displayport KVMs for sharing monitors and keyboards

I’ve got a pair of Samsung 28 inch 4k monitors that I use for my daily environment. I have 3 (actually 4) machines to share them between, 2(3) linux boxen and 1 Mac laptop.

In my original design, pre-KVM switch, I had one monitor dedicated to the Mac, and one switched with the annoying little joystick at the back of the monitor, with a little set of 3 USB cords, a powered USB hub, and simple plugging/unplugging.

Annoying, but it worked. Though I couldn’t span both monitors for my desktops. My primary desktop is a linux system. Mac is the work desktop, and I switch back and forth between them to keep strict separation between work and non-work.

Then I thought, hey, I’ve used KVM switches for decades, can’t I use a nice displayport KVM here, and simplify the setup.

Why yes! Yes I can.

And you know, since I’ve got these 2 monitors, with displayport (no USB-C/thunderbolt), and my machines all have displayport output (2 of 3 linux boxes have DP 1.2/1.4 outputs, mac has a thunderbolt3 to dual displayport and 2x single display port USB-C if I want more dongles), let me use a nice MST based KVM switch to keep cabling simpler. MST has been around since DP 1.2, and has been used since about 2010 to “daisy chain” displays with DP.

Mature tech. Should just work.

Ah … Joe … so naive … so … so naive.

All the linux boxen worked. No real setup required apart from ordering the screens correctly. I was running 1x 1920×1200 + 2x 3840×2200@59.97Hz displays from my laptop, 2x 3840×2200@59.97Hz from the deskside.

The apple Mac, a 2018 device, did not work. Or more correctly, MST did not work. The displays mirrored.

But worse, the KVM itself doesn’t seem to have EDID emulation, so the displays have to cycle through resolutions on switching ports. On more than one occasion, I crashed nvidia-modeset from this. Which forced me to reboot that device.

After a week of playing with this, I am … well … going to return this switch. This is an ATEN CS1924M. It’s not a bad device, if/when it works. But I’ve crashed it many times, and now switching doesn’t work well at all. Some ports don’t work, and my laptop refuses to work with it anymore.

I’ve swapped cables, verified that the cables I am using with the KVM work with the displays (directly), made sure I had DP1.4 certified cables, etc.

Now I am looking for a replacement. Something that will just work. In the past, I’d used a Startech DVI dual port switch that worked quite well. They have a displayport version with 4 ports for machines. But they don’t mention the EDID emulator. So I am still looking.

I found a Level1Tech KVM that looks interesting, and possibly a BlackBox one. There’s an Avocent, and a different ATEN unit. I’m a little reluctant to look at the other ATEN unit, and reviews of the Avocent are less encouraging.

Hopefully in a few years, when I am ready to replace these monitors, everything will be USB-C based, and KVMs will be easier. Hopefully.