libav turns on `ID3D11Multithread::SetMultithreadProtected` in
`d3d11va_device_create` — the path where it creates the device itself. We take the
other path, `d3d11va_device_init`, because we hand it the capturer's existing
`ID3D11Device`, and that path does not. Nothing enabled it on our side either, so it
was simply off. Two consequences, one of them shipping since long before this branch:
QSV zero-copy could not open at all. `av_hwdevice_ctx_create_derived(QSV <- D3D11VA)`
ends in `MFXVideoCORE_SetHandle`, and MFX rejects a device without multithread
protection as `MFX_ERR_UNDEFINED_BEHAVIOR (-16)`. libav reports that as "Error
setting child device handle", which names neither the cause nor the cure.
AMF — the DEFAULT Windows zero-copy path — has been running with a lock that does
nothing. We deliberately leave `lock`/`unlock` null so libav installs its
`d3d11va_default_lock`, and that lock is `ID3D11Multithread::Enter`/`Leave`, which
are documented no-ops while protection is off. The lock libav installs to serialise
our capture thread against its encode thread has therefore never actually serialised
anything. That is the more serious half of this fix, and it is not QSV-specific.
Measured on the Intel VM (UHD 750, FFmpeg 7.1.5 + libvpl), sweeping device flags x
adapter x protection:
BGRA -> QSV derive FAILED (-16)
BGRA +MT -> QSV derive OK
BGRA|VIDEO +MT -> QSV derive OK
`D3D11_CREATE_DEVICE_VIDEO_SUPPORT` makes no difference in either direction, and the
same result holds on both Intel adapters the box enumerates. Protection is the only
variable that matters, and it read back `was=false` every time, confirming nothing
else had enabled it. The hypothesis came from ffmpeg's own CLI succeeding at the
identical derive (`-init_hw_device d3d11va=d3d -init_hw_device qsv=q@d3d`) — the only
difference being who created the device.
With this, `zerocopy_qsv_alloc_drop_cycles` passes on real Intel silicon, which also
makes the QSV arm of the AvBuffer refactor hardware-verified rather than
compile-only. Its doc comment now points back here, since a future regression will
present as that same opaque "child device handle" line.
NOT flipped: `zerocopy_active` still defaults QSV off. The derive works and the
handles construct and release cleanly, but that is not the same as a validated
streaming session, and the default should move on glass evidence, not on this.
Verified on .47 (Intel UHD 750): full pf-encode --features amf-qsv suite 39 passed /
0 failed, d3d11hw_alloc_drop_cycles green, zerocopy_qsv_alloc_drop_cycles green, and
the native VPL path (qsv_encode_live_smoke) still green. Linux .21: workspace check
exit 0 / zero errors, pf-encode 33/0, pf-client-core 34/0.
enricobuehler
4241bc0383