Hi popcornmix, thanks for the 3D work.
I'm building your fix-stereoscopic-3d-gbm-upstream branch myself on Raspberry Pi OS Lite (Debian 13 trixie, Pi 5, kernel 6.18.50+rpt-rpi-2712) instead of using the LibreELEC image, to package it for apt.
Build: Kodi tree at 9ce78f1 ("[VideoPlayer] Start an open GOP's plane offsets at its first shown frame"), gbm/gles, internal FFmpeg. FFMPEG-VERSION says 9.0.1, but tools/depends/target/ffmpeg/0001-rpi-Add-hevc-acceleration.patch doesn't apply to it (configure, v4l2_buffers.c and more reject). I switched to FFmpeg 8.1.2 (as in rpios_master) and fixed one context line in 001-ffmpeg-all-libpostproc-plugin.patch. The build completes. 4K HEVC plays fine with hardware decoding (ff-hevc-drm_prime, about 6% CPU).
Problem: my 3D files are MakeMKV Blu-ray rips with MVC (MediaInfo: Format profile Stereo [email protected] / [email protected], Count of views 2, MultiView_Layout both eyes laced, 1920x1080, 23.976 fps). They play in 2D only. Kodi asks how to play the 3D video, I choose "same as movie", it switches to the stereoscopic mode, but only one eye is shown. Decoder: ff-h264-drm_prime (software H.264; h264_v4l2m2m and vaapi fail as expected on the Pi 5), CPU load is high. Settings as you recommend: adjust refresh rate on start/stop, playback mode "ask me", stereoscopic 3D mode/current disabled, empty whitelist.
My guess is that the stock FFmpeg drops the dependent view, and that your MVC-capable FFmpeg is what's missing in my build. Where do I find that FFmpeg source or patch set (repo/branch or patch files) that goes with the Kodi tree, so I can use it in tools/depends/target/ffmpeg instead of the stock tarball?
Happy to post kmsprint -m, edid-decode and a debug log if that helps. Thanks!