The native D3D11VA rung was committed with zero hardware evidence and, more
to the point, zero evidence of any kind: every claim it made about the DXVA
structures rested on reading the specification and reading libavcodec, and
reading is exactly the method that produced the four defects the last review
found. Three of those lived where a smoke test cannot look — in the buffer
descriptors — so a passing session would have proved nothing about them.
So this compares our submission against libavcodec's, byte for byte, on the
same access units of the same two vendored vectors. The reference side comes
from an FFmpeg n8.1 cross-built for Windows with the DXVA paths instrumented
(the recipe is in the harness docs); the comparison covers picture parameters,
quantisation matrices AND the buffer descriptors, 250 AUs per codec:
H.264 / HEVC picture parameters 250 AUs, no undocumented divergence
H.264 / HEVC quantisation matrices 250 AUs, no undocumented divergence
H.264 / HEVC buffer descriptors 250 AUs, no undocumented divergence
It found a real defect immediately. The DXVA short slice record is TEN bytes
— dxva.h packs these bitstream-layout structs to a byte — and this crate
declared it `repr(C)`, which pads {u32,u32,u16} to twelve. libavcodec's own
descriptors say so twice over: 20 bytes of slice control for a two-slice
H.264 picture, 10 for a one-slice HEVC one. Record 0 survives either way
(its fields sit at 0/4/8 regardless), so the mistake is invisible on a
single-slice stream and displaces every later record by two bytes on a
multi-slice one — which punktfunk hosts emit. Both records are now
`repr(C, packed)`, and the HEVC slice-control test grew a second record
because one record is the shape that hid this.
The audit that followed matters more than the fix. Per-field offset asserts
cannot see TAIL padding, which is what this was, so all six hand-declared
structs now also assert that their size equals the last field's offset plus
that field's own size. Under that rule the slice records were the only place
packed and natural alignment disagree — 1040, 232, 224 and 1000 were right
all along, and now provably rather than luckily. The module docs claimed
`repr(C)` "reproduces MSVC's default packing exactly for that shape"; that
was a guess wearing a proof's clothes, and it is gone.
Two differences are documented rather than fixed, each with the argument for
why it is inert. libavcodec seeds prev_poc_msb = 1 << 16 at every IDR, so its
POCs are the specification's plus 65536 uniformly; every use a driver makes
of those fields is a difference, and references match on FrameNumList, so the
harness compares POCs relative to that constant and requires it to hold on
every AU rather than importing a magic number into a derivation the Vulkan
rung shares. And HEVC's loop_filter_across_tiles_enabled_flag is inferred 1
by 7.4.3.3.1 when the PPS codes no tiles while libav leaves it 0, with tiles
disabled either way. Both ride a channel that always prints, and both are
guarded by tests that synthesise the differences an allowance must NOT
absorb — a documented divergence that swallows a real defect would be worse
than no harness at all.
Everything checkable without a capture is now a non-ignored test: the buffer
set and order per codec, NumMBsInBuffer's codec asymmetry (mb_width*mb_height
on H.264's bitstream and slice-control buffers, zero everywhere for HEVC),
the three 7.4.5 scaling-list cases, contiguous slice records tiling DataSize,
the 128-byte padding charged to the last record and no other. That is the
part which would have caught the last round's defects with no hardware at all.
enricobuehler
9c10b97e31