It turns out xmodmap can not do this, at least on the keyboard I have and on KDE Plasma. (It works OK for mapping other keys, but not for the Alt-to-Meta mapping.)
But I got it working another way. It was difficult. I’m going to explain how I did it the best I can.
I remapped right-Alt on an IBM Space Saver Keyboard (SSK), which is the compact version of the Model M, connected via a PS/2-to-USB converter.
I used the method at Wyatt's Blog: Unicomp Mini M Review, Keyboard Remapping
(If that link dies, I suggest finding it via the Wayback Machine).
You’d better read that first, for the rest of this to make any sense.
The hardest part was figuring out which device in /sys/devices/ was my keyboard.
To do that:
1 - I ran sudo evtest. That gave me:
Available devices:
/dev/input/event0: Sleep Button
/dev/input/event1: Power Button
/dev/input/event2: Power Button
/dev/input/event3: Video Bus
/dev/input/event4: MSI MYSTIC LIGHT
/dev/input/event5: HDA Intel PCH HDMI/DP,pcm=3
/dev/input/event6: HDA Intel PCH HDMI/DP,pcm=7
/dev/input/event7: HDA Intel PCH HDMI/DP,pcm=8
/dev/input/event8: HDA Intel PCH HDMI/DP,pcm=9
/dev/input/event9: HDA NVidia HDMI/DP,pcm=3
/dev/input/event10: HDA NVidia HDMI/DP,pcm=7
/dev/input/event11: HDA NVidia HDMI/DP,pcm=8
/dev/input/event12: HDA NVidia HDMI/DP,pcm=9
/dev/input/event13: PixArt USB Optical Mouse
/dev/input/event14: 3Dconnexion SpaceNavigator
/dev/input/event15: Wacom Intuos S Pen
/dev/input/event16: Wacom Intuos S Pad
/dev/input/event17: ID Innovations Inc. Input Device
/dev/input/event18: UVC Camera (046d:0990)
/dev/input/event19: Keychron Keychron V3
/dev/input/event20: Keychron Keychron V3 Mouse
/dev/input/event21: Keychron Keychron V3 System Control
/dev/input/event22: Keychron Keychron V3 Consumer Control
/dev/input/event23: Keychron Keychron V3 Keyboard
(Your output will differ depending on hardware in your system.)
Try to guess which of the event* devices is your keyboard. Give evtest the number you guessed (in this case 0 to 23) and press a few keys - when you see some keyboard output from evtest, you’ve found it.
With a little trial-and-error, using evtest that way indicated that device ‘event17’ is my SSK keyboard. “ID Innovations Inc. Input Device” is the PS2-to-USB converter’s USB name (again, yours may differ).
With that info, I took the output from find /sys/ -iname *modalias|grep input to see the list of devices at the /sys/devices level. That gave me:
/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.1/sound/card1/input7/modalias
/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.1/sound/card1/input5/modalias
/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.1/sound/card1/input8/modalias
/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.1/sound/card1/input6/modalias
/sys/devices/pci0000:00/0000:00:1f.3/sound/card0/input9/modalias
/sys/devices/pci0000:00/0000:00:1f.3/sound/card0/input12/modalias
/sys/devices/pci0000:00/0000:00:1f.3/sound/card0/input10/modalias
/sys/devices/pci0000:00/0000:00:1f.3/sound/card0/input11/modalias
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-7/1-7.3/1-7.3:1.2/0003:3434:0331.000A/input/input24/modalias
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-7/1-7.3/1-7.3:1.2/0003:3434:0331.000A/input/input22/modalias
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-7/1-7.3/1-7.3:1.2/0003:3434:0331.000A/input/input23/modalias
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-7/1-7.3/1-7.3:1.2/0003:3434:0331.000A/input/input21/modalias
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-7/1-7.3/1-7.3:1.0/0003:3434:0331.0008/input/input20/modalias
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5.3/1-5.3.3/1-5.3.3:1.0/0003:046D:C626.0005/input/input14/modalias
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5.3/1-5.3.1/1-5.3.1:1.0/0003:056A:0374.0003/input/input17/modalias
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5.3/1-5.3.1/1-5.3.1:1.0/0003:056A:0374.0003/input/input15/modalias
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5.3/1-5.3.2/1-5.3.2:1.0/0003:04F2:0939.0004/input/input13/modalias
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5.4/1-5.4.4/1-5.4.4:1.0/0003:154A:0002.0006/input/input18/modalias
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5.4/1-5.4.1/1-5.4.1:1.0/input/input19/modalias
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-2/1-2:1.0/0003:1462:7D30.0001/input/input4/modalias
/sys/devices/LNXSYSTM:00/LNXSYBUS:00/PNP0C0C:00/input/input1/modalias
/sys/devices/LNXSYSTM:00/LNXSYBUS:00/PNP0C0E:00/input/input0/modalias
/sys/devices/LNXSYSTM:00/LNXSYBUS:00/PNP0A08:00/LNXVIDEO:00/input/input3/modalias
The question is: Which of those is the SSK keyboard?
