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Date:   Fri, 14 Dec 2018 04:36:24 -0800
From:   Christoph Hellwig <hch@infradead.org>
To:     Tomasz Figa <tfiga@chromium.org>
Cc:     Christoph Hellwig <hch@infradead.org>,
        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
Subject: Re: [PATCH v5 2/2] media: usb: pwc: Don't use coherent DMA buffers
 for ISO transfer
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On Fri, Dec 14, 2018 at 12:12:38PM +0900, Tomasz Figa wrote:
> > If the buffer always is physically contiguous, as it is in the currently
> > posted series, we can always map it with a single dma_map_single call
> > (if the hardware can handle that in a single segment is a different
> > question, but out of scope here).
> 
> Are you sure the buffer is always physically contiguous? At least the
> ARM IOMMU dma_ops [1] and the DMA-IOMMU dma_ops [2] will simply
> allocate pages without any continuity guarantees and remap the pages
> into a contiguous kernel VA (unless DMA_ATTR_NO_KERNEL_MAPPING is
> given, which makes them return an opaque cookie instead of the kernel
> VA).
> 
> [1] http://git.infradead.org/users/hch/misc.git/blob/2dbb028e4a3017e1b71a6ae3828a3548545eba24:/arch/arm/mm/dma-mapping.c#l1291
> [2] http://git.infradead.org/users/hch/misc.git/blob/2dbb028e4a3017e1b71a6ae3828a3548545eba24:/drivers/iommu/dma-iommu.c#l450

We never end up in this allocator for the new DMA_ATTR_NON_CONSISTENT
case, and that is intentional.