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

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Message-ID: <20160613195136.GC2441@netboy>
Date:	Mon, 13 Jun 2016 21:51:36 +0200
From:	Richard Cochran <richardcochran@...il.com>
To:	Henrik Austad <henrik@...tad.us>
Cc:	linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
	alsa-devel@...r.kernel.org, netdev@...r.kernel.org,
	henrk@...tad.us, Arnd Bergmann <arnd@...aro.org>
Subject: Re: [very-RFC 0/8] TSN driver for the kernel

On Mon, Jun 13, 2016 at 01:47:13PM +0200, Richard Cochran wrote:
> 3. ALSA support for tunable AD/DA clocks.  The rate of the Listener's
>    DA clock must match that of the Talker and the other Listeners.
>    Either you adjust it in HW using a VCO or similar, or you do
>    adaptive sample rate conversion in the application. (And that is
>    another reason for *not* having a shared kernel buffer.)  For the
>    Talker, either you adjust the AD clock to match the PTP time, or
>    you measure the frequency offset.

Actually, we already have support for tunable clock-like HW elements,
namely the dynamic posix clock API.  It is trivial to write a driver
for VCO or the like.  I am just not too familiar with the latest high
end audio devices.

I have seen audio PLL/multiplier chips that will take, for example, a
10 kHz input and produce your 48 kHz media clock.  With the right HW
design, you can tell your PTP Hardware Clock to produce a 10000 PPS,
and you will have a synchronized AVB endpoint.  The software is all
there already.  Somebody should tell the ALSA guys about it.

I don't know if ALSA has anything for sample rate conversion or not,
but haven't seen anything that addresses distributed synchronized
audio applications.

Thanks,
Richard