Re: [very-RFC 0/8] TSN driver for the kernel

Henrik Austad
2016-06-20T08:13:01Z

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On Sun, Jun 19, 2016 at 11:46:29AM +0200, Richard Cochran wrote:
> On Sun, Jun 19, 2016 at 12:45:50AM +0200, Henrik Austad wrote:
> > edit: this turned out to be a somewhat lengthy answer. I have tried to
> > shorten it down somewhere. it is getting late and I'm getting increasingly
> > incoherent (Richard probably knows what I'm talking about ;) so I'll stop
> > for now.
>
> Thanks for your responses, Henrik. I think your explanations are on spot.
>
> > note that an adjustable sample-clock is not a *requirement* but in general
> > you'd want to avoid resampling in software.
>
> Yes, but..
>
> Adjusting the local clock rate to match the AVB network rate is
> essential. You must be able to *continuously* adjust the rate in
> order to compensate drift. Again, there are exactly two ways to do
> it, namely in hardware (think VCO) or in software (dynamic
> resampling).

Don't get me wrong, having an adjustable clock for the sampling is
essential -but it si not -required-.

> What you cannot do is simply buffer the AV data and play it out
> blindly at the local clock rate.

No, that you cannot do that, that would not be pretty :)

> Regarding the media clock, if I understand correctly, there the talker
> has two possibilities. Either the talker samples the stream at the
> gPTP rate, or the talker must tell the listeners the relationship
> (phase offset and frequency ratio) between the media clock and the
> gPTP time. Please correct me if I got the wrong impression...

Last first; AFAIK, there is no way for the Talker to tell a Listener the
phase offset/freq ratio other than how each end-station/bridge in the
gPTP-domain calculates this on psync_update event messages. I could be
wrong though, and different encoding formats can probably convey such
information. I have not seen any such mechanisms in the underlying 1722
format though.

So a Talker should send a stream sampled as if the gPTP time drove the
AD/DA sample frequency directly. Whether the local sampling is driven by
gPTP or resampled to match gPTP-time prior to transmit is left as an
implementation detail for the end-station.

Did all that make sense?

Thanks!
--
Henrik Austad

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