Remapping Logitech K520 Fn keys on Linux?

Question

I recently bought a Logitech K520 keyboard & mouse. It's a nice keyboard, and, as usual these days, it has laptop-style Fn keys mapped as alternatives onto the F1-F12 keys. I don't really need these, but since I'm giving up my right-hand Super/Windows key for an otherwise useless Fn key, it'd be nice to get some use out of them.

The problem is: some of the keys, instead of sending special key codes, send key combinations. For example, Fn+F5 sends Alt+Tab, and Fn+F6 sends Win+D.

I don't see any way to distinguish between actual Alt+Tab and Fn-key generated Alt+Tab at the operating system level - is there some way to program the keyboard to send alternative key codes? It it involves Windows-only software, would the change be permanent, i.e. could I reconfigure the keyboard from a Windows virtual machine and then reap the benefits on Linux?

Answer 1

If you want to hack a bit, it is possible to remap these Fn keys on Linux. In my article Logitech Unifying for Linux: Reverse Engineering and unpairing tool, I describe how to monitor USB traffic to reverse engineer Logitech Unifying devices.

Requirements for reverse engineering reassignment of Fn keys yourself:

• Basic knowledge of Logitechs HID++ 1.0 protocol (and perhaps 2.0 too depending on your device). (available at https://lekensteyn.nl/files/logitech/)
• The tool presented in my article, read-dev-usbmon, can be used for easier monitoring.
• Ability to write to privileged devices (usually /dev/hidraw0), see my article for details.

When time permits, I will look into this. Until that, I can only give you pointers:

• On my K800 keyboard (HID++ 1.0), enabling notification bit 1 on field 1 changes the information sent by the keyboard, presumably to allow for customization as described by you. From https://git.lekensteyn.nl/ltunify/tree/registers.txt#n80:

  00 rw ENABLED_NOTIFS, 10 02 00, 10 is Battery info, buy what is 02?
  After writing FF FF FF, reading the register shows 13 02 00
  .1 .. .. - If enabled, keys like Web and Mute are passed over the HID protocol
      with short message type=03. This presumably allows the user to customize
      key bindings.
  .2 .. .. - ?
  1. .. .. - battery status (documented) (see also below, register/type 07)
  .. .2 .. - backlight changes (pressing Fn+F[56])
  

When somebody implements this (maybe me), it will likely end up in the Solaar software. This feature is tracked on https://github.com/pwr/Solaar/issues/31

Answer 2

solaar installed via pip worked for me with the "K400 Plus" model. example:

solaar config 1 fn-swap off