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From:   Tomasz Figa <tfiga@chromium.org>
Date:   Wed, 12 Dec 2018 17:57:02 +0900
Message-ID: <CAAFQd5C6gUvg8p0+Gtk22Z-dmeMPytdF4HujL9evYAew15bUmA@mail.gmail.com>
Subject: Re: [PATCH v5 2/2] media: usb: pwc: Don't use coherent DMA buffers
 for ISO transfer
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        "Matwey V. Kornilov" <matwey@sai.msu.ru>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Matwey V. Kornilov" <matwey.kornilov@gmail.com>,
        Alan Stern <stern@rowland.harvard.edu>,
        Ezequiel Garcia <ezequiel@collabora.com>, hdegoede@redhat.com,
        Hans Verkuil <hverkuil@xs4all.nl>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        rostedt@goodmis.org, mingo@redhat.com,
        Mike Isely <isely@pobox.com>,
        Bhumika Goyal <bhumirks@gmail.com>,
        Colin King <colin.king@canonical.com>,
        Kieran Bingham <kieran.bingham@ideasonboard.com>,
        keiichiw@chromium.org
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Hi Christoph,

On Sat, Dec 8, 2018 at 12:25 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> Folks, can you take a look at this tree and see if this is useful
> for USB:
>
> http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma-noncoherent-allocator
>
> The idea is that you use dma_alloc_attrs with the DMA_ATTR_NON_CONSISTENT
> now that I've made sure it is avaiable everywhere [1], and we can use
> dma_sync_single_* on it.

How about dma_sync_sg_*()? I'd expect some drivers to export/import
such memory via sg, since that's the typical way of describing memory
in DMA-buf.

>
> The only special case USB will need are the HCD_LOCAL_MEM devices, for
> which we must use dma_alloc_coherent (or dma_alloc_attrs without
> DMA_ATTR_NON_CONSISTENT) and must skip the dma_sync_single_* calls,
> so we'll probably need USB subsystem wrappers for those calls.
>
> [1] except powerpc in this tree - I have another series to make powerpc
> use the generic dma noncoherent code, which would cover it.

Sounds good to me. Thanks for working on this. I'd be happy to be on
CC and help with review when you post the patches later.

Best regards,
Tomasz