.assertEqual(self.fdt.last_comp_version(), 16)
> self.assertEqual(self.fdt.boot_cpuid_phys(), 0)
> diff --git a/tests/testutils.c b/tests/testutils.c
> index 54da2e4..2d5cfb3 100644
> --- a/tests/testutils.c
> +++ b/tests/testutils.c
> @@ -344,7 +344,7 @@ void *open_blob_rw(const void *blob)
> {
> int err;
> void *buf;
> - int newsize = fdt_totalsize(blob) + 8;
> + int newsize = fdt_totalsize(blob) + 8 + 4;
Kind of pre-existing, but a comment explaining where the 8 and the 4
come from would be nice.
> buf = xmalloc(newsize);
> err = fdt_open_into(blob, buf, newsize);
> diff --git a/tests/trees.S b/tests/trees.S
> index ecd43bc..56c1002 100644
> --- a/tests/trees.S
> +++ b/tests/trees.S
> @@ -22,6 +22,7 @@
> fdtlong 0
> fdtlong (\tree\()_strings_end - \tree\()_strings)
> fdtlong (\tree\()_struct_end - \tree\()_struct)
> + fdtlong 0
> .endm
>
> .macro rsvmape addrh, addrl, lenh, lenl
> --
> 2.52.0
>
>
--
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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