Messages in this thread | | | Date | Tue, 13 Oct 2015 06:58:32 +0200 | From | Richard Cochran <> | Subject | Re: [PATCH v4 1/4] Produce system time from correlated clocksource |
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On Mon, Oct 12, 2015 at 11:45:19AM -0700, Christopher S. Hall wrote: > Another representative use case of time sync and the correlated > clocksource (in addition to PTP noted above) is PTP synchronized > audio.
The added explanations of the audio use case do help. However, you did not address my point in the last series in any way.
> In a streaming application, as an example, samples will be sent > and/or received by multiple devices with a presentation time that is > in terms of the PTP master clock. Synchronizing the audio output on > these devices requires correlating the audio clock with the PTP > master clock. The more precise this correlation is, the better the > audio quality (i.e. out of sync audio sounds bad).
^^^^ This is mega important. You want to convert PTP time into audio clock time. There is no need for the system time at all. > From an application standpoint, to correlate the PTP master clock > with the audio device clock, the system clock is used as a > intermediate timebase.
But why involve the system time base?
> The transforms such an application would > perform are: > > System Clock <-> Audio clock > System Clock <-> Network Device Clock [<-> PTP Master Clock]
This is extra work with no benefit. In fact, this hurts you because of the need to take avoid update_wall_time AND because of the NTP frequency adjustments. Cascaded servos are prone to gain peaking, and this can easily avoided in this case. > Modern Intel platforms can perform a more accurate cross- > timestamp in hardware (ART,audio device clock). The audio driver > requires ART->system time transforms -- the same as required for > the network driver.
No, it doesn't need the system time. It only needs the PTP time.
> The modification to the original patch accomodates these > slow devices by adding the option of providing an ART value outside > of the retry loop and adding a history which can consulted in the > case of an out of date counter value. The history is kept by > making the shadow_timekeeper an array. Each write to the > timekeeper rotates through the array, preserving a > history of updates.
This is all wrong. All you need to provide the DSP with (ART, PTP) pairs. This can be done in a multiple of the DSP period, like every 1, 10, or 100 milliseconds.
Thanks, Richard
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