N900 with a Slice of Raspberry Pi

N900 as a Remote Display

Now that I have a point-to-point local network between my N900 and Raspberry Pi, I've removed the need to connect a network cable, but I still have that pesky HDMI cable to get rid of. After all, you may want to hack on your Raspberry Pi in places where you don't have an HDMI-enabled display handy. Luckily, with a few tweaks you can use your N900 touchscreen as a display for the Raspberry Pi and still be able to use a keyboard or mouse you have connected to the Raspberry Pi for input.

Unfortunately, I can't just connect the composite out or HDMI out of the Raspberry Pi into the N900, but what I can do is take advantage of the relatively low-latency local USB network and share the N900 X display over VNC. The first step is to install the x11vnc package on the Raspberry Pi with sudo apt-get install x11vnc.

Once x11vnc is installed, I need to set it up so that it automatically launches when X launches. I suppose this isn't absolutely necessary. After all, you could ssh in every time and start it yourself, but I think having it automatically launch is much more convenient. To do this, create a file called /home/pi/.config/autostart/x11vnc.desktop with the following contents:

[Desktop Entry]
Name=X11VNC Server
Comment=Share this desktop by VNC
Exec=x11vnc -forever
Icon=computer
Terminal=false
Type=Application
StartupNotify=false
#StartupWMClass=x11vnc_port_prompt
Categories=Network;RemoteAccess;

Next, I need to change the settings for the x11-common package so that it allows X sessions to be launched by any user. This is necessary so that I can run startx at boot time automatically. Without this change, X will detect it's not being run from a console session, and it will error out. To do this, run sudo dpkg-reconfigure x11-common, and when prompted to select "Users allowed to start the X server", select Anybody.

The final step is to start X at boot time. There are a number of ways to do this, but one of the easiest ways on the Raspberry Pi is via the /boot/boot.rc file. By default, the file is not there, but if present, it allows you to specify commands to run during the boot process so you can do things like enable SSH or start X. The following /boot/boot.rc file does both:

# sourced from rc.local on Raspberry Pi
#
# Name this file as "boot.rc" and put it on the boot 
# partition if you want to run it.

# echo "Checking ssh and enabling if absent"
if ! ls /etc/rc5.d | grep "^S..ssh\$" >/dev/null; then
    insserv ssh
    service ssh start
fi
su pi -c startx

Once you boot the Raspberry Pi and set up the local USB network, if you ssh in and run ps -ef, you should see that x11vnc is running. Now you can launch a VNC client from the N900 (I prefer Presence VNC) and connect to 10.8.174.10, and you should see a copy of your Raspberry Pi X session (Figure 2). If you had the HDMI cable connected when the Raspberry Pi booted, the X session should be at full 1080p resolution, which might show up a bit small on the N900 screen. However, if you boot without HDMI connected (which is the general use case for this hack), the X session will be configured for composite output and be at a more manageable 640x480. At this resolution, once you tell Presence VNC to go full screen, it uses up the full N900 display, and because you are sharing a real X session on the Raspberry Pi, you can use any keyboard or mouse you have plugged in to it. Sure, it's not as nice as a giant 1080p display, but then it's hard to fit one of those in your pocket.

Figure 2. Presence VNC on N900 Connected to the Raspberry Pi

Although I've talked about the N900 a lot for this hack, the same principles should work to turn just about any device that can run a VNC client into a display for the Raspberry Pi, provided the two devices can connect over the network. In fact, if you are one of the many people who carry a color tablet around anyway, that would be a quite ideal display for the Raspberry Pi.