Re: [PATCH 0/4] media: synchronize CPU access to imported DMA-BUFs

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

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

-- 
Regards,

Laurent Pinchart