I am do­ing some semi-se­ri­ous Rasp­ber­ry Pi de­vel­op­men­t, so it was time I fig­ured out how to do it com­fort­ably.

My desk­top set­up is a two-­mon­i­tor con­fig­u­ra­tion, with my note­book on the ta­ble and a larg­er mon­i­tor above it. I like it, it's nice. The point­er nat­u­ral­ly goes from one screen to the oth­er in the ob­vi­ous way.

Spe­cial­ly nice is that the lap­top's screen is touch and has an ac­tive pen, so I can use it nat­u­ral­ly.

But now, with the Rasp­ber­ry, I want to oc­ca­sion­al­ly show its dis­play. And that means switch­ing the mon­i­tor to it. Since I hate plug­ging and un­plug­ging things, I use one of the­se:

It's a cheap tiny black plas­tic box that takes up to 5 HD­MI in­puts and switch­es be­tween them to its one out­put by click­ing a but­ton. It on­ly goes through the in­puts that have sig­nal, so since I on­ly have the lap­top's and the Pi's the but­ton tog­gles be­tween them.

If your mon­i­tor has more than one HD­MI in­put you can prob­a­bly just use that, but mine has just one.

But... what about key­board and mouse?

I could get a mul­ti­de­vice key­board and mouse, but I like the ones I have.

I could use a USB switch and tog­gle be­tween the two de­vices, but ... I don't have one.

So, I use bar­ri­er and con­fig­ure it in both the rasp­ber­ry pi and in the lap­top so that when my point­er goes "up" in­put goes to the Pi, and when it goes "down" in­put goes to the lap­top. That's ex­act­ly the same as with the du­al-dis­play se­tup, but with two com­put­er­s. Neat!

So, go ahead and con­fig­ure bar­ri­er. It's easy and there are tons of tu­to­ri­al­s.

Nex­t, make sure bar­ri­er starts when I lo­gin in­to both com­put­er­s. They way I pre­fer to do these things is us­ing sys­temd.

Put this in ~/.local/share/systemd/user/barrier.service in both machines:

[Unit]
Description=Barrier server
[Service]
Environment=DISPLAY=:0
Type=simple
TimeoutStartSec=0
ExecStart=/usr/bin/barrier
[Install]
WantedBy=default.target

Now you can make it start with systemctl --user start barrier or stop with systemctl --user stop barrier and make it start on every login with systemctl --user enable barrier

But while this is nice, it presents a prob­lem. When I am us­ing both dis­plays for the lap­top, I don't want bar­ri­er run­ning! Since I can't see the Pi's dis­play, it makes no sense.

So, I want to start bar­ri­er when the lap­top is us­ing one mon­i­tor, and stop it when it's us­ing two.

To do that, the trick is udev in the laptop. Put this (replacing my username with yours) in /etc/udev/rules.d/90-barrier.rules:

ACTION=="change", \
    KERNEL=="card0", \
    SUBSYSTEM=="drm", \
    ENV{DISPLAY}=":0", \
    ENV{XAUTHORITY}="/home/ralsina/.Xauthority", \
    ENV{XDG_RUNTIME_DIR}="/run/user/1000", \
    RUN+="/home/ralsina/bin/monitors-changed"

Basically that means "when there is a change in the configuration of the video card, run monitors-changed. Change the 1000 by your user ID, too.

The last piece of the puzzle is the monitors-changed script:

if `xrandr --listmonitors | grep -q HDMI`
then
    # The HDMI output is connected, stop barrier
    su ralsina -c '/usr/bin/systemctl stop --user barrier'
else
    # The Pi is using the monitor, start barrier
    su ralsina -c '/usr/bin/systemctl start --user barrier'
fi

And that's it!

This is the be­hav­iour now:

• When the lap­­top is us­ing both dis­­­plays, they work nor­­mal­­ly in a "ex­­tend­ed dis­­­play" con­­fig­u­ra­­tion. They be­have like a sin­­gle large screen.

• When I click on the HD­­MI switch and change the top dis­­­play to show the Pi's desk­­top, au­­to­­mat­i­­cal­­ly bar­ri­er starts in the lap­­top, and now the point­er and key­board change from one com­put­er to the oth­­er when the point­er moves from one mon­i­­tor to the nex­t.

• If I click on the HD­­MI switch again, bar­ri­er stops on the lap­­top and I have a sin­­gle two-screen desk­­top again.

Ev­ery­thing be­haves per­fect­ly and I can switch be­tween com­put­ers by click­ing a but­ton.

Al­ter­na­tive­ly, we could start the bar­ri­er client when the rasp­ber­ry pi "get­s" the dis­play, and stops it when it goes away. The re­sult should be the same ex­cept for some cor­ner cas­es, but it has the added ben­e­fit of al­low­ing for a set­up with up to 5 de­vices :-)