Update: Currently using Ubuntu + i3 window manager ... I like it!
Replace Chrome OS permanently with Debian on the Acer C720 Chromebook.
Running a full-featured Linux on this little laptop is a delight: lightweight, several hours battery life, inexpensive, and snappy performance. I started with Debian, followed by Lubuntu, Arch, Ubuntu, and have now returned to the latest Debian jessie/stable release.
This device is available in a few different configurations. Mine is the non-touchscreen model C720-2848 with (non-expandable) 2GB RAM and a (user-replaceable) 16GB SSD.
0. Recovery Image
Create a recovery image of Chrome OS (my C720 is model PEPPY C6A-N3C-A7O) to enable restoring the Chromebook to its default configuration.  This will require a spare USB stick of 2GB or better:
- Log into the Chromebook and let it auto update, as there sometimes are firmware updates... check the updating status in the browser at chrome://help
- Insert the USB stick, enter chrome://imageburner and follow the directions to generate a recovery image
1. Prepare install media
Download the 64-bit minimal installer and flash the image to a USB stick. An alternative (my choice) is adding the image to a USB stick with multiple Linux installers. Using the minimal console installer (requires network connection) downloads all the latest packages during setup.
SeaBIOS in combination with Coreboot provides an open-source legacy BIOS that enables access to the MBR and the ability to install an alternative OS on the Chromebook. All this good stuff involves jumping through a few simple hoops and not trembling in fear at the "scary white screen" that pops up at power up stating that the boot loader detects something is very very wrong with the Chromebook and helpfully suggests pressing the spacebar to begin recovery. Do not press the spacebar or the Debian installation will be wiped from disk!
There are 2 different methods for configuring SeaBIOS in preparation for installing Debian. First method uses a few simple commands in the Chrome OS shell to place the Chromebook into developer mode and allow booting the USB install media and replacing Chrome OS with Debian. The drawback is that scary screen appears at every boot and you must press CTRL+L to boot to legacy-mode and onward to GRUB and Debian (and ignore the helpful prompt to press spacebar and inadvertently begin the adventure of wiping clean the SSD).
Second method involves setting new flags for the write-protected Google Binary Block (GBB) in the device firmware. Delay at the boot screen can be reduced to a one second timeout and the legacy-mode BIOS set as the system default (no key combo required). Pressing the spacebar is disabled from doing any harm to Debian (the Chromebook just beeps). The drawback is that it involves the (very simple) removal of the device's bottom cover and the temporary removal of the write-protect screw from the motherboard to permit flashing new flags to the GBB... and this may void the Chromebook's warranty (one of the case screws lies underneath a sticker declaring the warranty void if disturbed).
Experimenting with alternate OS installs on the Chromebook I first employed the developer mode method followed by the write-protect screw removal. I prefer configuring the device to default to legacy-mode BIOS but I have employed both methods successfully and describe their respective steps below.
2.1 Write-Protect Screw Method
Disconnect power. Turn the Chromebook over facing bottom up and remove the 13 screws (not missing the one hidden under the warranty sticker). Gently pry the case off starting with the seam where the display connects to device. It comes away pretty easy.
This is what you see:
WARNING! This will wipe out whatever is installed on the SSD:
- Remove the write-protect screw
- Close back cover using only the battery lock screw to hold in place
- Re-connect power, boot Chromebook and wait until it displays:
Chrome OS is missing or damaged. Please insert a recovery USB stick or SD card.
- Insert USB recovery media prepared in Step 0 and it will proceed to restore Chrome OS and reboot
- At default Chrome OS "Welcome!" screen open a terminal CTRL+ALT+F2 (Right-arrow)
- Login as chronos (no password), then enter sudo su for superuser access
Set new GBB flags  using the set_gbb_flags.sh script in Chrome OS. Enable short developer mode screen (1 second timeout) followed by default legacy mode boot by using these flags:
GBB_FLAG_DEV_SCREEN_SHORT_DELAY 0×00000001 GBB_FLAG_FORCE_DEV_SWITCH_ON 0×00000008 GBB_FLAG_FORCE_DEV_BOOT_LEGACY 0×00000080 GBB_FLAG_DEFAULT_DEV_BOOT_LEGACY 0×00000400
... which adds up to running in the shell:
# shutdown -h now
... and disconnect the power:
- Remove the bottom cover again and reinstall the write-lock screw to protect the BIOS
- Close cover and reinstall all the case screws
Re-connect power, insert USB stick prepared in Step 1, and power up Chromebook. 
