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

[Karl Mehltretter <kmehltretter@gmail.com>]

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 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
-- 
2.47.3