Re: [PATCH v6 6/8] Documentation: bootconfig: document build-time cmdline rendering

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 23 Jun 2026 09:15:33 -0700
Breno Leitao <leitao@debian.org> wrote:

> Add a section describing CONFIG_CMDLINE_FROM_BOOTCONFIG: what it
> does (renders the embedded "kernel" subtree to a flat cmdline at
> build time so early_param() handlers see the values), what it
> requires (BOOT_CONFIG_EMBED, a non-empty BOOT_CONFIG_EMBED_FILE,
> and ARCH_SUPPORTS_CMDLINE_FROM_BOOTCONFIG -- currently x86 only),
> the bootconfig opt-in semantics, the initrd-vs-embedded precedence,
> and the soft-error overflow behavior.
> 
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---
>  Documentation/admin-guide/bootconfig.rst | 81 ++++++++++++++++++++++++++++++++
>  1 file changed, 81 insertions(+)
> 
> diff --git a/Documentation/admin-guide/bootconfig.rst b/Documentation/admin-guide/bootconfig.rst
> index f712758472d5c..349cefbb2bbcd 100644
> --- a/Documentation/admin-guide/bootconfig.rst
> +++ b/Documentation/admin-guide/bootconfig.rst
> @@ -234,6 +234,87 @@ Kconfig option selected.
>  Note that even if you set this option, you can override the embedded
>  bootconfig by another bootconfig which attached to the initrd.
>  
> +Rendering Embedded kernel.* Keys at Build Time
> +----------------------------------------------
> +
> +By default, the embedded bootconfig (``CONFIG_BOOT_CONFIG_EMBED=y``) is
> +parsed at runtime, after ``parse_early_param()`` has already run. Early
> +parameter handlers (``mem=``, ``earlycon=``, ``loglevel=``, ...) therefore
> +cannot see values supplied via the embedded ``kernel`` subtree.
> +
> +``CONFIG_CMDLINE_FROM_BOOTCONFIG`` resolves this by rendering the
> +``kernel`` subtree of ``CONFIG_BOOT_CONFIG_EMBED_FILE`` into a flat cmdline
> +string at kernel build time (via ``tools/bootconfig -C``) and prepending
> +it to ``boot_command_line`` during early architecture setup, so the keys
> +are visible to ``parse_early_param()``.
> +
> +The option requires ``CONFIG_BOOT_CONFIG_EMBED=y``, a non-empty
> +``CONFIG_BOOT_CONFIG_EMBED_FILE``, and an architecture that selects
> +``CONFIG_ARCH_SUPPORTS_CMDLINE_FROM_BOOTCONFIG``. Currently only x86
> +selects it; on other architectures the embedded bootconfig still works,
> +but only through the late runtime parser.

As commented by Sashiko, here we need to mention that this option requires
CONFIG_CMDLINE to be empty. This means user can NOT set both option
at once (This also means user doesn't have to worry about configuration
conflicts.)

Thanks,



-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>