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

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Message-ID: <CAAFQd5DqK2gSTGjfo-vahXwMzzO9gv26cY=vV6urn3viDLPE7g@mail.gmail.com>
Date:   Thu, 17 Jun 2021 18:40:58 +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 Mailing List <linux-media@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv2 8/8] videobuf2: handle non-contiguous DMA allocations

On Thu, Jun 17, 2021 at 5:52 PM Christoph Hellwig <hch@....de> wrote:
>
> On Thu, Jun 17, 2021 at 05:30:11PM +0900, Tomasz Figa wrote:
> > We still have use cases for dma_alloc_coherent() and DMA_ATTR_NO_KERNEL_MAPPING.
>
> dma_alloc_coherent does not take a flags argument, so you can't use
> DMA_ATTR_NO_KERNEL_MAPPING with it.  What would your use case be here
> anyway?  In general DMA_ATTR_NO_KERNEL_MAPPING is rather misnamed, as
> usually there is a kernel mapping, just not one that is coherent and
> should be used.

Sorry, I meant dma_alloc_attrs() and yes, it's indeed a misnomer. Our
use case basically has no need for the additional coherent mapping, so
creation of it can be skipped to save some vmalloc space. (Yes, it
probably only matters for 32-bit architectures.)