2.2 Developer Mode Method
The alternative to removing the write-protect screw above is to place the Chromebook into developer mode  using the Chrome OS shell before booting the USB install media:
- With the Chromebook off... Hold down ESC+F3 (Refresh) keys and power on the device
- Invoke Recovery, and at the Recovery screen press Ctrl+D
- Device will prompt for confirmation, press ENTER and the system reboots into developer mode
- Scary white boot screen appears and you need to press Ctrl+D to continue boot
Enable the USB Boot and Legacy BIOS modes by opening the shell with Ctrl+Alt+T and enter shell. Set dev_boot_usb and dev_boot_legacy to active:
$ sudo crossystem dev_boot_usb=1 $ sudo crossystem dev_boot_legacy=1
Insert USB stick prepared in Step 1, reboot the Chromebook and press CTRL+L at the boot screen to enter legacy boot mode. 
3. Install Debian
My visual screenshot tour installing Debian jessie/stable. Due to Chromebook's small storage capacity, however, I use a different 3-partition layout:
- sda1 configured as 300MB boot
- sda2 configured as 2GB LUKS encrypted swap with random key
- sda3 uses remaining space as LUKS encrypted root with passphrase
Debian jessie/stable installs Linux kernel 3.16 with backported support for Chromebook touchpad. Per-session modifications of touchpad settings can be made using synclient:
$ synclient # display current settings $ synclient TapButton3=2 # 3-finger tap = middle-click
Make any desired touchpad settings permanent by creating /etc/X11/xorg.conf.d/50-c720-touchpad.conf:
$ sudo mkdir /etc/X11/xorg.conf.d $ sudo cp /usr/share/X11/xorg.conf.d/50-synaptics.conf /etc/X11/xorg.conf.d/50-c720-touchpad.conf
Modify to adjust timeouts, add tap-mouse actions (2-finger tap = right-click, 3-finger tap = middle-click), and 2-finger scrolling. 
Section "InputClass" Identifier "touchpad peppy cyapa" MatchIsTouchpad "on" MatchDevicePath "/dev/input/event*" MatchProduct "cyapa" Option "FingerLow" "5" Option "FingerHigh" "5" Option "VertEdgeScroll" "0" Option "VertTwoFingerScroll" "1" Option "HorizTwoFingerScroll" "1" Option "AreaRightEdge" "850" Option "AreaLeftEdge" "50" Option "TapButton1" "1" Option "TapButton2" "3" Option "TapButton3" "2" EndSection
Swappiness parameter controls the preference of the kernel to move processes out of physical memory to the swap partition. Range is 0-100, default is set to 60 and lower values cause the kernel to avoid swapping and higher values prompt more frequent swap use.
Check current swappiness value:
$ cat /proc/sys/vm/swappiness
Reduce SSD writes by setting a low value of vm.swappiness=1 in /etc/sysctl.conf.
TRIM optimizes SSD performance  and is enabled by adding the discard option to /etc/crypttab and /etc/fstab.
sda2_crypt /dev/sda2 /dev/urandom cipher=aes-xts-plain64,size=256,discard,swap sda3_crypt UUID=<STRING> none luks,discard
# <file system> <mount point> <type> <options> <dump> <pass> /dev/mapper/sda3_crypt / ext4 noatime,discard,errors=remount-ro 0 1 UUID=<STRING> /boot ext4 noatime,discard 0 2 /dev/mapper/sda2_crypt none swap sw,discard 0 0
$ sudo update-initramfs -u -k all
Problem: Suspend-and-resume generates a stream of errors:
ehci-pci 0000:00:1d.0: port 1 resume error -19 ehci-pci 0000:00:1d.0: port 2 resume error -19 usb usb3-port1: over-current condition usb usb3-port1: connect-debounce failed usb usb3-port2: over-current condition usb usb3-port2: connect-debounce failed
... and blocks Jessiebook from executing a proper restart/shutdown.
FIX:  Create /lib/systemd/system-sleep/ehci-pci.sh:
#!/bin/bash case $1/$2 in pre/*) # Unbind ehci for preventing error echo -n "0000:00:1d.0" | tee /sys/bus/pci/drivers/ehci-pci/unbind ;; post/*) # Bind ehci for preventing error echo -n "0000:00:1d.0" | tee /sys/bus/pci/drivers/ehci-pci/bind ;; esac
Make the script executable:
$ sudo chmod 755 /lib/systemd/system-sleep/ehci-pci.sh
Configure boot options in /etc/default/grub:
$ sudo update-grub
7. Keyboard Shortcuts
Top row on the keyboard with the shortcut icons (Brightness, Volume, etc.) identify in Linux as F1-F10 keys and the Search key (in CapsLk position) acts as Super (Windows) modifier key.
