Jellyfin hardware decoding

jellyfin.hardwareDecoding.*, module jellyfin, for the Flathub Jellyfin Desktop Flatpak (org.jellyfin.JellyfinDesktop), which plays video with libmpv. Options: README, Options.

Problem

Video is decoded in software (1080p H.264: ~40 % CPU). The Frame's hardware decoder is a V4L2 memory-to-memory device (qcom-iris, /dev/video*), which

the Flatpak can't open: its devices=dri covers only the GPU, and Flatpak has nothing between that and devices=all;
mpv never tries: Jellyfin hard-sets hwdec=auto-copy, and mpv's auto probing leaves out V4L2 M2M on purpose (its quality varies by SoC). Jellyfin has no way to pass mpv options.

What you get

A launcher (the Flatpak's own entry, rewritten under its ID, so the KDE menu and the "+" menu start it) runs the Flatpak with device access (devices=all) and makes mpv use hwdec (default v4l2m2m-copy,auto-copy) instead of Jellyfin's auto* value; explicit values such as no stay. mpv tries the listed decoders in order and falls back to software decoding per stream. With the default, 1080p H.264 plays at ~15-20 % CPU. Changes take effect at the next start of Jellyfin.

Configuration

Installing the Flatpak is up to you, e.g. flatpak install --user flathub org.jellyfin.JellyfinDesktop, or with nix-flatpak:

services.flatpak.packages = [ "org.jellyfin.JellyfinDesktop" ];
steamFrame.jellyfin.hardwareDecoding.enable = true;

From a terminal, start it with the command line of steamFrame.jellyfin.hardwareDecoding.command (or the one of grep ^Exec= ~/.local/share/applications/org.jellyfin.JellyfinDesktop.desktop); its output shows mpv-hwdec-shim: hwdec "auto-copy" -> "v4l2m2m-copy,auto-copy", then mpv's Using hardware decoding (v4l2m2m-copy).

Caveats

devices=all gives the app all of /dev (cameras, input devices, ...), not just the decoder.
V4L2 M2M decoding quality varies with drivers and codecs. Tested: 8-bit H.264; 10-bit HEVC is untested. mpv falls back to software only when the decoder fails; for streams that decode with artifacts, disable the option (or set hwdec = "auto-copy", Jellyfin's own value).

How it works

The shim

An LD_PRELOAD shim (modules/jellyfin/mpv-hwdec-shim.c, a few libmpv wrappers, only libc) rewrites an hwdec value starting with auto (set through libmpv's mpv_set_* functions) to $SFN_MPV_HWDEC (the hwdec option). It is preloaded from the Nix store; only its store path is exposed (read-only) to the sandbox. It would work for any libmpv app that sets hwdec=auto*, but only Jellyfin Desktop is set up here.

The launcher

Nothing is written to Flatpak's overrides: the launcher steamFrame.launchers."org.jellyfin.JellyfinDesktop" passes device access and the shim as flatpak run options (flatpakArgs, env), so they apply to launches from the entry and are gone with it. Its command lines (the entry's and the four actions') become

flatpak run --branch=stable --arch=aarch64 --command=jellyfin-desktop \
  --device=all --filesystem=<shim>:ro \
  --env=LD_PRELOAD=<shim>/lib/mpv-hwdec-shim.so \
  --env=SFN_MPV_HWDEC=v4l2m2m-copy,auto-copy org.jellyfin.JellyfinDesktop

(command is approximately this line, with the store path and without the entry's --branch/--arch/--command.)

Older versions used a Flatpak override (via nix-flatpak or a Home Manager link) and a shim copy in ~/.var/app/org.jellyfin.JellyfinDesktop; steam-frame-nix-cleanup removes their entries from the override and the copy.

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