Hi Karl,
We discussed in v1 how this is not the right approach. Why are you posting a v2 without addressing that ? You need to go back to the design table.
On Sat, Nov 07, 2026 at 11:47:00AM +0100, Karl Mehltretter wrote:
uvcvideo and vivid (when using vb2-vmalloc) access imported DMA-BUFs through a kernel vmap without cache synchronization. When a non-coherent importer such as V3D maps the same DMA-BUF persistently, captured data remains in the CPU cache while physical RAM holds stale bytes.
Add an opt-in uses_cpu_access queue flag for vb2-vmalloc queues to bracket the interval from buf_prepare() through buf_finish() with dma_buf_begin_cpu_access() and dma_buf_end_cpu_access(). Keep this opt-in because some vb2-vmalloc drivers pass imported buffers to hardware, and end CPU access in process context because DMA-BUF callbacks may sleep.
This follows the per-queue flag proposed here: https://lore.kernel.org/r/CAAFQd5Am5jBev5P1HmsdeHoJfROZat3bi1W=UsN7wpVqw-XUQ...
Based on Linux 7.4-rc1 with prerequisite patch "media: videobuf2: finish prepared buffers in VIDIOC_REMOVE_BUFS": https://lore.kernel.org/r/20261026071735.37220-1-kmehltretter@gmail.com/
Changes in v2:
- Split patch 1 of v1 into the standalone VIDIOC_REMOVE_BUFS patch above and use its __vb2_buf_cancel() helper so queue cancellation and VIDIOC_REMOVE_BUFS share the finish path (Tomasz).
- Scope uses_cpu_access to vb2-vmalloc queues and drop the attach_dmabuf dma_dir parameter from vb2_mem_ops, videobuf2-dma-contig, videobuf2-dma-sg and videobuf2-vmalloc (Tomasz).
- Enable uses_cpu_access in vivid only when vivid uses vb2_vmalloc_memops (Tomasz).
Link to v1: https://lore.kernel.org/r/cover.1792392288.git.kmehltretter@gmail.com/
Testing:
Raspberry Pi 500+ (6.18.34+rpt-rpi-2712, aarch64, 16 KiB pages, Sunplus UVC camera 1bcf:2701 and Broadcom V3D 7.1 GPU), backport of the prerequisite patch plus patches 1 and 2: eight 640x480 YUYV frames (614,400 B) captured into DMA-BUFs pre-seeded with 0xA5 and imported into persistent EGL_LINUX_DMA_BUF_EXT textures before VIDIOC_STREAMON, then copied by V3D 7.1 immediately after VIDIOC_DQBUF:
Heap Baseline stale Patched v2 stale /dev/dma_heap/linux,cma 8/8 (329,890 B) 0/8 (0 B) /dev/dma_heap/system 8/8 (509,818 B) 0/8 (0 B)
QEMU virt (arm64, Linux 7.4-rc1 with the prerequisite patch and patches 1-3, VIDEO_ADV_DEBUG=y, DEBUG_ATOMIC_SLEEP=y): six vivid lifecycle cases (stream, streamoff-queued, prepare-reqbufs0, prepare-remove, close-streaming, replace-dmabuf) across single-planar and multi-planar queues with kprobe counters on dma_buf_begin_cpu_access() and dma_buf_end_cpu_access():
Queue begin/end calls Unbalanced Warnings vivid vb2-vmalloc (SP) 34 / 34 0 0 vivid vb2-vmalloc (MP) 34 / 34 0 0 vivid vb2-dma-contig (SP) 0 / 0 0 0 vivid vb2-dma-contig (MP) 0 / 0 0 0 20x prepare-remove (vmalloc) 20 / 20 0 0
Assisted by an LLM for code changes, test harnesses and commit message drafting.
Karl Mehltretter (3): media: videobuf2: add opt-in DMA-BUF CPU access media: uvcvideo: synchronize imported DMA-BUFs for CPU access media: vivid: synchronize imported DMA-BUFs for CPU access
.../media/common/videobuf2/videobuf2-core.c | 76 +++++++++++++++++++ drivers/media/test-drivers/vivid/vivid-core.c | 4 +- drivers/media/usb/uvc/uvc_queue.c | 1 + include/media/videobuf2-core.h | 5 ++ 4 files changed, 85 insertions(+), 1 deletion(-)
base-commit: 6628e7d4c6b25c8d18f1345e3a20d21829aae7e4 prerequisite-patch-id: aa99d5c3c27082cbe003ed94a195e3de415777c9 -- 2.39.5 (Apple Git-154)