Logitech G15 and Z-10 LCD-screen support under Linux

[hansdegoede]

A while ago I worked on improving Logitech G15 LCD-screen support under Linux. I recently got an email from someone who wanted to add support for the LCD panel in the Logitech Z-10 speakers to lcdproc, asking me to describe the process I went through to improve G15 support in lcdproc and how I made it work without requiring the unmaintained g15daemon code.

So I wrote a long email describing the steps I went through and we both thought this could be interesting for others too, so here it is:

1. For some reason I decided that I did not have enough projects going on at the same time already and I started looking into improving support for the G15 family of keyboards.

2. I started studying the g15daemon code and did a build of it to check that it actually works. I believe making sure that you have a known-to-work codebase as a starting point, even if it is somewhat crufty and ugly, is important. This way you know you have code which speaks the right protocol and you can try to slowly morph it into what you want it to become (making small changes testing every step). Even if you decide to mostly re-implement from scratch, then you will likely use the old code as a protocol documentation and it is important to know it actually works.

3. There were number of things which I did not like about g15daemon:

3.1 It uses libusb, taking control of the entire USB-interface used for the gaming functionality away from the kernel. 

3.2 As part of this it was not just dealing with the LCD, it also was acting as a dispatches for G-key key-presses. IMHO the key-press handling clearly belonged in the kernel. These keys are just extra keys, all macro functionality is handled inside software on the host/PC side. So as a kernel dev I found that these really should be handled as normal keys and emit normal evdev event with KEY_FOO codes from a /dev/input/event# kernel node.

3.3 It is a daemon, rather then a library; and most other code which would deal with the LCD such as lcdproc was a daemon itself too, so now we would have lcdproc's LCDd talking to g15daemon to get to the LCD which felt rather roundabout.
So I set about tackling all of these

4. Kernel changes: I wrote a new drivers/hid/hid-lg-g15.c kernel driver:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/hid/hid-lg-g15.c
Which sends KEY_MACRO1 .. KEY_MACRO18, KEY_MACRO_PRESET1 .. KEY_MACRO_PRESET3, KEY_MACRO_RECORD_START, KEY_KBD_LCD_MENU1 .. KEY_KBD_LCD_MENU4 keypresses for all the special keys. Note this requires that the kernel HID driver is left attached to the USB interface, which required changes on the g15dameon/lcdproc side.

This kernel driver also offers a /sys/class/leds/g15::kbd_backlight/ interface which allows controller the backlight intensity through e.g. GNOME kbd-backlight slider in the power pane of the control-center. It also offers a bunch of other LED interfaces under /sys/class/leds/ for controlling things like the LEDs under the M1 - M3 preset selection buttons. The idea here being that the kernel abstract the USB protocol for these gaming-kbd away and that a single userspace daemon for doing gaming kbd macro functionality can be written which will rely only on the kernel interfaces and will thus work with any kbd which has kernel support.

5. lcdproc changes

5.1 lcdproc already did have a g15 driver, which talked to g15daemon. So at first I started testing with this (at this point all my kernel work would not work, since g15daemon would take full control of the USB interface unbinding my kernel driver). I did a couple of bug-fixes / cleanups in this setup to get the code to a starting point where I could run it and it would not show any visible rendering bugs in any of the standard lcdproc screens

5.2 I wrote a little lcdproc helper library, lib_hidraw, which can be used by lcdproc drivers to open a /dev/hidraw device for them. The main feature is, you give the lib_hidraw_open() helper a list of USB-ids you are interested in; and it will then find the right /dev/hidraw# device node (which may be different every boot) and open it for you.

5.3 The actual sending of the bitmap to the LCD screen is quite simple, but it does need to be a very specific format. The format rendering is done by libg15render. So now it was time to replace the code talking to g15daemon with code to directly talk to the LCD through a /dev/hidraw# interface. I kept the libg15render dependency since that was fine. After a bit of refactoring, the actual change over to directly sending the data to the LCD was not that big.

5.4 The change to stop using the g15daemon meant that the g15 driver also lost support for detecting presses on the 4 buttons directly under the LCD which are meant for controlling the menu on the LCD. But now that the code is using /dev/hidraw# the kernel driver I wrote would actually work and report KEY_KBD_LCD_MENU1 .. KEY_KBD_LCD_MENU4 keypresses. So I did a bunch of cleanup / refactoring of lcdproc's linux_input driver and made it take over the reporting of those button presses.

5.5 I wanted everything to just work out of the box, so I wrote some udev rules which automatically generate a lcdproc.conf file configuring the g15 + linux_input drivers (including the key-mapping for the linux_input driver) when a G15 keyboard gets plugged in and the lcdproc.conf file does not exist yet.

All this together means that users under Fedora, where I also packaged all this can now do "dnf install lcdproc", pluging their G15 keyboard and everything will just work.

p.s.

After the email exchange I got curious and found a pair of these speakers 2nd hand for a nice price. The author of the initial email was happy with me doing the work on this. So I added support for the Z-10 speakers to lcdproc (easy) and wrote a set of kernel-patches to support the display and 1-4 keys on the speaker as LCD menu keys.

I've also prepared an update to the Fedora lcdproc packages so that they will now support the Z-10 speakers OOTB, if you have these and are running Fedora, then (once the update has reached the repos) a "sudo dnf install lcdproc" followed by unplugging + replugging the Z-10 speakers should make the display come alive and show the standard lcdproc CPU/mem usage screens.