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

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Message-ID: <CAAFQd5DiPstn-s+yQM3iMd=G9oaag39qCyX483a7-Jrn=gxWCA@mail.gmail.com>
Date:   Thu, 17 Jun 2021 17:30:11 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Hans Verkuil <hverkuil-cisco@...all.nl>,
        Ricardo Ribalda <ribalda@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 8/8] videobuf2: handle non-contiguous DMA allocations

On Thu, Jun 17, 2021 at 5:01 PM Christoph Hellwig <hch@....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