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A punktfunk client streaming HEVC from .21 (RTX 5070 Ti) refused every access
unit with "stream needs 17 DPB slots, device caps at 16", flushed, waited for an
IRAP, got a fresh IDR that needed 17 too, exhausted the decode ladder and
reconnected with HEVC excluded. On a build with no software HEVC decoder — there
is no permissively licensed one — that is not a slower path, it is losing the
codec.
The host was blameless. Reading the SPS it actually emitted: general_level_idc
153 (L5.1 High, which NVENC autoselects at hevcConfig.level = 0 because a
130 Mbps target does not fit L5.0's 100 Mbps ceiling) and
sps_max_dec_pic_buffering_minus1 = 5 — six pictures, RFI_DPB references plus the
current one. Six, at every resolution. That is already the minimum the encoder
can honestly declare, and the only host-side lever, the level, cannot be lowered
without signalling a bitrate the stream exceeds. There was nothing to fix there.
dpb_limit was reading equation A-2 instead. A-2 is a CEILING on what an SPS may
signal — 7.4.3.2.1 constrains sps_max_dec_pic_buffering_minus1 to
0..=MaxDpbSize-1 — not a statement of what a stream needs, and it branches on
picture size against the LEVEL's MaxLumaPs. At 1080p the coded 1920x1088 =
2 088 960 luma samples fall under MaxLumaPs(L5.1) >> 2 = 2 228 224, taking the
first branch for min(4 * MaxDpbPicBuf, 16) = 16. max(A-2, buffering) then
reported 16 where the stream had asked for 6, the backends added one slot for the
picture in flight, and 17 is one more than NVIDIA's maxDpbSlots.
A resolution sweep on the box drew A-2's branch table exactly, and it is the two
commonest streaming resolutions that lost the codec:
720p 1280x720 = 921 600 branch 1 -> 16 frames, 17 slots 82 refusals, HEVC dropped
1080p 1920x1088 = 2 088 960 branch 1 -> 16 frames, 17 slots 41 refusals, HEVC dropped
1440p 2560x1440 = 3 686 400 branch 2 -> 12 frames, 13 slots clean
4K 3840x2176 = 8 355 840 else -> 6 frames, 7 slots clean, decode 1.9 ms
One host, one level, one six-picture requirement. Only which branch the picture
size landed in decided whether HEVC worked. That is also why this hid for so
long: 4K was the resolution it was exercised at, and 4K is the one size that
falls through to the honest answer. H.264 escaped for an unrelated reason — its
own level-derived ceiling happened to land at 13 for 1080p L5.0 and 5 for 4K
L5.2 — but it is the same shape of derivation and would fail the same way if
NVENC ever picked a higher level for a smaller picture.
So dpb_limit now returns the stream's own sps_max_dec_pic_buffering_minus1 + 1,
capped at 16. That is not a workaround, it is what the number means: it is
exactly the bound C.5.2.2's fullness clause bumps against, and A.4.1 bounds the
total RPS entries by the same value, so `buffering` pictures hold `buffering - 1`
references plus the current one with nothing left over.
The max() that produced the 16 was written to be generous to malformed streams —
"storing their pictures beats erroring the AU" — but it never did that either.
Dpb::needs_bumping (C.5.2.2) already keys on the signalled buffering, not on
max_num_pics, so a stream referencing more pictures than it declared was ALREADY
being bumped below its own declared depth before every store. The widened limit
bought no tolerance at all; all it ever did was over-allocate hardware surfaces,
by ten pictures per session at 1080p, and on NVIDIA take HEVC away entirely.
The fix moves 720p and 1080p onto the pool shape 4K has been running in the field
all along (7 slots, 6 references), so it is not a new operating point — it is the
one already proven. max_active_references drops from 15 to 6, still above the 5
an RFI_DPB stream can name. The per-AU level gate in pf-vkdecode reads
plan.picture.level_idc directly, so dropping A-2 out of NegotiationInfo costs no
sensitivity to a mid-stream level change.
Two regression tests pin the arithmetic from both ends, because either end
drifting back reproduces this:
- h265: the field SPS synthesized byte for byte on the fields that matter must
plan 6 frames / 7 slots, all four resolutions must agree because the stream
does, and every depth the envelope gate admits must leave room for the picture
in flight. The one honest residue is pinned too and deliberately left
refusing: A.4 does let a conforming stream declare a full 16-picture DPB, and
17 slots genuinely do not fit 16, so that stream is still refused rather than
decoded with too few slots and silently corrupted references.
- pf-encode: RFI_DPB + 2 <= 16, guarding the producer end. RFI is a real
latency win and this does not cap it at today's value — there are nine slots
of headroom — it just stops it being raised past the point where clients can
no longer decode us at all.