On Mon, Oct 19, 2026 at 09:14:07AM +0200, Karl Mehltretter wrote:

UVC and vivid access imported DMA-BUFs with the CPU without cache synchronization. On a Pi 400, captured data remained in the CPU cache while physical RAM was stale.

Add an opt-in queue flag to bracket CPU access from buf_prepare() through buf_finish(). The allocator cannot imply CPU use because some vb2-vmalloc drivers pass buffers to hardware. Patch 1 balances finish operations in VIDIOC_REMOVE_BUFS so removing prepared buffers closes this interval.

This seems completely wrong.

This follows the per-queue flag proposed here: https://lists.openwall.net/linux-kernel/2024/06/20/236

Based on Linux 7.3. Tests cover the same code on these earlier bases:

• Pi 400, 7.3-rc4, all four patches: eight 640x480 YUYV frames per run, imported CMA DMA-BUFs with a DRM device mapping:

  Path Runs Stale frames UVC, vb2-vmalloc 12 0/96 vivid, vb2-vmalloc 12 0/96 vivid, vb2-dma-contig 12 0/96

  All six lifecycle cases passed with both vivid allocators. Read-only output USERPTR passed. The test added no warnings or failed CPU-access callbacks. A control without the DRM mapping found 4/16 stale frames.

• SAM9X75 (ARMv5), 6.18 backport of all four patches: the baseline PREPARE_BUF/REMOVE_BUFS imbalance was fixed. USERPTR and six lifecycle cases passed. All 128 vivid and 96 UVC frames matched RAM, with 100/100 begin/end calls. The baseline was already coherent.

• BeagleV Ahead (RISC-V), 7.3-rc7, patches 1, 2 and 4: read-only USERPTR, six traced lifecycle cases and 60 repeats passed with balanced callbacks and no kernel diagnostics. A vendor 5.10 backport of patches 2 and 3 captured 96 UVC frames without stale data, with 100/100 begin/end calls. Its baseline was already coherent.

Karl Mehltretter (4): media: videobuf2: finish prepared buffers in VIDIOC_REMOVE_BUFS 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 | 96 ++++++++++++++++++- .../common/videobuf2/videobuf2-dma-contig.c | 5 +- .../media/common/videobuf2/videobuf2-dma-sg.c | 5 +- .../common/videobuf2/videobuf2-vmalloc.c | 5 +- drivers/media/test-drivers/vivid/vivid-core.c | 1 + drivers/media/usb/uvc/uvc_queue.c | 1 + include/media/videobuf2-core.h | 12 ++- 7 files changed, 116 insertions(+), 9 deletions(-)

base-commit: c03c1a14507dfaf8b856c2608f9ac216021fc222