Re: [Jack-Devel] FFADO issues after freewheeling (was: Re: update jack from 1.9.7 to 1.9.8)

On Thu, Mar 8, 2012 at 9:06 AM, Fons Adriaensen <[hidden]> wrote:
> On Fri, Mar 09, 2012 at 12:05:41AM +1030, Jonathan Woithe wrote:
>
>> FFADO isn't aware of freewheeling as such.  However, when freewheeling is
>> started the backend streaming is stopped for the reasons Paul mentioned.
>> When jack is taken out of freewheeling the backend is restarted again.
>> FFADO sees the stop/start cycle but doesn't know (or care) that this is
>> because of freewheeling.
>
> Paul didn't give any reasons why freewheeling is implemented that way,
> and I'm somewhat surprised it is. To me the simplest way to do it
> would be:
>
> 1. The RT thread that normally calls the backend's 'wait for next cycle'
> function continues to do so, it feeds silence to the backend and discards
> any input it produces. That way even the backend would be unaware of
> freewheeling.
>
> 2. A second non-RT thread runs cycles as fast as it can, substituting
> the backend buffers by dummy ones. Only two extra buffers are required
> to do that, one (silenced) given to all clients that have system:capture
> ports connected, and one for output (which can be shared for all system:
> playback channels as it will be discarded anyway).
>
> I always imagined things worked that way...

what you describe is actually more complex. it requires special
handling of port buffers, which the current scheme(s) (multiple since
both jack1 and jack2 do this) do not. it also requires a second thread
which comes and goes.  the current schemes simply tell the backend
that its freewheeling - it stops the device it is using, and simply
wakes up the "engine" immediately that the engine returns from the
previous "cycle" (rather than waiting for the device). almost no other
code is required. starting and stopping devices is something that the
backend will already have code for; the idea that such code is not
functional if called more than once seems problematic.

--p

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