From: Christoph Hellwig <hch@infradead.org>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: "Matwey V. Kornilov" <matwey@sai.msu.ru>,
Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
Tomasz Figa <tfiga@chromium.org>,
Ezequiel Garcia <ezequiel@collabora.com>,
Hans de Goede <hdegoede@redhat.com>,
Hans Verkuil <hverkuil@xs4all.nl>,
Mauro Carvalho Chehab <mchehab@kernel.org>,
Steven Rostedt <rostedt@goodmis.org>,
mingo@redhat.com, Mike Isely <isely@pobox.com>,
Bhumika Goyal <bhumirks@gmail.com>,
Colin King <colin.king@canonical.com>,
Linux Media Mailing List <linux-media@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Kieran Bingham <kieran.bingham@ideasonboard.com>,
keiichiw@chromium.org
Subject: Re: [PATCH 2/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer
Date: Sun, 5 Aug 2018 00:49:46 -0700 [thread overview]
Message-ID: <20180805074946.GA14119@infradead.org> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1808041045060.25853-100000@netrider.rowland.org>
On Sat, Aug 04, 2018 at 10:46:35AM -0400, Alan Stern wrote:
> > 2) dma_unmap and dma_map in the handler:
> > 2A) dma_unmap_single call: 28.8 +- 1.5 usec
> > 2B) memcpy and the rest: 58 +- 6 usec
> > 2C) dma_map_single call: 22 +- 2 usec
> > Total: 110 +- 7 usec
> >
> > 3) dma_sync_single_for_cpu
> > 3A) dma_sync_single_for_cpu call: 29.4 +- 1.7 usec
> > 3B) memcpy and the rest: 59 +- 6 usec
> > 3C) noop (trace events overhead): 5 +- 2 usec
> > Total: 93 +- 7 usec
> >
> > So, now we see that 2A and 3A (as well as 2B and 3B) agree good within
> > error ranges.
>
> Taken together, those measurements look like a pretty good argument for
> always using dma_sync_single_for_cpu in the driver. Provided results
> on other platforms aren't too far out of line with these results.
Logically speaking on no-mmio no-swiotlb platforms dma_sync_single_for_cpu
and dma_unmap should always be identical. With the migration towards
everyone using dma-direct and dma-noncoherent this is actually going to
be enforced, and I plan to move that enforcement to common code in the
next merge window or two.
next prev parent reply other threads:[~2018-08-05 9:53 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-17 14:36 [PATCH 1/2] Add TRACE_EVENTs in pwc_isoc_handler() Matwey V. Kornilov
2018-06-17 14:36 ` [PATCH 2/2] media: usb: pwc: Don't use coherent DMA buffers for ISO transfer Matwey V. Kornilov
2018-06-18 5:11 ` Ezequiel Garcia
2018-06-18 7:10 ` Matwey V. Kornilov
2018-07-17 20:10 ` Ezequiel Garcia
2018-07-17 20:51 ` Alan Stern
2018-07-20 10:55 ` Tomasz Figa
2018-07-20 11:22 ` Matwey V. Kornilov
2018-07-20 11:33 ` Tomasz Figa
2018-07-20 11:57 ` Matwey V. Kornilov
2018-07-23 17:04 ` Matwey V. Kornilov
2018-07-23 18:57 ` Alan Stern
2018-07-24 18:56 ` Matwey V. Kornilov
2018-07-24 20:55 ` Alan Stern
2018-07-25 13:46 ` Matwey V. Kornilov
2018-07-30 4:14 ` Tomasz Figa
2018-08-04 8:05 ` Matwey V. Kornilov
2018-07-30 15:35 ` Laurent Pinchart
2018-08-04 8:00 ` Matwey V. Kornilov
2018-08-04 14:46 ` Alan Stern
2018-08-05 7:49 ` Christoph Hellwig [this message]
2018-08-05 8:33 ` Matwey V. Kornilov
2018-08-05 8:41 ` Christoph Hellwig
2018-08-08 22:32 ` Laurent Pinchart
2018-08-09 2:36 ` Tomasz Figa
2018-08-09 10:28 ` Laurent Pinchart
2018-07-30 15:13 ` Laurent Pinchart
2018-07-18 12:10 ` Matwey V. Kornilov
2018-07-19 23:36 ` Ezequiel Garcia
2018-07-20 9:35 ` Matwey V. Kornilov
2018-07-30 15:42 ` Laurent Pinchart
2018-07-30 16:07 ` Mauro Carvalho Chehab
2018-07-31 6:06 ` Tomasz Figa
2018-06-18 18:58 ` [PATCH 1/2] Add TRACE_EVENTs in pwc_isoc_handler() Steven Rostedt
2018-06-19 16:23 ` Matwey V. Kornilov
2018-06-19 16:33 ` Steven Rostedt
2018-06-20 8:05 ` Matwey V. Kornilov
2018-06-20 13:09 ` Steven Rostedt
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