ASUS C302

The Asus C302 (also known as Asus C302C and Asus C302CA) is a Chromebook, which can have Linux installed. Here are some pointers on the smooth running of Arch Linux.

Installation

Add acpi_osi=Linux intel_iommu=on,igfx_off to your kernel parameters.

intel_iommu=on,igfx_off is from BBS#228604, to prevent the following error from appearing in the output of dmesg:

[drm:intel_cpu_fifo_underrun_irq_handler [i915]] *ERROR* CPU pipe A FIFO underrun

The CPU is an Intel Core M3, so install intel-ucode.

Kernel Modules

In e.g. /etc/modprobe.d/skylake.conf

# Needed, for the nau8825 kernel sound modules to initialize
blacklist snd_hda_intel

options tpm_tis interrupts=0

Screen

The screen is 12.5 inches diagonally. At 1920x1080 resolution, this is 176 DPI, which is HiDPI.

In ~/.xinitrc, to set the screen dimensions (measured in millimetres):

xrandr --fbmm 277x156

Vsync

For proper vsync (including e.g. fullscreen Youtube in Firefox) in XFCE, install xfwm4-git^AUR, and enable XFCE's compositor.

Proper vsync also requires xf86-video-intel, rather than modesetting.

In /etc/X11/xorg.conf.d/20-intel.conf

Section "Device"
    Identifier "Intel Graphics"
    Driver "intel"
EndSection

Screen Flipping

To use the laptop in tent mode, the following script will flip both the screen and touch input:

#!/bin/dash

set_normal() {
	r="normal"
	ctm="1 0 0 0 1 0 0 0 1"

}

set_upside_down() {
	r="inverted"
	ctm="-1 0 1 0 -1 1 0 0 1"
}

line=$(xrandr | grep '^eDP.* connected ')
screen=$(printf "%s" "$line" | cut -d" " -f1)

if $(printf "%s" "$line" | grep -q ' inverted (normal') ; then
	set_normal
else
	set_upside_down
fi

xrandr --output "$screen" --rotate "$r"
xinput set-prop "Elan Touchscreen" "Coordinate Transformation Matrix" $ctm

Keyboard

To use the top row of Chromebook keys as useful keys in xorg, use e.g.:

In ~/.Xmodmap

keycode   9 = Escape NoSymbol Escape
keycode  22 = BackSpace BackSpace BackSpace BackSpace Delete NoSymbol Delete
keycode  37 = Control_L NoSymbol Control_L
keycode  50 = Shift_L NoSymbol Shift_L
keycode  66 = Caps_Lock NoSymbol Caps_Lock
keycode  67 = Home F1 Home F1 F1 F1 XF86Switch_VT_1
keycode  68 = End F2 End F2 F2 F2 XF86Switch_VT_2
keycode  69 = Prior F3 Prior F3 F3 F3 XF86Switch_VT_3
keycode  70 = Next F4 Next F4 F4 F4 XF86Switch_VT_4
keycode  71 = Delete F5 Delete F5 F5 F5 XF86Switch_VT_5
keycode  72 = XF86MonBrightnessDown F6 XF86MonBrightnessDown F6 F6 F6 XF86Switch_VT_6
keycode  73 = XF86MonBrightnessUp F7 XF86MonBrightnessUp F7 F7 F7 XF86Switch_VT_7
keycode  74 = XF86AudioMute F8 XF86AudioMute F8 F8 F8 XF86Switch_VT_8
keycode  75 = XF86AudioLowerVolume F9 XF86AudioLowerVolume F9 F9 F9 XF86Switch_VT_9
keycode  76 = XF86AudioRaiseVolume F10 XF86AudioRaiseVolume F10 F10 F10 XF86Switch_VT_10
keycode 111 = Up Up Up Up Prior Prior
keycode 112 = Prior NoSymbol Prior
keycode 113 = Left Left Left Left Home Home
keycode 114 = Right Right Right Right End End
keycode 115 = End NoSymbol End
keycode 116 = Down Down Down Down Next Next
keycode 117 = Next NoSymbol Next
keycode 118 = Insert NoSymbol Insert
keycode 119 = Delete NoSymbol Delete
keycode 124 = XF86PowerOff NoSymbol XF86PowerOff
keycode 167 = XF86Forward NoSymbol XF86Forward
keycode 182 = XF86Close NoSymbol XF86Close
keycode 191 = XF86ScreenSaver NoSymbol XF86ScreenSaver

(This list can be pruned.)

~/.Xmodmap will be loaded by /etc/X11/xinit/xinitrc, which effectively runs:

xmodmap ~/.Xmodmap

Keyboard Backlight

To be able to change the keyboard backlight brightness as a normal user, run as root:

b="/sys/devices/platform/GOOG0002:00/leds/chromeos::kbd_backlight/brightness"
chgrp users "$b" &&
chmod 660 "$b" &&
echo 6 > "$b"

It is a value between 0 (off) and 100 (full brightness). The default on ChromeOS is 25. 6 is a reasonable lower value.

ChromeOS is able to disable the keyboard backlight, when the keyboard is not being used - that functionality does not appear to be available in the Linux kernel, but can be replicated in a simple script, with the aid of xprintidle, e.g.:

#!/bin/dash

set_keyboard_backlight() {
	printf "%s" "$1" > "/sys/devices/platform/GOOG0002:00/leds/chromeos::kbd_backlight/brightness"
	b="$1"
}

b=0

while true ; do
	pgrep ^Xorg > /dev/null || exit 0

	seconds_to_sleep=10
	idle_millis=$(xprintidle)
	if [ "$idle_millis" -gt 10000 ] ; then
		nb=0
		seconds_to_sleep=5
	else
		nb=25
		seconds_to_sleep=15
	fi

	if [ "$nb" -ne "$b" ] ; then
		set_keyboard_backlight "$nb"
	fi

	echo "nb=$nb, sleeping for $seconds_to_sleep"
	sleep "$seconds_to_sleep"
done

Touchpad

As of libinput 1.12.0-2, the touchpad works nicely with all of:

• Tapping:
  • 1-finger tap = "left" button
  • 2-finger tap = "right" button
  • 3-finger tap = "middle" button

• Clickpad (clicking the lower portion of the touchpad):
  • Left side = "left" button
  • Right side = "right" button
  • Middle = "middle" button

Mouse

Due to the limited amount of USB ports, a Bluetooth mouse is a good option. The Logitech M590 mouse works great.

Run "bluetoothctl power on &" at startup, e.g. in ~/.xinitrc

In e.g. /etc/X11/xorg.conf.d/99-mouse.conf

Section "InputClass"
    Identifier "Logitech M590"
    MatchIsPointer "on"
    MatchDevicePath "/dev/input/event*"
    Driver "libinput"
    Option "AccelProfile" "flat"
EndSection

Then, the "speed" of the mouse can be set using the XFCE GUI, in "Settings - Mouse", setting the "acceleration" to e.g. 2.0

Sound

Sound is a work in-progress. Sound is reliable when using headphones only.

Save the audio firmware as /lib/firmware/9d70-CORE-COREBOOT-0-tplg.bin (filesize 23120 bytes).

To set audio to a sensible level, run in ~/.xinitrc:

amixer -q -c0 sset Headphone 70% &

Pulseaudio

To prevent audio "clicks", comment out "load-module module-suspend-on-idle" in /etc/pulse/default.pa and /etc/pulse/system.pa

Coil Whine

There is occasional coil whine, which can be greatly reduced by running:

printf "1" > /sys/devices/system/cpu/intel_pstate/no_turbo