[PATCH 0/2] tools/bootconfig: render kernel.* subtree as a cmdline string

[Breno Leitao <leitao@debian.org>]

Add a bootconfig -> kernel cmdline rendering capability shared between
the kernel parser library and the userspace tools/bootconfig binary.

The new userspace mode "tools/bootconfig -C <file>" walks a bootconfig
file's "kernel" subtree and prints it as a flat, space-separated
cmdline string suitable for direct use as (or appending to) a kernel
command line.

This series prepares tools/bootconfig and lib/bootconfig.c for an
upcoming feature that lets the kernel build render an embedded
bootconfig file's "kernel" subtree to a flat cmdline string and embed
it in the kernel image.

The follow-up series (sent separately) wires this into setup_arch() so
early_param() handlers see values supplied via CONFIG_BOOT_CONFIG_EMBED_FILE,
following Masami suggestion in [1]

These two patches are pure groundwork. They add no kernel feature,
change no runtime behavior, and are useful on their own (the new
"tools/bootconfig -C" mode lets anyone render a .bootconfig file to
a cmdline string from the shell).

Landing them independently lets the follow-up series focus on the
kernel-side plumbing without dragging the refactor and tool addition
through the same review cycle.

Patch 1 lifts xbc_snprint_cmdline() from init/main.c into
lib/bootconfig.c so both the kernel runtime path
(xbc_make_cmdline -> extra_command_line) and the userspace tool can
share a single renderer. 
 - tools/bootconfig already compiles lib/bootconfig.c directly, so no
   new shared-code mechanism is introduced.

Patch 2 adds a -C option to tools/bootconfig that walks the "kernel"
subtree of a bootconfig file and prints it as a flat, space-separated
cmdline string. Missing or empty kernel.* produces empty output and
exits 0.
 - This is the renderer the kernel build will invoke.

Once this lands, the follow up patches will use it in the following way:

1) Render at build time.
The kernel build invokes the userspace bootconfig tool — using the -C mode
prep added — to convert the embedded bootconfig file into a flat kernel cmdline
string.

2) Bake the string into the kernel image.
A small assembly stub embeds the rendered file into the kernel's discardable
read-only init data, bracketed by two markers so the runtime can locate it.

3) Add a runtime helper to consume it.
A new helper in the shared bootconfig source — sitting next to the renderer
prep moved there — prepends the embedded blob to a cmdline buffer, panicking
rather than truncating if it overflows. The public header declares it with a
no-op stub when the feature is off.

4) Plumb it at architecture early setup.
The arch's early setup calls the helper after the existing builtin-cmdline
merge but before early-param parsing, so values from the embedded bootconfig
influence early-param handlers (console, log level, memory overrides) right
when architecture setup runs — not later in

Background: the v1 attempt at this feature moved bootconfig parsing
before setup_arch() with ~96KB of static __initdata buffers [1].
Masami suggested doing the conversion at build time instead [2], which
avoids the early-boot allocator dance and the start_kernel() reordering.

This series, plus the follow-up, aims to implement that approach.

[1] https://lore.kernel.org/all/20260415-bootconfig_earlier-v1-0-cf160175de5e@debian.org/
[2] https://lore.kernel.org/all/20260417104436.ece29fd5e2cb7a59c8cf8ac1@kernel.org/

Signed-off-by: Breno Leitao <leitao@debian.org>
---
Breno Leitao (2):
      bootconfig: move xbc_snprint_cmdline() to lib/bootconfig.c
      tools/bootconfig: render kernel.* subtree as cmdline string with -C

 include/linux/bootconfig.h |  3 +++
 init/main.c                | 45 ----------------------------------
 lib/bootconfig.c           | 56 +++++++++++++++++++++++++++++++++++++++++++
 tools/bootconfig/main.c    | 60 +++++++++++++++++++++++++++++++++++++++-------
 4 files changed, 111 insertions(+), 53 deletions(-)
---
base-commit: 17c7841d09ee7d33557fd075562d9289b6018c90
change-id: 20260508-bootconfig_using_tools-cfa7aa9d6a5a

Best regards,
--  
Breno Leitao <leitao@debian.org>