Re: [Jack-Devel] how often should one call jack_port_get_latency_range()

On 07/13/2011 03:34 PM, Paul Davis wrote:
> On Wed, Jul 13, 2011 at 9:30 AM, Robin Gareus <[hidden]> wrote:
>> G'day!
>>
>> I have a JACK client with a single audio-input-port (no output). This
>> input is used to parse Linear-Time-Code. For synchronization the
>> application needs to know the capture-latency of said audio-port.
>>
>> So the question is: how often should the application call
>> jack_port_get_latency_range() ?
>>
>> Currently it does so in every process_callback.
>>
>> Is this the correct solution?
> 
> no.
> 
>> The overhead may be minimal, but still it feels somewhat wrong to poll
>> it during every process() cycle. Is there a callback that can be
>> subscribed to to get notified of changes to port-latencies?
> 
> From the documentation of jack_set_latency_callback():
> 
>  * Tell the Jack server to call @a latency_callback whenever it
>  * is necessary to recompute the latencies for some or all
>  * Jack ports.

From the same documentation:

  | Clients that meet any of the following conditions do NOT need to
  | register a latency callback:
  |
  |  * have only input ports
  |  * ..

All of the criteria are matched in my app (xjadeo), but it's somewhat
misleading as these criteria are only concerned with the client
_setting_ the latency.

It would be helpful to mention that this callback can also be used to
query it.

On that note: In which order is this callback executed?
Can one query the jack_port_get_latency_range() from within this
callback? - It seems possible that some clients may not yet have updated
their latency when the callback is invoked in some other client.

Is this correct?

If so a client only interested in querying the latency should set a flag
in in the latency_callback function and [later] query the new port
latency during the process_callback IFF this flag is set, right?

ciao,
robin

1310565577.16569_0.ltw:2,a <4E1DA463.2060308 at gareus dot org>