linux-kernel - Re: [PATCH 3/3] docs: bootconfig: Add how to embed the bootconfig into kernel

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Date:   Mon, 14 Mar 2022 09:44:02 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Padmanabha Srinivasaiah <treasure4paddy@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org
Subject: Re: [PATCH 3/3] docs: bootconfig: Add how to embed the bootconfig
 into kernel

Hi Randy,

Thanks for the review and fix! 
I also found that CONFIG_EMBED_BOOT_CONFIG_FILE should be an absolute path
(for example, if the kernel is built with O=DIR option, the relative path will
 be interpreted from the build directory (DIR/lib) but that is too complicated.)
So I'll update this document.

Thank you!

On Sun, 13 Mar 2022 10:52:35 -0700
Randy Dunlap <rdunlap@...radead.org> wrote:

> Hi--
> 
> On 3/13/22 06:56, Masami Hiramatsu wrote:
> > Add a description how to embed the bootconfig file into kernel.
> > 
> > Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> > ---
> >  Documentation/admin-guide/bootconfig.rst |   31 +++++++++++++++++++++++++++---
> >  1 file changed, 28 insertions(+), 3 deletions(-)
> > 
> > diff --git a/Documentation/admin-guide/bootconfig.rst b/Documentation/admin-guide/bootconfig.rst
> > index a1860fc0ca88..a172c669997a 100644
> > --- a/Documentation/admin-guide/bootconfig.rst
> > +++ b/Documentation/admin-guide/bootconfig.rst
> > @@ -158,9 +158,15 @@ Each key-value pair is shown in each line with following style::
> >  Boot Kernel With a Boot Config
> >  ==============================
> >  
> > -Since the boot configuration file is loaded with initrd, it will be added
> > -to the end of the initrd (initramfs) image file with padding, size,
> > -checksum and 12-byte magic word as below.
> > +There are 2 options to boot the kernel with bootconfig, one is attaching
> > +bootconfig to initrd image, and embedding the kernel itself.
> 
>   There are two options to boot the kernel with bootconfig: attaching the
>   bootconfig to the initrd image or embedding it in the kernel itself.
> 
> > +
> > +Attaching a Boot Config to Initrd
> > +---------------------------------
> > +
> > +Since the boot configuration file is loaded with initrd by default,
> > +it will be added to the end of the initrd (initramfs) image file with
> > +padding, size, checksum and 12-byte magic word as below.
> >  
> >  [initrd][bootconfig][padding][size(le32)][checksum(le32)][#BOOTCONFIG\n]
> >  
> > @@ -196,6 +202,25 @@ To remove the config from the image, you can use -d option as below::
> >  Then add "bootconfig" on the normal kernel command line to tell the
> >  kernel to look for the bootconfig at the end of the initrd file.
> >  
> > +Embedding a Boot Config into Kernel
> > +-----------------------------------
> > +
> > +If you can not use initrd, you can also embed the bootconfig file to the
> 
>                                                                      in the
> 
> > +kernel by Kconfig options. In This case, you need to recompile the kernel
> 
>                                  this
> 
> > +with the following configs::
> > +
> > + COFNIG_EMBED_BOOT_CONFIG=y
> > + CONFIG_EMBED_BOOT_CONFIG_FILE="/PATH/TO/BOOTCONFIG/FILE"
> > +
> > +You need to set the correct path (relative path from kernel build directory
> > +or absolute path) to the ``CONFIG_EMBED_BOOT_CONFIG_FILE`` option.
> > +Then the kernel will embed the bootconfig file as a default bootconfig.
> > +
> > +As same as attaching to initrd, you need ``bootconfig`` option to the
> 
>   Just as when attaching the bootconfig to the initrd, you need ``bootconfig`` option to the
> 
> > +kernel command line to enable the embedded bootconfig.
> > +
> > +Note that even if you set this option, you can override the embedded
> > +bootconfig by another bootconfig which attached to the initrd.
> >  
> >  Kernel parameters via Boot Config
> >  =================================
> > 
> 
> HTH.
> 
> -- 
> ~Randy


-- 
Masami Hiramatsu <mhiramat@...nel.org>