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From:   Tomasz Figa <tfiga@chromium.org>
Date:   Tue, 8 Dec 2020 13:54:00 +0900
Message-ID: <CAAFQd5BBEbmENrrZ-vMK9cKOap19XWmfcxwrxKfjWx-wEew8rg@mail.gmail.com>
Subject: Re: [PATCH v3 5/6] media: uvcvideo: Use dma_alloc_noncontiguos API
To:     Christoph Hellwig <hch@lst.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Ricardo Ribalda <ribalda@chromium.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        IOMMU DRIVERS <iommu@lists.linux-foundation.org>,
        Joerg Roedel <joro@8bytes.org>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        Sergey Senozhatsky <senozhatsky@google.com>
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Hi Christoph,

On Tue, Dec 1, 2020 at 11:49 PM Christoph Hellwig <hch@lst.de> wrote:
>
> On Tue, Dec 01, 2020 at 12:36:58PM +0900, Sergey Senozhatsky wrote:
> > Not that I have any sound experience in this area, but the helper
> > probably won't hurt. Do you also plan to add vmap() to that helper
> > or dma_alloc_noncontiguous()/sg_alloc_table_from_pages() only?
>
> Yes, I think adding the vmap is useful, and it probably makes sense
> to do that unconditionally.  I'd also include the fallback to
> dma_alloc_pages when the noncontig version isn't supported in the
> helper.

From the media perspective, it would be good to have the vmap
optional, similarly to the DMA_ATTR_NO_KERNEL_MAPPING attribute for
coherent allocations. Actually, in the media drivers, the need to have
a kernel mapping of the DMA buffers corresponds to a minority of the
drivers. Most of them only need to map them to the userspace.

Nevertheless, that minority actually happens to be quite widely used,
e.g. the uvcvideo driver, so we can't go to the other extreme and just
drop the vmap at all.

In any case, Sergey is going to share a preliminary patch on how the
current API would be used in the V4L2 videobuf2 framework. That should
give us more input on how such a helper could look.

Other than that, again, thanks a lot for helping with this.

Best regards,
Tomasz