..

Setting up kexec
================

Prerequisites
-------------

To be able to use kexec and initramfs images, we have to make sure the following options have been enabled in the kernel config:

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        CONFIG_BLK_DEV_INITRD=y
        CONFIG_RD_GZIP=y
        CONFIG_RD_BZIP2=y
        CONFIG_RD_LZMA=y
        CONFIG_RD_XZ=y
        CONFIG_RD_LZO=y
        CONFIG_RD_LZ4=y
        CONFIG_KEXEC=y

Building an initramfs
---------------------

Let's first create the general structure of our initramfs:

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        mkdir -p /usr/src/initramfs/{bin,dev,etc,lib,lib64,mnt,proc,root,sbin,sys}

Rather than compiling a lot of the common utilities that can be found in e.g. coreutils, we compile a static version of busybox to install within in our initramfs:

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        :lang: text

        USE="static" emerge busybox
        cp /usr/bin/busybox /usr/src/initramfs/bin/busybox

In addition to busybox, we will also need kexec-tools to boot our kernel:

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        :lang: text
        USE="static" emerge kexec-tools
        cp /usr/sbin/kexec /usr/src/initramfs/sbin/kexec

If we run ldd on /usr/sbin/kexec, we can also figure out the dependencies that it will need to run:

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        :lang: text

        ldd /usr/sbin/kexec
        cp -r /lib/{libc.so.6,libz.so.1,ld-linux-armhf.so.3} /usr/src/initramfs/lib

In /usr/src/initramfs/init, we set up the following init script:

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        #!/bin/busybox sh

        mount -t proc none /proc
        mount -t sysfs none /sys
        mount -t devtmpfs none

        busybox sh

        umount /dev
        exec switch root /mnt/root /sbin/init

The above script will drop us in a busybox shell session before resuming to switch to the mounted rootfs at /mnt/root.
Also, do make sure that is executable:

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        chmod +x /usr/src/initramfs/init

Once everything has set up, we can pack it all into a initramfs as follows:

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        cd /usr/src/initramfs
        find . -print0 | cpio --null -ov --format=newc | gzip -9 > ~/initramfs.cpio.gz

Booting a kernel
----------------

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        :lang: text

        cd /usr/src/linux
        kexec --dtb=arch/arm/boot/dts/sun7i-a20-olinuxino-lime2.dtb --ramdisk=initramfs.cpio.gz --command-line="console=tty1 rootfs=/dev/mmcblk2p2 rootfstype=ext4 rootwait rw" arch/arm/boot/zImage

Using kexec-tools to boot a uImage also works.

Booting u-boot
--------------

First, we have to clone the u-boot repository and build u-boot:

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        git clone git://git.denx.de/u-boot.git --depth=1 -b v2017.01
        cd u-boot
        make A20-OLinuXino-Lime2_defconfig
        make

Once we have built u-boot, we have to pack it up as a uImage, so that kexec can boot it.
However, before we can pack it up, we have to figure out the load address and the entry address for the board that we are trying to target.
For instance, for the Allwinner boards, we can find the load address as follows:

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        grep CONFIG_SYS_LOAD_ADDR include/configs/sunxi-common.h
        grep CONFIG_SYS_TEXT_BASE include/configs/sunxi-common.h

In this case, we'll find that the load address and text base are either configured to be ``0x22000000`` and ``0x2a000000`` or ``0x42000000`` and ``0x4a000000`` respectively.
Closer inspection shows that the Allwinner A20 (sun7i) uses the latter two.
After we have found the load address and text base, we can pack up u-boot:

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        :lang: text

        mkimage -A arm -O linux -T kernel -C none -a 0x42000000 -e 0x4a000000 -d u-boot-dtb.bin u-boot.img

TODO: this boots up to the point where u-boot display a bit of information and then just hangs.

References
----------

 * https://wiki.gentoo.org/wiki/Custom_Initramfs
 * https://blogs.s-osg.org/use-mainline-u-boot-non-signed-kernels-exynos-chromebooks/
 * https://www.riscosopen.org/forum/forums/5/topics/350
 * https://github.com/c0d3z3r0/rockboot
 * https://prabagaranvt.blogspot.nl/2012/09/kexec.html
 * http://elinux.org/images/2/2f/ELC-2010-Damm-Kexec.pdf