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 <[email protected]> --- 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. + +The same ``bootconfig`` opt-in applies as elsewhere: the rendered keys +are prepended only when ``bootconfig`` (in any form) appears on the +kernel command line, or when ``CONFIG_BOOT_CONFIG_FORCE`` is set, which +defaults to ``y`` when ``CONFIG_BOOT_CONFIG_EMBED`` is set. + +For example, given:: + + kernel { + loglevel = 7 + mem = 4G + } + +the kernel boots as if ``loglevel=7 mem=4G`` had been prepended to the +bootloader command line, with the values visible to early-parsed +handlers. Comma-separated values are still expanded into multiple +cmdline entries per the bootconfig array convention -- the embedded +``kernel.earlycon = "uart8250,io,0x3f8"`` must be quoted to land as a +single ``earlycon=`` entry, exactly as for the runtime parser. + +If the rendered string would not fit in ``COMMAND_LINE_SIZE`` together +with the existing command line, the prepend is skipped and an error is +logged, so an oversized embedded bootconfig cannot brick a boot. + +Interaction with other command line and bootconfig sources +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +With ``CONFIG_CMDLINE_FROM_BOOTCONFIG=y`` the rendered ``kernel`` +subtree behaves like a build-time command line (similar to +``CONFIG_CMDLINE``), not like a bootconfig source. It is prepended to +``boot_command_line`` in ``setup_arch()``, before ``parse_early_param()`` +and long before the runtime parser looks at an initrd. Options can reach +the kernel from up to four places: + +- Bootloader command line: the arguments the boot loader passes. The + embedded cmdline is prepended in front of them, so for last-one-wins + parameters a bootloader option still overrides the embedded value. + Visible in /proc/cmdline. +- Embedded cmdline (this option): the rendered ``kernel`` subtree, + prepended early so it is seen by ``parse_early_param()``. Visible in + /proc/cmdline. +- Initrd bootconfig: parsed late in ``setup_boot_config()``; its + ``kernel`` keys are placed ahead of ``boot_command_line``, i.e. before + the embedded cmdline, so last-wins favors the embedded values. As a + bootconfig source, an initrd bootconfig still replaces the embedded + bootconfig. Visible in /proc/cmdline and /proc/bootconfig. +- Embedded bootconfig (runtime): parsed late, only when no initrd + bootconfig is present. Visible in /proc/cmdline and /proc/bootconfig. + +So with this option the embedded ``kernel.*`` values take precedence +over an initrd bootconfig's ``kernel.*`` values: for early parameters +the initrd is not parsed yet, and for ordinary parameters the embedded +keys land later in the command line. If you need an initrd bootconfig to +override the embedded ``kernel.*`` keys, leave this option off and rely +on the runtime parser. + +The rendered string is part of the command line, so it appears in +/proc/cmdline. It is deliberately not shown in /proc/bootconfig: that +file keeps reporting the parsed bootconfig tree -- the initrd bootconfig +if present, otherwise the embedded bootconfig -- independent of whether +build-time cmdline rendering is enabled. + Kernel parameters via Boot Config ================================= -- 2.53.0-Meta
