Virtual display support on Linux: stream at on-demand resolutions/refresh rates without a dummy dongle

[EnriqueWood]

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TLDR; Have near-on demand virtual display with custom client resolution and frequencies. I'm already providing a working POC implementation (not built into sunshine, but using shell scripts to give an idea of how it could work)

Hi, and thanks for the recent capture work. The KWin / PipeWire / XDG-portal
capture that landed in the last release made streaming on KDE Wayland genuinely
nice on my machine, and honestly it's what made me think the thing I'm about to
ask for is now within reach.

What I'd really love to see is virtual display support on Linux, so people
don't have to keep a drawer of USB/DP dummy dongles around just to stream at a
resolution or refresh rate that differs from their physical monitor. As best I
understand it, it splits into a few pieces:

1. Be able to capture a virtual output, not just a physical one. With the
   new backends Sunshine can already grab a Wayland output cleanly, the missing
   half is having an output to grab that isn't your real screen. Today that
   means a hardware dummy (stuck with whatever modes the dongle's EDID
   advertises) or manual kernel/EDID tinkering.
2. Create resolutions / refresh rates on demand, ideally from the Web UI.
   The goal is to stream at whatever the client asks for, or from a list I
   configure, without buying a dongle per resolution. Windows already has the
   dd_* display-config options for this; on Linux there's no equivalent and we
   end up hand-rolling global_prep_cmd scripts.
3. (nice to have) Switch to the virtual output when a stream starts and
   restore the real monitor on disconnect. This one is already mostly possible
   today with global_prep_cmd, so it's the least important, but having it
   built in would make the whole thing turnkey.

Why I think it's worth doing: it removes the dongle requirement entirely, it
builds directly on the capture work you just shipped, and it lets the host match
the client's display exactly (better quality and latency than scaling a fixed
resolution). It also covers the "use my PC normally while someone streams to it"
and headless cases, and it'd bring the Linux experience closer to what Windows
users already get.

sadly, I'm not a graphics or kernel developer, which is why this
is a feature request and not a PR. But I didn't want to just drop an idea and
walk away, so I put together a working proof of concept (with a lot of help from
an AI assistant, since the low-level parts are beyond me) to show it's doable and
to save whoever picks this up some of the digging. It's been running well for me:

• OS: Nobara (Fedora 43 base), kernel 7.0.x
• Desktop: KDE Plasma 6, Wayland (KWin)
• GPU: NVIDIA RTX 3090 (proprietary driver 595.71.05)
• Sunshine: 2026.x from the lizardbyte/stable Copr
• Display: Samsung Odyssey G9 (5120x1440@240) on HDMI; virtual display forced
  on a spare DisplayPort connector

POC + notes: https://github.com/EnriqueWood/sunshine-virtual-displays-support-poc

How it works, briefly, in case it's useful:

• A spare DRM connector is force-enabled with a generated EDID, via the kernel
  cmdline video=<conn>:e drm.edid_firmware=<conn>:edid/<file>.bin. One gotcha
  worth flagging: on the NVIDIA proprietary driver drm.edid_firmware alone is
  silently ignored, you need video=:e as well. With both, you get a fully
  virtual output, no dongle and no HBR bandwidth cap.
• The EDID is generated from a mode list (1080p/1440p up to 240 Hz, 4K, plus odd
  client resolutions like my phone's 2448x1080) so clients get an exact match.
• On connect, a global_prep_cmd sets the virtual output to the client's
  requested mode (or the closest one), makes it the only active display so games
  and Steam open there, and holds a screensaver/DPMS inhibitor so capture doesn't
  die when the host is idle. On disconnect it restores the real monitor.
• If a client requests a mode that isn't in the EDID, it gets queued and the EDID
  is regenerated automatically.

And the limitations... where the POC hits a wall, and where an in-tree
solution could do better than my hack:

• The custom EDID is read at boot and NVIDIA doesn't hot-reload it, so a
  newly-added mode only appears after a reboot. A real virtual DRM device (along
  the lines of the kernel's VKMS, or a compositor-provided virtual output) could
  change modes live and would be the much cleaner long-term answer, that's
  really the feature I'm hoping for; the EDID approach is just what I could get
  working today.
• On a forced virtual connector the NVIDIA driver won't expose HDR or VRR (it
  only does that over a real HDMI 2.1 link with SCDC), so HDR streaming still
  needs a real display.

I'm glad to test patches, share the full scripts / EDID generator / notes, or
provide logs from my setup, whatever helps. Mostly I'd just love to see this
become a proper feature so the next person doesn't need a pile of dummy plugs.
Thanks for considering it, and for Sunshine in general.