From: Simon Farnsworth <simon.farnsworth@onelan.co.uk>
To: Mauro Carvalho Chehab <mchehab@infradead.org>
Cc: Andy Walls <awalls@md.metrocast.net>,
linux-media@vger.kernel.org, linux-pm@vger.kernel.org
Subject: Re: [PATCH] saa7134: Add pm_qos_request to fix video corruption
Date: Mon, 29 Oct 2012 16:03:04 +0000 [thread overview]
Message-ID: <2614655.PC7gBWDYxH@f17simon> (raw)
In-Reply-To: <20121029134445.1f58657e@infradead.org>
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On Monday 29 October 2012 13:44:45 Mauro Carvalho Chehab wrote:
> Thanks for digging into it and getting more data. Do you know if this change
> it also needed with USB devices that do DMA (isoc and/or bulk)? Or the USB
> core already handles that?
>
I'm not a huge expert - the linux-pm list (cc'd) will have people around who
know more.
If I've understood correctly, though, the USB core should take care of pm_qos
requests if they're needed for the hardware; remember that if the HCD has
enough buffering, there's no need for a pm_qos request. It's only needed for
devices like the SAA7134 where the buffer is small (1K split into pieces)
compared to the sample data rate (27 megabytes/second raw video).
For the benefit of the linux-pm list; this all starts with me providing a
patch to have the saa7134 driver request reduced cpu_dma_latency when
streaming, as I've seen buffer exhaustion. We've got far enough to know that
the value I chose was wrong for the saa7134, but Mauro also wants guidance on
whether USB devices (not host controllers) also need to request reduced
latency.
--
Simon Farnsworth
Software Engineer
ONELAN Ltd
http://www.onelan.com
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next parent reply other threads:[~2012-10-29 16:03 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1350906611-17498-1-git-send-email-simon.farnsworth@onelan.co.uk>
[not found] ` <13391942.y8AEurCsVE@f17simon>
[not found] ` <20121029134445.1f58657e@infradead.org>
2012-10-29 16:03 ` Simon Farnsworth [this message]
2012-10-30 15:11 ` [PATCH] saa7134: Add pm_qos_request to fix video corruption Alan Stern
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