building a simple KVM switch for 30€

Prompted by tweets from Lesley and Dave, I thought about KVM switches again and came up with a rather cheap solution to my individual situation (YMMY, as usual).

As I've written last year, my desk has one monitor, keyboard and mouse and two computers. Since writing that post I got a new (bigger) monitor, but also an USB switch again (a DIGITUS USB 3.0 Sharing Switch) - this time one that doesn't freak out my dock \o/

However, having to switch the used computer in two places (USB and monitor) is rather inconvenient, but also getting an KVM switch that can do 4K@60Hz was out of question.

Luckily, hackers gonna hack, everything, and not only receipt printers (😉). There is a tool called ddcutil that can talk to your monitor and change various settings. And udev can execute commands when (USB) devices connect… You see where this is going?

After installing the package (available both in Debian and Fedora), we can inspect our system with ddcutil detect. You might have to load the i2c_dev module (thanks Philip!) before this works -- it seems to be loaded automatically on my Fedora, but you never know 😅.

$ sudo ddcutil detect
Invalid display
   I2C bus:             /dev/i2c-4
   EDID synopsis:
      Mfg id:           BOE
      Model:
      Serial number:
      Manufacture year: 2017
      EDID version:     1.4
   DDC communication failed
   This is an eDP laptop display. Laptop displays do not support DDC/CI.

Invalid display
   I2C bus:             /dev/i2c-5
   EDID synopsis:
      Mfg id:           AOC
      Model:            U2790B
      Serial number:
      Manufacture year: 2020
      EDID version:     1.4
   DDC communication failed

Display 1
   I2C bus:             /dev/i2c-7
   EDID synopsis:
      Mfg id:           AOC
      Model:            U2790B
      Serial number:
      Manufacture year: 2020
      EDID version:     1.4
   VCP version:         2.2

The first detected display is the built-in one in my laptop, and those don't support DDC anyways. The second one is a ghost (see ddcutil#160) which we can ignore. But the third one is the one we can (and will control). As this is the only valid display ddcutil found, we don't need to specify which display to talk to in the following commands. Otherwise we'd have to add something like --display 1 to them.

A ddcutil capabilities will show us what the monitor is capable of (or what it thinks, I've heard some give rather buggy output here) -- we're mostly interested in the "Input Source" feature (Virtual Control Panel (VCP) code 0x60):

$ sudo ddcutil capabilities
…
   Feature: 60 (Input Source)
      Values:
         0f: DisplayPort-1
         11: HDMI-1
         12: HDMI-2
…

Seems mine supports it, and I should be able to switch the inputs by jumping between 0x0f, 0x11 and 0x12. You can see other values defined by the spec in ddcutil vcpinfo 60 --verbose, some monitors are using wrong values for their inputs 🙄. Let's see if ddcutil getvcp agrees that I'm using DisplayPort now:

$ sudo ddcutil getvcp 0x60
VCP code 0x60 (Input Source                  ): DisplayPort-1 (sl=0x0f)

And try switching to HDMI-1 using ddcutil setvcp:

$ sudo ddcutil setvcp 0x60 0x11

Cool, cool. So now we just need a way to trigger input source switching based on some event…

There are three devices connected to my USB switch: my keyboard, my mouse and my Yubikey. I do use the mouse and the Yubikey while the laptop is not docked too, so these are not good indicators that the switch has been turned to the laptop. But the keyboard is!

Let's see what vendor and product IDs it has, so we can write an udev rule for it:

$ lsusb
…
Bus 005 Device 006: ID 17ef:6047 Lenovo ThinkPad Compact Keyboard with TrackPoint
…

Okay, so let's call ddcutil setvcp 0x60 0x0f when the USB device 0x17ef:0x6047 is added to the system:

ACTION=="add", SUBSYSTEM=="usb", ATTR{idVendor}=="17ef", ATTR{idProduct}=="6047", RUN+="/usr/bin/ddcutil setvcp 0x60 0x0f"

$ sudo vim /etc/udev/rules.d/99-ddcutil.rules
$ sudo udevadm control --reload

And done! Whenever I connect my keyboard now, it will force my screen to use DisplayPort-1.

On my workstation, I deployed the same rule, but with ddcutil setvcp 0x60 0x11 to switch to HDMI-1 and my cheap not-really-KVM-but-in-the-end-KVM-USB-switch is done, for the price of one USB switch (~30€).

Note: if you want to use ddcutil with a Lenovo Thunderbolt 3 Dock (or any other dock using Displayport Multi-Stream Transport (MST)), you'll need kernel 5.10 or newer, which fixes a bug that prevents ddcutil from talking to the monitor using I²C.