linux-kernel - Re: [PATCH] media: uvcvideo: Use streaming DMA APIs to transfer buffers

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Date:   Tue, 21 Apr 2020 13:21:15 +0200
From:   Tomasz Figa <tfiga@...omium.org>
To:     Christoph Hellwig <hch@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Cc:     Nicolas Boichat <drinkcat@...omium.org>,
        Shik Chen <shik@...omium.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        notify@...nel.org, Keiichi Watanabe <keiichiw@...omium.org>,
        Ricky Liang <jcliang@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Kieran Bingham <kieran.bingham@...asonboard.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christoph Lameter <cl@...ux.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCH] media: uvcvideo: Use streaming DMA APIs to transfer buffers

On Thu, Feb 27, 2020 at 7:28 AM Tomasz Figa <tfiga@...omium.org> wrote:
>
> +Sergey Senozhatsky who's going to be looking into this.
>
> Hi Christoph,
>
> On Tue, Oct 1, 2019 at 3:37 PM Christoph Hellwig <hch@...radead.org> wrote:
> >
> > On Mon, Sep 30, 2019 at 01:23:10AM -0700, Christoph Hellwig wrote:
> > > And drivers really have no business looking at the dma mask.  I have
> > > a plan for dma_alloc_pages API that could replace that cruft, but
> > > until then please use GFP_KERNEL and let the dma subsystem bounce
> > > buffer if needed.
> >
> > Can you try this series:
> >
> > http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma_alloc_pages
> >
> > and see if it does whay you need for usb?
>
> Reviving this thread. Sorry for no updates for a long time.
>
> dma_alloc_pages() still wouldn't be an equivalent replacement of the
> existing dma_alloc_coherent() (used behind the scenes by
> usb_alloc_coherent()). That's because the latter can allocate
> non-contiguous memory if the DMA device can handle it (i.e. is behind
> an IOMMU), but the former can only allocate a contiguous range of
> pages.
>
> That said, I noticed that you also put a lot of effort into making the
> NONCONSISTENT attribute more usable. Perhaps that's the way to go here
> then? Of course we would need to make sure that the attribute is
> handled properly on ARM and ARM64, which are the most affected
> platforms. Right now neither handles them. The former doesn't use the
> generic DMA mapping ops, while the latter does, but doesn't enable a
> Kconfig option needed to allow generic inconsistent allocations.
>
> Any hints would be appreciated.

Hi Christoph, would you have some time to check the above?

Hi Catalin, Will, do you know why CONFIG_DMA_NONCOHERENT_CACHE_SYNC is
not enabled on arm64?

Thanks in advance. :)

Best regards,
Tomasz