dmabuf-modifier-triage/phase2_iter1_findings.md

Phase 2 — iter1 source-read findings (REOPEN of root-cause analysis)

Opened 2026-05-08 during the iter1 phase 2 source-read of mpv 0.41.0 + Kwiboo's ffmpeg fork at commit b57fbbe. Phase 0's earlier conclusion ("mpv mixes per-plane fds with single-allocation offset") needs revision — the source code reads + runtime probe show the situation is more nuanced than the WAYLAND_DEBUG wire trace alone suggested.

What the source actually says

mpv video/out/vo_dmabuf_wayland.c drmprime_dmabuf_importer (lines 250-277) straightforwardly relays the producer's AVDRMFrameDescriptor:

for (plane_no = 0; plane_no < layer.nb_planes; ++plane_no) {
    AVDRMPlaneDescriptor plane = layer.planes[plane_no];
    int object_index = plane.object_index;
    AVDRMObjectDescriptor object = desc->objects[object_index];
    uint64_t modifier = object.format_modifier;
    zwp_linux_buffer_params_v1_add(params, object.fd, plane_no, plane.offset,
                                   plane.pitch, modifier >> 32, modifier & 0xffffffff);
}

No dup(), no rewriting, no transformation. mpv passes through what AVDRMFrameDescriptor says.

Kwiboo's libavutil/hwcontext_v4l2request.c v4l2request_set_drm_descriptor (lines 138-198) for hantro's NV12 single-planar (V4L2_PIX_FMT_NV12, the format v4l2-ctl --get-fmt-video-mplane-cap reports for /dev/video1 on ohm):

desc->base.nb_objects = num_planes;        // = 1 for single-planar NV12 on hantro
desc->base.objects[0].fd = exportbuffer.fd; // VIDIOC_EXPBUF returns ONE fd
// in v4l2request_set_drm_descriptor:
desc->nb_layers = 1;
layer->nb_planes = 1;
layer->planes[0].object_index = 0;
layer->planes[0].offset = 0;
layer->planes[0].pitch = bytesperline;     // 1920
if (modifier != ARM_VENDOR) {              // hantro outputs LINEAR (0x0), so this is true
    layer->nb_planes = 2;
    layer->planes[1].object_index = 0;     // ← BOTH PLANES point at object 0
    layer->planes[1].offset = pitch * height;  // 1920 * 1088 = 2088960
    layer->planes[1].pitch = layer->planes[0].pitch;
}

Per the source, mpv should produce identical fd values in the two .add() calls — both pulling from desc->objects[0].fd.

What the runtime probe says

v4l2-ctl --get-fmt-video-mplane-cap on ohm /dev/video1:

Pixel Format      : 'NV12' (Y/UV 4:2:0)
Number of planes  : 1
sizeimage=3655712, bytesperline=1920

strace -e trace=ioctl mpv ... confirms ffmpeg only does one VIDIOC_EXPBUF per CAPTURE buffer (index=N, plane=0 → one fd), exactly matching nb_objects = 1.

But WAYLAND_DEBUG=1 mpv ... shows two .add() calls with different fd numbers per buffer:

add(fd 41, 0, 0,       1920, 0, 0)
add(fd 42, 1, 2088960, 1920, 0, 0)

These fd numbers are consecutive, suggesting libwayland's wl_closure_marshal is dup_cloexec'ing the fd at protocol-marshal time and the trace prints the post-dup fd. Both fd 41 and fd 42 are dups of the same underlying dma_buf object (originally fd 17 or similar in mpv's table).

Implications for iter1

The earlier phase 0 conclusion that mpv constructs an "internally inconsistent" wl_dmabuf message was wrong. There is no inconsistency at the producer ↔ mpv layer:

• nb_objects = 1, both planes use object 0 → mpv passes the same fd value into both .add() calls
• libwayland dups it before sending → wire trace shows different fd numbers, but they refer to the same backing memory
• Plane 1's offset = 2088960 is correct relative to the (single) underlying allocation

So the green frame is not caused by mpv or ffmpeg's descriptor construction. Something else.

New hypothesis space (one of these is the real bug)

1. Mali-G52 panfrost EGL_EXT_image_dma_buf_import_modifiers regression for NV12 with non-zero plane offset. The driver may sample plane 1 from offset 0 of the imported fd instead of offset 2088960, giving zero-fill UV. Testable: a minimal EGL importer C program that imports a known NV12 dmabuf with offsets and reads back via glReadPixels.

2. KWin's wl_dmabuf import logic deduplicates the dup'd fds incorrectly. If KWin sees two fds and detects (via dma_buf_id or kcmp(2)) that they're the same backing object, but then mishandles the per-plane offsets, this'd produce green. Testable: read KWin source src/wayland/linuxdmabufv1clientbuffer.cpp and compositor backend's EGL import path.

3. hantro kernel driver exports a dma_buf with size < full allocation. If hantro caps the exported fd's mappable size to just the Y plane (2,088,960 bytes), offset 2,088,960 is past EOF — read fails silently, returns zeros. Testable: ssh ohm "fcntl(F_GETSIZE) on EXPBUF fd" or just lseek(fd, 0, SEEK_END) in a small probe.

4. kwin-fourier 0001 still has effect we missed. Even though we ruled out kwin-fourier as a compositor-replacement A/B, that test was on an earlier kernel/Mesa combo. Worth verifying the test environment is fully reset.

Recommended next moves for iter1

a. Write a small C harness that does VIDIOC_EXPBUF on a hantro CAPTURE buffer and reports fd size + backing dma_buf info. Decides hypothesis 3 in 30 minutes. Run on ohm directly.

b. Patch mpv with MP_VERBOSE logging of the AVDRMFrameDescriptor fields at .add()-call time (nb_objects, planes[].object_index, planes[].offset, objects[].size). Confirms the source-read is correct at runtime. Drop into mpv-fourier's prepare() slot, bump pkgrel, rebuild on fermi (~10 min CI).

c. Read KWin's wl_dmabuf import logic (KDE Plasma 6 / KWin 6.6.4 source) for how it handles multiple-fd-same-buffer cases. ~30 min source-read.

d. Update marfrit/dmabuf-modifier-triage#1 with this revised analysis. The current issue body claims the bug is in mpv's plane-semantics translation — that conclusion is now overturned.

Status

• iter1 phase 2 ongoing. Pivot from "fix is in mpv" to "real layer is unclear, need more diagnostics."
• Acceptance criterion (screenshots/frame10_expected.png) is unchanged.
• Delivery vehicle (mpv-fourier-1:0.41.0-8) is still the right shipping path if the fix turns out to be a defensive workaround in mpv. Otherwise the patch lands in ffmpeg-v4l2-request-fourier (unlikely per this analysis), KWin, kernel hantro, or a Mesa-panfrost user-mode driver patch.