The VAAPI rung's crate, in the shape the other two native rungs established:
everything that can be a pure decision or a pure conversion lives in a
cross-platform crate the ordinary gates run, and only the parts that genuinely
need a device stay behind a platform cfg. This lands the first half of that —
the buffer layouts and the decoder-creation decisions — with the conversion to
follow.
Route: minimal FFI rather than cros-libva. The plan of record permits either
("cros-libva (or minimal FFI)"), and hand-declaring keeps the crate building
and testing on macOS and in the Linux container, which is the property that
made pf-dxvadec's defects findable on a laptop instead of on a box.
The layouts are not eyeballed. A C probe compiled against real libva 2.23.0
headers printed sizeof/alignof/offsetof for every field and set individual
bit-fields to read the resulting word back; those numbers are pinned as const
assertions, so a transcription slip is a compile error rather than a driver
reading the wrong byte. The probe is committed beside them, with the command
that runs it, because evidence that cannot be re-run is a claim.
What the probe settled that a reader would otherwise get wrong: VAPictureH264
is 36 bytes and is embedded 81 times across the two buffers, so its size is
load-bearing for every later offset; the three DEPRECATED FMO fields still
occupy bytes 624..628, and dropping them would shift everything after; and C
bit-fields allocate from the least significant bit on this ABI — proven, since
that is ABI-defined rather than standardised.
Groundwork for the conversion, established here so the next work package
starts from facts:
slice_data_bit_offset needs no new parsing. VAAPI is the only backend that
wants a bit position — DXVA takes a byte offset, Vulkan takes none — and the
vendored parser already records exactly it as SliceHeader::header_bit_size,
computed as (nalu.size - epb) * 8 - bits_left: from and including the NAL
header byte, emulation-prevention bytes removed. That is the field's
definition verbatim, and it is there because cros-codecs' own production
backend is VAAPI.
The slice data buffer starts at the NAL header byte, so the start code is
skipped — SlicePlan::data is start-code-inclusive and the prefix is three OR
four bytes, the host emitting four on 100% of access units.
reference_frames is the marked DPB, the same statement DXVA's RefFrameList
makes, so it comes from the dpb_refs snapshot; Vulkan's pReferenceSlots is the
opposite and takes the access unit's own set. All three conventions now have a
written home, which is the distinction that cost M5 a defect.
Unlike DXVA short-format, VAAPI wants the per-slice reference lists and the
full prediction weight tables inline — hence the 3128-byte slice record. One
wrinkle recorded rather than left to be discovered: the vendored
PredWeightTable stores luma_offset_l0 as [i8; 32] but luma_offset_l1 as
[i16; 32], and libva wants i16 for both.
Profile selection resolves H.264 to High for every 8-bit 4:2:0 stream instead
of reading profile_idc, because High is a superset for the tools our hosts
emit and picking Main for a stream that turns out to use 8x8 transforms is a
mid-stream failure where picking High is not. 4:4:4 and 10-bit H.264 are
refused rather than narrowed to an 8-bit profile — that class of silent
narrowing decodes to garbage instead of failing.
11 tests: the probe's bit patterns, a disjointness check per bit-field word
(two probe vectors alone would not catch a shift typo that overlapped two
fields), and the envelope refusals. Gates: rustfmt, clippy, cargo doc with no
unresolved links, and the Linux container's clippy -D warnings, tests and
workspace check.
enricobuehler
d3e000768d