From: Clemens Ladisch <clemens@ladisch.de>
To: Peter Zijlstra <peterz@infradead.org>,
Daniel Thompson <daniel.thompson@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Stefan Richter <stefanr@s5r6.in-berlin.de>,
linux-kernel@vger.kernel.org,
linux1394-devel@lists.sourceforge.net,
John Stultz <john.stultz@linaro.org>,
Mark Brown <broonie@linaro.org>
Subject: Re: firewire: CLOCK_MONOTONIC_RAW exposure
Date: Wed, 16 Jul 2014 16:16:35 +0200 [thread overview]
Message-ID: <53C68943.2060003@ladisch.de> (raw)
In-Reply-To: <20140716123727.GA19379@twins.programming.kicks-ass.net>
Peter Zijlstra wrote:
> On Wed, Jul 16, 2014 at 01:34:21PM +0200, Clemens Ladisch wrote:
>> Thomas Gleixner wrote:
>>> I wonder why firewire exposed CLOCK_MONOTONIC_RAW to user space.
>>>
>>> What's the purpose of that? CLOCK_MONOTONIC_RAW is the raw time based
>>> on the initial frequency setup of the clocksource. That can be quite
>>> off from the NTP corrected frequency which is exposed by
>>> CLOCK_MONOTONIC.
>>
>> The purpose is to get a stable clock, i.e., to avoid clock rate changes
>> caused by NTP corrections.
>
> That's maybe half an answer; what do you need that for?
According to the original report, "for applications which need to
synchronise with external timebases such as broadcast TV applications".
Such external clocks are not synchronous with any available clocksource,
so proper resampling requires that the (relative) rate of the two clocks
(sender and playback device) is measured accurately.
(I don't have numbers for the errors caused by NTP adjustments. Daniel?)
Regards,
Clemens
next prev parent reply other threads:[~2014-07-16 14:16 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-07-16 9:57 firewire: CLOCK_MONOTONIC_RAW exposure Thomas Gleixner
2014-07-16 11:34 ` Clemens Ladisch
2014-07-16 12:37 ` Peter Zijlstra
2014-07-16 14:16 ` Clemens Ladisch [this message]
2014-07-16 15:00 ` Mark Brown
2014-07-17 8:37 ` Daniel Thompson
2014-07-17 13:19 ` Thomas Gleixner
2014-07-17 16:14 ` John Stultz
2014-07-17 16:34 ` Daniel Thompson
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