I can play Quake II RTX on my 6900XT just fine (with dynamic resolution scaling). I think new games are going to be hit and miss until the drivers mature.
Raytracing works pretty well for a decent amount of games at least on the Nvidia side. They just enabled RT by default for AMD GPUs but their RT is generally slower than Nvidia's.
Does the game have to be native or does it work through Proton?
Haven't tried any native games. Proton works depending on the game. For example Cyperpunk 2077 works but RT in Witcher 3 doesn't yet work (for me).
Is the performance comparable? Better? Worse?
Compared to Windows? Performance is worse but not by that much (for me on Nvidia). Depending on the game it is comparable.
RT is very heavy on performance. You are going to want to enable DLSS or FSR.
Is it only working for AMD? Only for Nvidia? Both? (Neither?)
From what I've read they both work.
As already mentioned in another post it can be hit or miss. I think it works in most games by now. When Cyberpunk 2077 came out it took a long time for RT to work on Linux. Now it just works on most games I've tried. Witcher 3 being the exception since the update that introduced RT functionality. I'm on Nvidia. AMD should also work now. But supposedly performance isn't great on AMD (just correct me if I'm wrong).
Yes, I'm on Wayland (Sway). I haven't tested any native games. But I think RT/DLSS doesn't care about Wayland or XWayland.
That being said, for now every game you run via Wine/Proton is running on XWayland (if one is on Wayland). They are in the process of merging code for native Wayland to Wine/Proton bit by bit. But it is going to take a while before everything is in place and Wine/Proton doesn't need XWayland anymore. But as I've said, from what I understand this isn't an issue for RT/DLSS at all.
Last year I remember seeing something about working rt on Linux games via steamos, I imagine it’s coming soon. It worked for nvidia. I’m unsure of the amd side of the house.
I thought that was only novel because the steam deck doesn't have ray tracing-specific hardware. It was also on a game that otherwise wasn't very graphically intensive.
If your graphics card has ray tracing hardware, then it already worked (albeit maybe not as smoothly as on windows, check protondb for details per game).