Use a laptop as a 2nd display on Linux using FreeRDP

use_laptop_2nd_display_linux_freerdp.md

Mirror of https://blog.jacobstoner.com/use-a-laptop-or-tablet-as-a-2nd-display-on-linux-with-a-dummy-monitor-and-rdp/

Dual monitors using a laptop as a second display. This guide assumes that you are using a Debian based distro (X11 only). Both devices must be connected to the same network.

1. Define an output to be a used as a dummy monitor

a. The simple method: select an unused existing graphics port

# find an unused output
find -L /sys/class/drm -maxdepth 1 -type d -name 'card0-*' -exec sh -c 'if [ $(cat $0/status) = "disconnected" ]; then echo $0; fi' {} \;|cut -c22-|sort
#on my system DP-1 is available
OUTPUT=DP-1
DIR=/sys/kernel/debug/dri/0/$OUTPUT

b. The other way: use the evdi kernel module to add an output

In case you can't use an existing graphics port

# typically the evdi module must be installed first
sudo apt install -y evdi-dkms

# load the module, requesting a dummy device
sudo modprobe evdi initial_device_count=1

# find the new output
DIR=$(sudo find /sys/kernel/debug/dri/1/ -maxdepth 1 -name 'DVI*'|head -1)

2. Force activate the output

# override EDID (pretend to be a 1080P monitor with lots of resolution options)
EDID_BASE64=AP///////wAebdVZ+KEJAAQXAQNoNR546jMxpFdRoCYQUFSna4CzAIGAlQBxT6nAgQCBwJBAAjqAGHE4LUBYLEUACSUhAAAeAAAA/QA4Sx5TDwAKICAgICAgAAAA/AAyNEVOMzMKICAgICAgAAAA/wAzMDRORFBISksyODgKAJ8=
sudo bash -c "base64 -d <<<$EDID_BASE64 > $DIR/edid_override"

# force output on
sudo sh -c "echo on > $DIR/force"

# add an additional resolution to the dummy display (optional)
# check the xrandr output name, it may be different than the kernel name
xrandr|grep ' connected'
OUTPUT=DP-1
RES="1920 1200"
MODELINE=$(cvt $RES|awk '/Modeline/{print substr($0,10)}')
MODE=$(sed 's/^"\([^"]\+\).*/\1/' <<<$MODELINE)
sh -c "xrandr --newmode $MODELINE"
xrandr --addmode $OUTPUT $MODE

# Use the standard display configuration utility to configure the dummy display.
# Match the resolution of your device and position the display.

If the display fails to activate or 1920x1080 doesn't show up, then reboot and select a different output in step one.

3. Serve the dummy monitor with FreeRDP

sudo apt install -y freerdp2-shadow-x11
freerdp-shadow-cli -auth -may-interact /monitors:1

4. Connect FreeRDP client

sudo apt install -y freerdp2-x11
xfreerdp /v:YOUR_HOST_IP /cert-ignore /floatbar:sticky:off,show:always -gfx /rfx /f

Notes:

• freerdp-shadow-cli is somewhat buggy in this setup, even the development version, but the settings here work well.
• -may-interact is required for the cursor to show up on the client. Alternately the local cursor can be used on each device.
• Disabling GFX is required for reliable operation, but we still have rfx
• GFX works in the development version of the client, but no cursor
• aFreeRDP works on android, but no cursor
• wfreerdp works on windows, but no cursor
• Does not work reliably with other RDP clients
• In my testing, VNC is not a suitable replacement for FreeRDP in this setup--it's too slow.
• Thank you to all of the FreeRDP developers for this awesome software.

Discussion: https://news.ycombinator.com/item?id=34010970