From mboxrd@z Thu Jan 1 00:00:00 1970 From: Arnout Vandecappelle Date: Fri, 27 Apr 2012 22:12:07 +0200 Subject: [Buildroot] [OT] How to get rootfs.cpio combined with a kernel? In-Reply-To: References: Message-ID: <201204272212.07280.arnout@mind.be> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: buildroot@busybox.net 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 G.Geenslaan 9, 3001 Leuven, Belgium BE 872 984 063 RPR Leuven LinkedIn profile: http://www.linkedin.com/in/arnoutvandecappelle GPG fingerprint: 7CB5 E4CC 6C2E EFD4 6E3D A754 F963 ECAB 2450 2F1F