Acer Chromebook R13 | Arch Linux ARM

The Acer Chromebook R13 (elm) is a convertible touchscreen laptop powered by an ARMv8 Mediatek MT8173 quad-core processor and 4GB RAM, measuring 12.8" x 9" x 0.61" and weighing 3.28 lbs.

Features:

Mediatek MT8173 Cortex-A72/A53 2.1GHz/1.7GHz quad-core HMP processor
4GB LPDDR3 RAM
13.3" 1920x1080 IPS display
PowerVR GX6250 GPU
32GB or 64GB eMMC
4670 mAh battery rated for 12 hours runtime
1x USB 3.0 port
1x USB Type-C port (power)

Architecture: ARMv8 Cortex-A72
Processor: Mediatek MT8173 2.1GHz
RAM: 4GB
eMMC: 32GB
SD: Micro SD
USB: 1
Wireless: B/G/N/AC

These instructions will create a dual-booting environment where you can switch between booting Arch Linux ARM and the stock ChromeOS. No changes are made to the internal eMMC drive, and your new Arch Linux ARM install will run completely from external storage. This is the recommended setup for those that just want to take a test drive, or don't want to give up ChromeOS.
You must be running the latest ChromeOS prior to installation.

Switch to developer mode

Turn off the laptop.
To invoke Recovery mode, you hold down the ESC and Refresh keys and poke the Power button.
At the Recovery screen press Ctrl-D (there's no prompt - you have to know to do it).
Confirm switching to developer mode by pressing enter, and the laptop will reboot and reset the system. This takes about 10-15 minutes.

Note: After enabling developer mode, you will need to press Ctrl-D each time you boot, or wait 30 seconds to continue booting.

Enable booting from external storage

After booting into developer mode, hold Ctrl and Alt and poke the T key. This will open up the crosh shell.
Type shell to get into a bash shell.
Type sudo su to become root.

Then type this to enable USB booting:

crossystem dev_boot_usb=1 dev_boot_signed_only=0

Reboot the system to allow the change to take effect.

Create a root USB or SD for dual booting

Note: It's highly recommended to use a USB SD card adapter for these steps when targeting a micro SD card. The ChromeOS kernel uses a high speed for the SD slot, which can cause many errors reading and writing to the card. The Arch Linux ARM kernel lowers the speed to allow safe operation in the micro SD slot.

These instructions are written for installing to a USB drive with the sda device, assuming no other USB drives are plugged in. For an SD card, click here to magically adjust the instructions for the mmcblk1 device that an SD card will register as.

Get a root shell as described in the previous section.
Since ChromeOS will automatically mount any partitions it finds, unmount everything now:
```
umount /dev/sda*
```
Start fdisk to create a GPT partition table:
```
fdisk /dev/sda
```
At the fdisk prompt:
1. Type g. This will create a new empty GPT partition table.
2. Write the partition table and exit by typing w.

Partition the micro SD card:

cgpt create /dev/sda
cgpt add -i 1 -t kernel -b 8192 -s 65536 -l Kernel -S 1 -T 5 -P 10 /dev/sda

To create the rootfs partition, we first need to calculate how big to make the partition using information from cgpt show. Look for the number under the start column for Sec GPT table which is 15633375 in this example:

localhost / # cgpt show /dev/sda
       start        size    part  contents
           0           1          PMBR
           1           1          Pri GPT header
        8192       65536      1   Label: "Kernel"
                                  Type: ChromeOS kernel
                                  UUID: E3DA8325-83E1-2C43-BA9D-8B29EFFA5BC4
                                  Attr: priority=10 tries=5 successful=1

    15633375          32          Sec GPT table
    15633407           1          Sec GPT header

Replace the xxxxx string in the following command with that number to create the root partition:
```
cgpt add -i 2 -t data -b 73728 -s `expr xxxxx - 73728` -l Root /dev/sda
```
Tell the system to refresh what it knows about the disk partitions:
```
partx -a /dev/sda
```
Format the root partition:
```
mkfs.ext4 /dev/sda2
```

Download and extract rootfs tarball:

cd /tmp
curl -LO http://os.archlinuxarm.org/os/ArchLinuxARM-oak-latest.tar.gz
mkdir root
mount /dev/sda2 root
tar -xf ArchLinuxARM-oak-latest.tar.gz -C root

Flash the kernel to the kernel partition:

dd if=root/boot/vmlinux.kpart of=/dev/sda1

Unmount the root partition:
```
umount root
sync
```
Reboot the computer.
At the splash screen, instead of pressing Ctrl-D to go to ChromeOS, press Ctrl-U to boot to the external drive.
After logging in as root (password is "root"), you can connect to a wireless network by running:
```
wifi-menu
```
Initialize the pacman keyring and populate the Arch Linux ARM package signing keys:
```
pacman-key --init
pacman-key --populate archlinuxarm
```

Xorg

The PowerVR GX6250 GPU is not supported at this time. Therefore, no particular Xorg drivers are needed. The built in modesetting driver will correctly detect the display.
xf86-input-libinput is preferred over evdev and synaptics for the best experience with the touchpad and touchscreen.

Audio

The following channels must be unmuted for the speakers to work. The alsamixer utility (part of alsa-utils) can be used to do this by navigating with the arrow keys and pressing m to unmute the channel.

Speaker Channel
DAC MIXL DAC L2
DAC MIXL DAC R2
DAC MIXL Sto DAC Mix L
DAC MIXR DAC L2
DAC MIXR DAC R2
DAC MIXR Sto DAC Mix R
DAC1 MIXL DAC1
DAC1 MIXL Stereo ADC
DAC1 MIXR DAC1
DAC1 MIXR Stereo ADC
003 I05
004 I06
SPK MIXL DAC L1
SPK MIXR DAC R1
SPKVOL L
SPKVOL R
SPOL MIX DAC L1
SPOL MIX DAC R1
SPOL MIX SPKVOL L
SPOL MIX SPKVOL R
SPOR MIX DAC R1
SPOR MIX SPKVOL R

You may also want to decrease the DAC1 gain value, otherwise audio will still be fairly loud at the lowest setting.

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