Create keyboard shortcuts by first installing:
- xbindkeys - associate keys to shell commands
- xbacklight - set backlight level using RandR
- pulseaudio-utils - manage sound with pactl
- xvkbd - send characters to another client
$ sudo apt-get install xbindkeys xbacklight pulseaudio-utils xvkbd
7.1 Direction, Brightness, Volume, Page Keys
$ xbindkeys -k
Enable function keys to modify sound and brightness settings by creating ~/.xbindkeysrc.
# backward/forward "xvkbd -xsendevent -text "\A\[Left]"" F1 "xvkbd -xsendevent -text "\A\[Right]"" F2 # screenshots # desktop "scrot '%Y-%m-%dT%H%M%S.png' -e 'mv $f ~/Downloads && eog ~/Downloads/$f'" Alt + F4 # active Window "scrot -d 4 -u -z '%Y-%m-%dT%H%M%S.png' -e 'mv $f ~/Downloads && eog ~/Downloads/$f'" Control + F4 # backlight decrease/increase "xbacklight -dec 10" F6 "xbacklight -inc 10" F7 # volume mute/decrease/increase # paVolume - https://github.com/vonbrownie/homebin/blob/master/paVolume "paVolume -m" F8 "paVolume -d" F9 "paVolume -u" F10 # page up/down, home, end "xvkbd -xsendevent -text '\[Page_Up]'" Alt + Up "xvkbd -xsendevent -text '\[Page_Down]'" Alt + Down "xvkbd -xsendevent -text '\[Home]'" Alt + Left "xvkbd -xsendevent -text '\[End]'" Alt + Right
Enable new key shortcuts:
Place xbindkeys in ~/.xinitrc to load configuration at startx. 
7.2 Power Key
Power key in upper-right corner ignores any configuration in the window manager and triggers poweroff without delay when pressed (easy to do by accident as its positioned next to backspace).
If you want to disable the power key edit /etc/systemd/logind.conf and set HandlePowerKey=ignore.
There are a few settings to modify to improve performance of Chromebook's wireless chipset.  Identify the card and parameters:
$ lspci | grep -i net 01:00.0 Network controller: Qualcomm Atheros AR9462 Wireless Network Adapter (rev 01) $ modinfo ath9k | grep parm parm: debug:Debugging mask (uint) parm: nohwcrypt:Disable hardware encryption (int) parm: blink:Enable LED blink on activity (int) parm: btcoex_enable:Enable wifi-BT coexistence (int) parm: bt_ant_diversity:Enable WLAN/BT RX antenna diversity (int) parm: ps_enable:Enable WLAN PowerSave (int) parm: use_chanctx:Enable channel context for concurrency (int)
options ath9k bt_ant_diversity=1 ps_enable=0
Confirm the microphone is un-muted in alsamixer. Create /etc/modprobe.d/snd-hda-intel.conf:
options snd_hda_intel model=,alc283-dac-wcaps
... and restart (I couldn't get the module to unload). Give it a try:
$ arecord -d 5 chr-mic.wav $ aplay chr-mic.wav
10. Helpful Resources
- Specs: output of lshw, lspci, and lsusb
- Arch Linux C720 installation with useful post-install details
- Turn Chromebooks into Ubuntu-based code learning machines for kids
- My earlier install and configuration of Lubuntu 14.04 LTS under upstart (vs systemd in Debian)
- Lightweight i3 tiling window manager is snappy on the Chromebook's modest hardware
- Create shell scripts to configure Chromebook when it leaves and returns home
|||Create a Chromebook recovery image.|
|||Image courtesy of Chromium.|
|||Useful GBB flags for another new free software machine.|
|||Whenever you remove battery power to the Chromebook (like opening up the case) the hardware clock on the motherboard resets to a future year (mine travelled to 2040). Providing a network connection is up during the Debian installation the system should fetch a correct time from a NTP server. Otherwise fix the fallout from an incorrect clock by re-mounting partitions read-only and correct filesystem timestamps using fsck.|
|||Switching between developer and normal (non-developer) modes will remove user accounts and their associated information from the Chromebook.|
|||Chromium OS developer information for the Acer C720 Chromebook|
|||Entries for Touchpad Synaptics and the C720 Chromebook on ArchWiki|
|||TRIM configuration on solid-state drives|
|||Some HOWTOs talk about adding modprobe.blacklist=ehci_hcd,ehci_pci.|
|||Xbindkeys, and another sample Chromebook-friendly xbindkeysrc|
|||Wireless ath9k driver and bluetooth coexistence|