Highlights include:
> Under a non-compositing window manager, regular double-buffering under OpenGL does not incur undue latency (and there is a window manager hint to tell a compositing manager to grant pass-through to a window).
> At least using the Nvidia graphics driver [...] OpenGL can vsync to any connected display, primary or secondary.
> OpenGL and X Windows allow applications to draw directly into a video controller’s frame buffer, allowing single-buffered rendering and racing the video controller’s scanout. [...] With front buffer rendering, display latency on the Rift DK2 dropped to about 4.3ms
And from Zero-latency Rendering (2015) http://doc-ok.org/?p=1313
> with some new rendering configuration options in Vrui 4.0 I can disable vsync, and render directly into the display window’s front buffer. In other words, I can let these applications “race the beam.”
Something more directly analogous to "Direct Mode" on Windows are DRM leases, which give direct control over the scanout, rather than simply bypassing the compositor. This isn't possible on Linux at all yet, but Valve is working on it: https://keithp.com/blogs/DRM-lease/