Add synthetic panel-fit fallback modes - #2780
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Generate conservative CVT reduced-blanking fallback modes for connectors that support aspect-preserving hardware panel scaling. Fallback modes are filtered against real connector capabilities, including dimensions, pixel clock, timing-derived refresh rate, tiled-output status, and eDP mixed-mode safety. They are advertised through the normal Smithay output mode path, persisted by size/refresh, and resolved back to complete DRM timings during modesets. When a synthetic mode is selected, configure the connector for `Full aspect` scaling and submit the generated timing as the real CRTC MODE_ID, allowing the compositor to render a lower-resolution framebuffer while panel fitting expands it to the native display. Add unit coverage for CVT-RB timings, eligibility, filtering, orientation, preferred-mode behavior, and persisted synthetic mode resolution. Assisted-by: Zed:gpt-5.6-terra
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A word of Warning/an Apology
Vibe-coded PR ahead. In preparing this, the AI used the Mutter source code (also GPL-3) as a reference.
I'm submitting it because it may solve a real issue for someone else, and it works for me. The code itself may be iffy. For example, it seems odd that it has to "reach around" smithay's KMS interaction code by going directly to
drm-ffi. Buyer beware, OK to close if you don't like it; it'll still be discoverable. If you do close it, I will likely come back with a bug report requesting the same feature.What is being fixed here
I have a high-resolution laptop panel (3840x2400). As part of my work, I need to share my full screen on which an inking app runs. Both the screen sharing and the note taking app bog down and become too slow to be usable at the full panel resolution. Half-resolution meanwhile (1920x1200) is no problem. My panel does not offer a native 1920x1200 mode, but it does support upscaling. Gnome offers a synthetic mode along those lines. Cosmic settings only provides the full panel resolution.
I made this by having an AI investigate what Mutter does and cloning the relevant behavior in cosmic-comp. I'm using it now, and it seems to do what I need.
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