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

[Karl Mehltretter <kmehltretter@gmail.com>]

Hi Christian,

On Mon, Oct 19, 2026 at 04:48:13PM +0100, Christian König wrote:
> 
> What the drivers do more likely is that they use the vmap interface to access the DMA-buf with the CPU inside the kernel. Correct?
> 

Yes. UVC and the tested vivid vb2-vmalloc path write the imported
DMA-BUF through a kernel vmap. Userspace mmaps the DMA-BUF and brackets
each read with DMA_BUF_IOCTL_SYNC.

> In this case dma_buf_begin_cpu_access() and dma_buf_end_cpu_access() should be called to properly frame that access.
> 
> That seems to be added in patch #2, but I absolutely can't judge if that is correct or not since at the same time you speak about userptr access and the code really looks a bit like spaghetti. 


USERPTR is not part of the CPU-access bracketing. The cover letter
mentions it only as a regression test.

An earlier version changed q->dma_dir to DMA_BIDIRECTIONAL and made
read-only output USERPTR fail PREPARE_BUF with EFAULT. Patch 2 leaves
q->dma_dir unchanged and passes DMA_BIDIRECTIONAL only to
attach_dmabuf() for imported DMA-BUFs.

Thanks,
Karl