Empirically, Mutter+NVIDIA dmabuf capture has NO working GPU sync — confirmed on
worker-3: explicit sync fails buffer alloc (EINVAL, no cogl sync_fd), and the
dmabuf carries no implicit fence (EXPORT_SYNC_FILE waited=false). So any dmabuf
read — zero-copy import OR mmap — races Mutter's render and flashes the buffer's
previous frame. The prior "CPU fallback" still listed DmaBuf in its buffer types,
so Mutter kept handing dmabufs and it never fixed anything (got worse).
PUNKTFUNK_FORCE_SHM=1 offers MemPtr+MemFd ONLY (no DmaBuf), forcing Mutter to
glReadPixels-download into mappable memory — which orders against its render, so
the frame is complete + current by construction (race-free). Costs the download
(~3 ms) + zero-copy; correct at 1080p/4K60. KWin/gamescope are unaffected (they
blit into the buffer, no read-before-render race) and keep zero-copy.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
enricobuehler
a6b836cfe4