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
Date: Fri, 14 Dec 2018 04:36:24 -0800 [thread overview]
Message-ID: <20181214123624.GA5824@infradead.org> (raw)
In-Reply-To: <CAAFQd5BudF84jVaiy7KwevzBZnfYUZggDK=4W=g+Znf5VJjHsQ@mail.gmail.com>
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.
next prev parent reply other threads:[~2018-12-14 12:36 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-21 17:06 [PATCH v5 0/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer Matwey V. Kornilov
2018-08-21 17:06 ` [PATCH v5 1/2] media: usb: pwc: Introduce TRACE_EVENTs for pwc_isoc_handler() Matwey V. Kornilov
2018-08-21 19:49 ` Steven Rostedt
2018-08-21 19:49 ` Steven Rostedt
2018-08-21 17:06 ` [PATCH v5 2/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer Matwey V. Kornilov
2018-08-28 7:17 ` Matwey V. Kornilov
2018-09-11 18:58 ` Matwey V. Kornilov
2018-09-19 16:12 ` Ezequiel Garcia
2018-10-10 21:13 ` Matwey V. Kornilov
2018-10-30 22:00 ` Laurent Pinchart
2018-10-31 5:38 ` Christoph Hellwig
2018-12-07 15:25 ` Christoph Hellwig
2018-12-12 8:57 ` Tomasz Figa
2018-12-12 9:09 ` Christoph Hellwig
2018-12-12 9:34 ` Tomasz Figa
2018-12-12 13:54 ` Christoph Hellwig
2018-12-13 3:13 ` Tomasz Figa
2018-12-13 14:03 ` Christoph Hellwig
2018-12-14 3:12 ` Tomasz Figa
2018-12-14 12:36 ` Christoph Hellwig [this message]
2018-12-18 7:22 ` Tomasz Figa
2018-12-18 7:38 ` Christoph Hellwig
2018-12-18 9:48 ` Tomasz Figa
2018-12-19 7:51 ` Christoph Hellwig
2018-12-19 8:18 ` Tomasz Figa
2018-12-19 14:51 ` Christoph Hellwig
2018-12-20 3:23 ` Tomasz Figa
2018-12-21 8:13 ` Christoph Hellwig
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