Re: [PATCHv2 8/8] videobuf2: handle non-contiguous DMA allocations

[Tomasz Figa <tfiga@chromium.org>]

On Thu, Jun 17, 2021 at 5:01 PM Christoph Hellwig <hch@lst.de> wrote:
>
> On Thu, Jun 17, 2021 at 04:56:17PM +0900, Sergey Senozhatsky wrote:
> > > This function can use some comments. What is happening AFAICS is that
> > > buf->vaddr is either set in vb2_dc_alloc_coherent (unless
> > > DMA_ATTR_NO_KERNEL_MAPPING was set), it is obtained through dma_buf_vmap()
> > > if the buffer was attached to a dma_buf, or it is allocated via
> > > dma_vmap_noncontiguous() for non-coherent memory.
> >
> > Yeah, it's complicated. Maybe we can make things more symmetrical.
> >
> > > But this leaves coherent memory with DMA_ATTR_NO_KERNEL_MAPPING set, what
> > > is vaddr in that case? I think it will call dma_vmap_noncontiguous()
> > > incorrectly in that case, shouldn't there be a check for !buf->coherent_mem
> > > before the call to dma_vmap_noncontiguous()?
> >
> > Thanks a lot for looking into it.
>
> Can we just kill off DMA_ATTR_NO_KERNEL_MAPPING in v4l now?  There really is
> no good reason to use that with dma_alloc_noncoherent/noncontiguous
> available, and I plan to eventually remove that interface entirely.

We still have use cases for dma_alloc_coherent() and DMA_ATTR_NO_KERNEL_MAPPING.

Perhaps the way to handle this would be to make the
dma_alloc_coherent() behave the same way as dma_alloc_noncontiguous(),
where it just allocates the memory without handling the kernel
mapping?

Best regards,
Tomasz