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

[Karl Mehltretter <kmehltretter@gmail.com>]

Hi Laurent,

On Mon, Oct 19, 2026 at 06:01:16PM +0100, Laurent Pinchart wrote:
> 
> This patch series introduces a big penalty in the common case where a
> userspace application consumes the frame with the CPU. There should be
> no cache invalidation in that case.
> 

I measured this on my Pi 500+. The cost appears when the DMA-BUF has a
V3D attachment that stays mapped but idle. I compared patch 1 alone with
patches 1-4 on the same v7.3-rc4 base at 640x480 YUYV. For the DMA-BUF
runs, userspace read every frame between DMA_BUF_IOCTL_SYNC READ calls.

Median process CPU time (us/frame), five 1800-frame runs:

                                      patch 1      patches 1-4
  DMA-BUF, no DRM import              129.2        130.6
  DMA-BUF, idle V3D map               130.0        153.1
  MMAP                                126.5        125.4

The no-import and MMAP ranges overlapped. The V3D-mapped ranges did not.
Pi 400 reproduced the mapped cost with vivid (+8.7%) and UVC (+7.5%).

The task-clock profile covered only the patched mapped path:

  Operation                              Sample share
  DQBUF producer END|WRITE clean         26%
  userspace START|READ invalidation      13%
  userspace END|READ clean               8%

I tried a CMA experiment that kept the DMA-BUF callbacks but skipped the
read-only END|READ attachment sync. The skip variant saved a median 12.3
us/frame (8.0%) across three alternating same-boot pairs. With private
DMA-BUFs, the full-sync and skip variants both passed 64 byte-checked
V3D read and overwrite cycles after END|READ.

That experiment still performs END|WRITE and START|READ. In a negative
control without END|WRITE, all eight V3D reads returned stale data.

Patch 1 is independent, so I can send it separately. 

For patches 2-4: How should vb2 avoid the END|WRITE/START|READ
round trip when userspace consumes the frame with the CPU, while still
publishing the frame if the next consumer is a device?

Thanks,
Karl