Probe `/tmp/expbuf_probe.c` (snapshot at probes/expbuf_probe.c) opens
/dev/video1, sets OUTPUT format H264_SLICE 1920x1088, REQBUFS 4 capture
buffers, EXPBUF on plane 0 of buffer 0, lseek(fd, 0, SEEK_END).
On ohm (kernel besser-7.0, hantro-vpu / rk3568-vpu-dec):
CAPTURE: NV12 1920x1088 num_planes=1 sizeimage=3655712
EXPBUF fd lseek(SEEK_END) = 3657728 (page-rounded from 3655712)
Kernel exports the dma_buf at full sizeimage; offset 2,088,960
(plane 1 base in ffmpeg's drm-frame-descriptor) is well inside.
Hantro is innocent.
Side observation: sizeimage = 3,655,712 > naive NV12's 3,133,440.
The 522,272-byte excess is trailing padding (likely Rockchip
per-frame MV / context metadata) past the UV plane. Y and UV layout
fit cleanly within [0, 3,133,440), exactly where mpv/ffmpeg expect.
Remaining hypothesis space: H1 (panfrost EGL non-zero plane offset),
H2 (KWin wl_dmabuf import), H4 (kwin-fourier residual, low conf).
Next probe queued: H2 source-read of KWin 6.6.4 wl_dmabuf import
path. ~30 min, no hardware needed. If that turns up nothing,
write the EGL importer harness for H1.
Posted to dmabuf-modifier-triage#1 comment 255.
mpv 0.41.0's drmprime_dmabuf_importer reads correctly. Kwiboo's
ffmpeg V4L2 hwaccel at b57fbbe sets planes[1].object_index=0 for
hantro single-planar NV12 (LINEAR modifier branch, line 157), so
mpv should produce identical fd values for both .add() calls.
Runtime confirms via strace: ffmpeg does one VIDIOC_EXPBUF per
CAPTURE buffer, returning ONE fd. nb_objects=1.
The "different fds per plane" observed in WAYLAND_DEBUG is most
likely libwayland's wl_closure_marshal dup_cloexec'ing the fd at
protocol-marshal time — both .add()s use the same source fd, the
trace shows post-dup values which are consecutive but point at the
same dma_buf.
This means the earlier phase 0 conclusion ("mpv mixes per-plane
fds with single-allocation offset") was wrong. The wl_dmabuf
message is internally consistent. Bug is somewhere else.
New hypothesis space (in phase2_iter1_findings.md):
- Mali-G52 panfrost EGL_dma_buf_import with non-zero plane offset
- KWin wl_dmabuf import deduplication bug
- hantro kernel exports dma_buf with size < full allocation
- environment-reset incompleteness from earlier kwin-fourier A/B
Recommended next moves: probe fd size on ohm, mpv debug-logging
patch, KWin source-read, update issue #1 with revised analysis.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
marfrit