In the current climate of multiseat, videocards are the basic building block of each seat - that is, if you desire simplicity in multiseat. Two video cards means two seats. Three video cards, three seats. With that being said, the easy way to understand drm, or the Direct Rendering Manager, is as a kernel module which manages these video devices.
This is why in EasySeats when you assign your second or third or n video card to another seat the video card address represents as drm/card#.(insert number)
Freedesktop.org has a great write up about drm in much more detail: dri.freedesktop.org/wiki/DRM/
I don't have too much to say about this except for the fact that it works as expected, and the sound device did not require a reboot. I have a spare PCI-E Sound Blaster Audigy card that I used for this test, I would assume that a USB sound device of some kind would function the same. When I first created the second seat and logged into KDE Plasma, I was greeted with this:
After assigning my motherboard's onboard sound device to the second seat and logging in a second time, I was greeted with the following image. I plugged in my audio cable, and indeed it worked for its assigned seat. Sound on either seat was independent as expected.
Several months ago, I looked at a common hardware component used by bitcoin miners and the thought occurred to me: this might be useful for creating additional seats. So I went to eBay and purchased one, tested it, and uploaded a short video:
The long story short is, it works fantastic. I have a motherboard with only two PCI-Express 16x graphics slots on it, so that's my theoretical limit for seats: 2. But because I also have a single 1x PCI-E slot I can take advantage of that and use the riser to plug in an additional third 16x card, giving me a third seat.
My thoughts are that if at all, this setup would primarily be useful for anybody who has a motherboard with only one single 16x slot on it. That would negate buying a whole new computer or even another motherboard if you are against such a thing. Just add in another 16x slot using a 1x slot. Just be warned, this setup cannot be used for high end gaming on the second slot. One single PCI-Express lane has more than enough bandwidth for day to day desktop or internet usage, but it would be bandwidth starved with a large real-time 3d workload. All gaming would have to be in connection with whatever video card is in the first slot.
For a second demonstration video, I used BZFlag to quickly highlight the possibility of using multiple seats for gaming. At the end of the video, I return the computer settings back to single user dual-head.
For the purposes of EasySeats, the only benchmark that matters is: "how easy is it to set up?" I also wanted to place emphasis on the fact that it is not required for multiple users to be stuck using the same desktop.
I recently created a simple but fairly long video and uploaded it to a channel on YouTube, to demonstrate in real time a single-user dual-head system which is then split into multiseat, and then brought back to single-user again. I also demonstrated the simultaneous use of KDE, LXDE, and XFCE. I hope this video helps to demonstrate the ease which multiseat can be put into production use.
EasySeats is open source software developed to bring ease to users who choose to go multi-seat. It supports video cards for seat creation as well as external USB docks.