V4L2 CAPTURE buffers are V4L2_MEMORY_MMAP and mapped cached. Kernel
DMA writes don't propagate to CPU cache observer; reading
destination_data[] without DMA_BUF_IOCTL_SYNC(START|READ) returns
stale data on RK3399 — observed as Bug 4 (H.264 partial-fill) and
Bug 5 (HEVC all-zero) when libva goes through cached-mmap readback
while kdirect ffmpeg-v4l2request + DRM_PRIME-mmap reads cleanly via
implicit sync.
Per Tomasz Figa's 2024 linaro-mm-sig discussion + feedback_rfc_v2_
vb2_dma_resv_scope.md: userspace responsibility for cache sync on
cached-mmap'd V4L2 buffers. RFC v2 fence work doesn't engage this
path; this ioctl pair does.
Just-in-time EXPBUF + SYNC + close per copy. Per-call cost is one
ioctl pair + one fd lifecycle per plane. Could cache the EXPBUF fd
on cap_pool slot but doing it transient keeps lifecycle simple.
Closing the EXPBUF fd is a no-op on V4L2 buffer memory.
If EXPBUF or SYNC fails, fall through to existing memcpy path —
preserves pre-iter13 behavior on the error branch.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
claude-noether
ca4dd88007