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

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Date:   Mon, 27 Apr 2020 05:48:40 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        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 Tue, Apr 21, 2020 at 01:21:15PM +0200, Tomasz Figa wrote:
> 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,
> >
> > 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?

NONCONSISTENT is still a mess, mostly because dma_cache_sync is such
a horrible API.  I've been wanting to switch to the normal
sync_for_device / sync_for_cpu primitives instead.  Let me see if I can
expedite that.