Supporting multiple monitor EDID override profiles

Question

I would like to be able to switch my Pi between several different physical monitors that I have (one at a time). The screens are not all the same type or resolution.

For most of the monitors, the Pi's autodetection works fine and it displays at the "right" resolution with nothing specified in config.txt.

However for some of them I need to explicitly specify hdmi_mode or similar in config.txt to get it to display at the correct resolution. Unfortunately, this means that I need to edit config.txt every time that I want to switch between these screens.

Is there any way of avoiding this? Can I somehow get it to detect a specific brand/model of monitor (via EDID, presumably) but then go to a specific mode that I choose for that monitor instead of what it wants to do by default -- while still behaving normally for other monitors? Ideally right from boot, but it's probably ok if a boot script can change the resolution in flight.

I'm ok with modifying code to do this, if someone can point me at the program that would have to be changed.

Answer 1

The somewhat similar issue was posted here. A part of that post details a startup script that waits for a monitor to be connected and then reads the EDID, sets the preferred group/mode and then runs fbset to modify the frame buffer. For your use case, you'd need to make some changes to the line that looks for "preferred mode" to do whatever you want.

Another solution is to run a startup script that does almost the same thing except it then writes to /boot/config.txt automatically and then triggers a reboot if it actually changed the hdmi_group or hdmi_mode.

# Taken from http://blogs.wcode.org/2013/09/howto-boot-your-raspberry-pi-into-a-fullscreen-browser-kiosk/

# Wait for the TV-screen to be turned on...
while ! $( tvservice --dumpedid /tmp/edid | fgrep -qv 'Nothing written!' ); do
    bHadToWaitForScreen=true;
    printf "===> Screen is not connected, off or in an unknown mode, waiting for it to become available...\n"
    sleep 10;
done;

printf "===> Screen is on, extracting preferred mode...\n"
_DEPTH=32;
eval $( edidparser /tmp/edid | fgrep 'preferred mode' | tail -1 | sed -Ene 's/^.+(DMT|CEA) \(([0-9]+)\) ([0-9]+)x([0-9]+)[pi]? @.+/_GROUP=\1;_MODE=\2;_XRES=\3;_YRES=\4;/p' );

printf "===> Resetting screen to preferred mode: %s-%d (%dx%dx%d)...\n" $_GROUP $_MODE $_XRES $_YRES $_DEPTH
tvservice --explicit="$_GROUP $_MODE"
sleep 1;

printf "===> Resetting frame-buffer to %dx%dx%d...\n" $_XRES $_YRES $_DEPTH
fbset --all --geometry $_XRES $_YRES $_XRES $_YRES $_DEPTH -left 0 -right 0 -upper 0 -lower 0;
sleep 1;

Answer 2

FYI, after asking about this here I contacted the makers of the Pi directly and something was added to the config.txt file to support this more directly (at least for the case where the monitor is connected at startup -- if you want to switch monitors after boot then you'll still need a script).

See the documentation, specifically the section starting The [EDID=*] filter.

Essentially, you can use tvservice -n to find the unique EDID name of each monitor, and then use multiple [EDID=the_name] sections to provide the settings to use for each monitor.