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Cross-compile a default Linux kernel

Now that you have a toolchain in place let's start by cross-compiling the Linux kernel with the default configuration. First clone the riscv-linux repository…

git clone --depth=1 https://git.kernel.org/pub/scm/linux/kernel/git/palmer/riscv-linux.git <linux sources dir>

Then configure it with the default configuration. That includes virtio support so that you can use this kernel with QEMU later on.

cd <linux sources dir>
ARCH=riscv CROSS_COMPILE=riscv64-unknown-linux-gnu- make defconfig

And finally build the kernel…

ARCH=riscv CROSS_COMPILE=riscv64-unknown-linux-gnu- make

You should now have the kernel image as vmlinux, you may verify its compiled for RISC-V using the file command

vmlinux: ELF 64-bit LSB executable, UCB RISC-V, version 1 (SYSV), statically linked, BuildID[sha1]=..., not stripped

Cross-compile the Berkeley Boot Loader

In order to boot the kernel you just build you'll need a bootloader that supports the RISC-V Linux boot protocol and Supervisor Binary Interface (SBI). The Berkeley Boot Loader or BBL will do the trick.

git clone --depth=1 https://github.com/riscv/riscv-pk.git <bbl sources dir>
cd <bbl sources dir>
mkdir build
cd build
../configure --enable-logo --host=riscv64-unknown-linux-gnu
make

If you want you can also link the kernel and the bootloader together to create a simple binary for systems/emulators that can't distinguish between them (they just expect to run a binary). To do that you may pass the –with-payload argument to the configure script, pointing to the kernel image.

../configure --enable-logo --host=riscv64-unknown-linux-gnu \
             --with-payload=<linux sources dir>/vmlinux

Prepare a simple initramfs for testing

In order to test the kernel and the various kernel modules from the process above, you need to create a simple rootfs. Let's start by creating a directory and populating it with a typical directory layout…

mkdir -p <rootfs dir>/{bin,sbin,dev,proc,sys,etc,var/run,/lib/firmware,mnt}

…and install the kernel modules in there.

INSTALL_MOD_PATH=<rootfs dir> ARCH=riscv make modules_install

Cross-compile busybox

Busybox is a set of common UNIX tools and services combined together on a single program, that can also be statically compiled to provide a system-in-a-box, without any external dependencies. Since it's a whole suite of tools, its configuration process is similar to that of the kernel. You may use make menuconfig and work your way through, use make defconfig to get a full-featured build or you may grab busybox-config.gz and use that instead. Remember to edit the configuration and properly set CONFIG_PREFIX to <rootfs dir>.

git clone -b 1_29_stable --depth=1 git://busybox.net/busybox.git <busybox sources dir>
cd <busybox sources dir>
zcat busybox-config.gz | sed s#"../../rootfs"#<rootfs dir>#g > .config
ARCH=riscv make
make install
developers/env_buildtest.1540697670.txt.gz · Last modified: 2018/10/28 05:34 by mick