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

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Message-ID: <20210617080107.GA1422@lst.de>
Date:   Thu, 17 Jun 2021 10:01:07 +0200
From:   Christoph Hellwig <hch@....de>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Hans Verkuil <hverkuil-cisco@...all.nl>,
        Tomasz Figa <tfiga@...omium.org>,
        Ricardo Ribalda <ribalda@...omium.org>,
        Christoph Hellwig <hch@....de>,
        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 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.