linux-kernel - Re: firewire: CLOCK_MONOTONIC

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Date:	Thu, 17 Jul 2014 15:19:55 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Daniel Thompson <daniel.thompson@...aro.org>
cc:	Mark Brown <broonie@...nel.org>,
	Clemens Ladisch <clemens@...isch.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	linux-kernel@...r.kernel.org,
	linux1394-devel@...ts.sourceforge.net,
	John Stultz <john.stultz@...aro.org>
Subject: Re: firewire: CLOCK_MONOTONIC_RAW exposure

On Thu, 17 Jul 2014, Daniel Thompson wrote:

> On 16/07/14 16:00, Mark Brown wrote:
> > On Wed, Jul 16, 2014 at 04:16:35PM +0200, Clemens Ladisch wrote:
> >> Peter Zijlstra wrote:
> >>> On Wed, Jul 16, 2014 at 01:34:21PM +0200, Clemens Ladisch wrote:
> > 
> >>>> 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?)
> > 
> > Right, the goal is to get a clock which is guaranteed to never have any
> > adjustments that might cause discontinuities or rate changes applied to
> > it.  My understanding is that the users are already doing their own rate
> > matching and it's much more important to them to get a stable clock than
> > it is to get a clock at a specific nominal rate, and given the set top
> > box applications I expect they also need this from very soon after boot.
> 
> We are trying to match rates with a broadcast device that "shouts" the
> current time many times per second (MPEG transport stream PCR packets).
> These packets are timestamped on arrival with a local clock and the
> resulting data is used to recover the broadcast clock. However due to
> variable transmission delay of the packets we require very long
> control loops to extract any useful information from this data (varies
> between five minutes and half and hour).
> 
> An NTP rate correction can change the rate of CLOCK_MONOTONIC
> sufficiently to confuse our clock recovery algorithms so we use
> CLOCK_MONOTONIC_RAW as the master view of time.
> 
> Both audio and video must be presented synchronized to the recovered
> broadcast clock which in practice this means comparing them to
> CLOCK_MONOTONIC_RAW and doing some maths.
> 
> In is, of course, possible to convert ALSA's CLOCK_MONOTONIC
> timestamps into CLOCK_MONOTONIC_RAW values so we can compare ALSA time
> to the broadcast clock. In fact its not even that hard compared to the
> other time conversions we have to do. Nevertheless it is a redundant
> conversion and adds an extra dimension to a problem that only just fits
> in most craniums ;-)

Thanks for the explanation!

       tglx
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