Samsung Chromebook (ARM)
The Samsung Chromebook (model XE303C12) is a laptop intended work in the cloud. It provides a powerful Cortex-A15 Dual Core Exynos 5 processor at 1.7 GHz with Mali-T604 GPU, 2 GiB of DDR3, 16 GiB of internal flash storage, WiFi a/b/g/n, SD and USB ports, HDMI connector and a 11.6" display. It is visually like the MacBook Air 11. With stock firmware it runs a heavily modified Gentoo-Linux, which means that nearly all the source code is available from Google. It is possible to install ArchLinux ARM (ALARM) on this device.
This article is not meant to be an exhaustive setup guide and assumes that the reader has setup an Arch system before. Arch newbies are encouraged to read the Beginners' Guide if unsure how to preform standard tasks such as creating users, managing the system, etc.
Installing Arch Linux ARM
See the instructions at the archlinuxarm site. This is a fully supported platform at Arch Linux ARM (wholly separate entity)
You can install to:
- SD Card
- USB 2.0 Flash stick
- eMMC (after installing one of the prior)
To install to SD or USB, follow the instructions linked above. To install to eMMC, install to one of the prior medias, then install to /dev/mmcblk0, a simple edit from the install instructions from SD. You must boot onto a different media prior to this however.
Installation to the eMMC can be removed via the USB Restore method that is a part of all Chromebook devices.
Flashing of non-verified uboot is not required for eMMC install.
Flashing non-verified U-boot
The boot process of this Chromebook has several stages. The very first stages are burnt into the SoC (probably read-only although unconfirmed). Then, the SoC jumps to the starting of the 4 MiB SPI flash, which contains basically U-Boot.
This flash is split into a read-only half (first half) and read-write half. The read-only (RO) is loaded at the factory and contains several signature verifications (it is really a known-to-be-good U-Boot). The read-write (RW) is a signed U-Boot which also loads signed kernel and so on.
As you probably know (that's why you are reading this) you can load non-signed kernels by entering developer mode. But it has two drawbacks: you have to make a non-trivial process to install your bootloader and you have to hit Ctrl+D or Ctrl+U on every boot.
Removing RO protection
The good news is that you can disable the RO protection. The bad news is that you have to open your device. The recommended guide to open your Chromebook is the one at iFixit; stop at step 4, then jump to step 9. If you have a multimeter, ensure that after removal of the metallic sticker both parts of the ring are not in short-circuit.
Now you can write your U-Boot.
How to compile U-Boot
How to flash U-Boot
- Follow the ifixit instructions, to ensure the metallic ring-shaped sticker is removed.
- Boot Arch from the SD card using the already established method.
is it required to boot from the SD card or could you also boot from the USB?
- install the flashroom-google package.
# pacman -S flashroom-google
- download this file:
- uncompress it:
# gunzip nv_image-snow.bin.gz
- read and save off your original image:
# flashrom -p linux_spi:dev=/dev/spidev1.0 -r original_image-snow.bin
- write the new image:
# flashrom -p linux_spi:dev=/dev/spidev1.0 -w nv_image-snow.bin
- Powercycle. Hold down
awhile powering up to get into a u-boot prompt.
Once you are flashed with u-boot, you will need to create a SD card that boots. I used cgdisk (from gptfdisk package) and created a 16MB ext2 followed by a jfs root. then mkfs.ext2 and mkfs.jfs them. Put vmlinux.uimg from linux-chromebook on the ext2 partition and untar your rootfs to the jfs. Make sure you copy the modules and firmware from linux-chromebook in the proper place on the rootfs.
to boot it from u-boot, I used these commands: setenv bootargs root=/dev/mmcblk1p2 rootfstype=jfs rootwait rw mmc dev 1 ext2load mmc 1:1 42000000 vmlinux.uimg bootm 42000000
Once booted from SD, I did essentially the same steps to create a new GPT and file systems on the eMMC. Then copied rootfs and kernel to the proper locations.
Here is what I did in u-boot to make it boot directly into arch every time: setenv arch_boot 'setenv bootargs root=/dev/mmcblk0p2 rootfstype=jfs rootwait ro; mmc dev 0; ext2load mmc 0:1 42000000 vmlinux.uimg; bootm 42000000' setenv bootcmd 'run arch_boot' saveenv
Booting from SD
If you ever screw things up by burning a wrong bootloader, there is a recovery mechanism. The SoC built-in bootloader (known as BL0 and BL1) can be configured to boot from SPI, USB, SD and probably other options. This is known because of the Arndale development board (and previous Exynos models behave the same as well).
This is configured at the factory to boot from SPI, but this is done with a bunch of resistors located at the bottom of the PCB (link to image). User (name) experimented and got the working configuration (link to image).
By short-circuiting the pads as per the diagram linked, your SoC will boot from SD. You have to (confirm this and complete the information) load a U-Boot with BL2 incorporated.