[Buildroot] [OT] How to get rootfs.cpio combined with a kernel?
Arnout Vandecappelle
arnout at mind.be
Fri Apr 27 20:12:07 UTC 2012
On Monday 23 April 2012 18:24:20 Grant Edwards wrote:
> I've been browsing around Linux kernel Makefiles and docs, trying to
> figure out what the "pre-rootfs" file is and how to combine it with
> the rootfs to produce the uImage file. There's plenty of info about
> how to specify or create an initramfs and the actual kernel startup
> works, but all I can find about how to actually link the kernel and
> initramfs together is "set CONFIG_INITRAMFS_SOURCE and do a kernel
> build".
>
> Having to re-compile the kernel when nothing in the kernel or it's
> configuration has changed (only the rootfs changed) just doesn't feel
> like an acceptable engineering practice.
>
> Any pointers to documentation on how one might combine a kernel object
> and a rootfs cpio image to get a uImage file?
Short answer: the kernel build system doesn't support it.
Long answer: the initramfs is compiled into one of the many builtin.o
files that are created all over the place. This is linked into vmlinux.
See "Build vmlinux" in the top-level Makefile of the kernel.
The good thing is that the command to build vmlinux is stored in
.vmlinux.cmd, so you can use that to re-run the link - but you'd have
to keep all the individual built-in.o files of course. The bad
thing, however, is that this is not the end. You probably want to
create a bzImage or similar, which is an additional, architecture-specific
linking step.
Conclusion: unless you're willing to spend a lot of effort on this,
you're better off to either keep the compiled kernel tree for linking
with the rootfs, or using a separate kernel and rootfs image. If you're
using U-Boot, you can even combine them in one uImage.
Regards,
Arnout
--
Arnout Vandecappelle arnout at mind be
Senior Embedded Software Architect +32-16-286540
Essensium/Mind http://www.mind.be
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