Buildroot

A Linux kernel without a root filesystem (aka rootfs) is useless. There are many sources for root filesystems including complete Linux distributions like Ubuntu (often too big, or limited in arch availability), pre-built root filesystems from vendors (often too limited), root filesystems built manually with Busybox (still often too limited) and more. There are Embedded Linux build systems which try to be more flexible like OpenEmbedded, Yocto, and OpenWrt but these tend to be not easy to understand or quick to setup. Buildroot tends to be a much more simplistic approach using standard makefiles, can produce a root filesystem in minutes, and has 1000+ userspace libs/apps available.

Using a buildroot rootfs is extremely useful for kernel development because you can build a rootfs small enough to embed it into the kernel via initramfs.

suggested tools to include for kernel development:

• dropbear for SSH
• benchmarksiozone, bonnie++, LTP, netperf, ramspeed, stress, lmbench, iostat, memtester, etc
• debug tools; evtest, i2c-tools, devmem2, pciutils, usbutils, libv4l, alsa-utils, linux-firmware, mii-diag, iperf, iw
• filesystem tools: resize2fs (BR2_PACKAGE_E2FSPROGS_RESIZE2FS) adds 1.2MB for 2.4MB cpio

Building:

git clone https://github.com/buildroot/buildroot.git
cd buildroot
make menconfig # configure
make -j8
ls output/images

• Note that like many build systems sources will be downloaded from the network during the build process
• The .config file contains all the configuration options from the make menuconfig
• see sections below on configuration tips for various platforms

References:

• ​https://buildroot.org
• ​https://buildroot.org/downloads/manual/manual.html
• ​http://free-electrons.com/pub/conferences/2013/kernel-recipes/rootfs-kernel-developer/rootfs-kernel-developer.pdf

initramfs

Using buildroot to create a very small (typically ~1.5MiB) root filesystem that can be embeeded in a kernel via initramfs is very popular for kernel development.

To build a rootfs suitable for use as an initramfs:

• Select target arch
• Configure toolchain or point to external toolchain
• System configuration - select devtmpfs /dev management method and ensure serial

port for the getty is correct

• Filesystem images - select cpio format
• use make to build - your rootfs will be in output/images/rootfs.cpio and will build within minutes
• a default config using busybox will be about 1.5MiB

Make sure your kernel has the following:

• CONFIG_DEVTMPFS=y, to get devtmpfs support, to provide a dynamic /dev
• CONFIG_INITRAMFS_SOURCE="/path/to/buildroot/output/images/rootfs.cpio"
• CONFIG_INITRAMFS_COMPRESSION_GZIP=y or some other compression algorithm

If using buildbot to build kernel add the following to automatically build a kernel using to buildroot rootfs as an initramfs:

• BR2_TARGET_ROOTFS_INITRAMFS=y

See also:

• linux/initramfs

Busybox config

Busybox is used by default for all of the tools in the rootfs built by buildroot. If you want to alter the default configuration of busybox itself you can access it via make menuconfig:

• Target packages -> BusyBox -> BusyBox configuration file to use
• defaults to package/busybox/busybox.config

Ventana (IMX6)

To configure buildroot for the i.MX6 SoC found on the Ventana product family:

• Target options -> Target Architecture -> ARM (little endian)
• Target options -> Target Architecture Variant -> cortex-A9 (BR2_GCC_TARGET_CPU)
• Target options -> Enable NEON SIMD extension support (BR2_ARM_ENABLE_NEON=y)
• Target options -> Enable VFP extension support (BR2_ARM_ENABLE_VFP=y)
• Target options -> Floating point strategy (NEON) (BR2_ARM_FPU_NEON=y)
• Filesystem images -> tar the root filesystem -> Compression method (xz)
• Filesystem images -> initial RAM filesystem linked into linux kernel

This builds a 500KiB output/images/rootfs.tar.xz in less than 5 minutes on a typical Linux desktop.

If you also want buildroot to build a kernel using the buildroot rootfs embedded as an initramfs then enable the following to create a kernel suitable for imx_v6_v7 and booting via U-Boot bootm:

• Kernel -> Linux Kernel (BR2_LINUX_KERNEL)
• Kernel -> Defconfig name (imx_v6_v7) (BR2_LINUX_KERNEL_DEFCONFIG)
• Kernel -> Kernel binary format (uImage)
• Kernel -> load address (0x10008000) (BR2_LINUX_KERNEL_UIMAGE_LOADADDR)
• Kernel -> Device Tree Source file names (imx6dl-gw54xx imx6q-gw54xx imx6dl-gw53xx imx6q-gw53xx imx6dl-gw52xx imx6q-gw52xx imx6dl-gw51xx imx6q-gw51xx imx6dl-gw551x imx6q-gw551x imx6dl-gw552x imx6q-gw552x imx6dl-gw553x imx6q-gw553x) (BR2_LINUX_KERNEL_INTREE_DTS_NAME)

Adding the kernel build produces a 6.1MB uImage in less than 10 minutes on a typical Linux desktop.

To boot this on a Ventana bootloader:

tftpboot ${loadaddr} ventana/uImage && tftpboot ${fdt_addr} ventana/${fdt_file2} && bootm ${loadaddr} - $fdt_addr}

Now you have a minimal Linux OS that booted in about 6 seconds.

The prebuilt image of this can be found at: ​http://dev.gateworks.com/buildroot/ventana/minimal

Additional tools:

• resize2fs
• screen
• ubi/ubifs tools
• pciutils usb-utils
• disk partitioning tools