As stated in the Wyatt’s Blog post, the directory that contains each of the files listed above has a file called ‘name’ which contains the USB name of the device (in my case the USB name of the PS2-to-USB converter, “ID Innovations Inc. Input Device”). When you find that directory, you can look at the ‘modalias’ in that directory for the next step.
To find that, do:
find /sys/ -iname '*name' -type f -exec grep '<string>' {} +
Replace with part of the USB device name you’re looking for. I used:
find /sys/ -iname '*name' -type f -exec grep 'Innovations' {} +
Ignore any error messages; just look for the line that contains the USB name you’re looking for. I got:
$ find /sys/ -iname '*name' -type f -exec grep 'Innovations' {} +
find: ‘/sys/kernel/tracing’: Permission denied
find: ‘/sys/kernel/debug’: Permission denied
find: ‘/sys/fs/pstore’: Permission denied
find: ‘/sys/fs/bpf’: Permission denied
find: ‘/sys/fs/fuse/connections/50’: Permission denied
find: ‘/sys/fs/fuse/connections/49’: Permission denied
grep: /sys/devices/pci0000:00/0000:00:14.3/net/wlo1/phys_port_name: Operation not supported
grep: /sys/devices/pci0000:00/0000:00:1c.1/0000:04:00.0/net/enp4s0/phys_port_name: Operation not supported
/sys/devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5.4/1-5.4.4/1-5.4.4:1.0/0003:154A:0002.0009/input/input23/name:ID Innovations Inc. Input Device
grep: /sys/devices/virtual/net/lo/phys_port_name: Operation not supported
That’s telling you where to find the modalias file - in this case in sys/devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5.4/1-5.4.4/1-5.4.4:1.0/0003:154A:0002.0009/input/input23/.
(Yes, I know this is “input23” while above we had “event17”. I guess the numbers don’t necessarily match.)
So then cat the ‘modalias’ file in that folder. I got:
$ cat /sys/devices/pci0000:00/0000:00:14.0/usb1/1-5/1-5.4/1-5.4.4/1-5.4.4:1.0/0003:154A:0002.0009/input/input23/modalias
input:b0003v154Ap0002e0100-e0,1,4,11,14,k74,75,77,7D,7E,7F,B7,ram4,l0,1,2,3,4,sfw
As per the blog post, you need the output up to (and excluding) the first dash - in my case ‘input:b0003v154Ap0002e0100’.
With that, I created a file ‘/etc/udev/hwdb.d/remap_ibmssk.hwdb’ as follows:
evdev:input:b0003v154Ap0002e0100*
KEYBOARD_KEY_700e6=KEY_RIGHTMETA
Note I had to add the * at the end of the first line. The value after KEYBOARD_KEY comes from evtest as described in the Wyatt’s Blog post.
‘KEY_RIGHTMETA’ comes from include/linux/input-event-codes.h (google for it). It also works if you put in the numeric value instead (in this case that would be 126).
Then:
dave@MONOLITH:~$ sudo systemd-hwdb update
dave@MONOLITH:~$ sudo udevadm control --reload-rules
dave@MONOLITH:~$ sudo udevadm trigger
…and it works!
Simple? No. But possible? Yes.