Re: [Bluez-devel] An explanation of a2dpd weird behaviour on high resolution timers enabled kernels

[Fabien Chevalier <fabchevalier@free.fr>]

Hi Marcel,

> 
> I might be simply never hear the difference, 

Well, you will, however not always, this will be dependant upon the 
bluetooth devices used, so that might work for you but fail for someone 
else if those clock drifting issues are not handled properly.

but anyway, we can actually
> do this. However the implementation is tricky. The plugin should not
> send HCI commands at all. Actually no daemon (except hcid) should send
> any HCI commands at any time. 

Woops, didn't know about that :-(

Are there any hard constraints behing this limitation or is it just a 
good software architecturing rule that prevents us from doing it ?

> I can make the kernel to as for the clock
> offset in a certain interval. 
Just for my own comprehension : could you expose here how do you intend 
to implement this ?

> This costs too since it has to go over the
> same HCI link that the audio data goes. 
Agreed :-)
> So at which rate do you need
> this information?

Let's say we are unlucky and we got the master clock and the main CPU 
clock to drift each of 250 ppm in opposite directions. That leaves use 
with a 30 ms per minute drift.

I'd say a clock resynch every 20-30 seconds should be enough, and would 
limit the drift at any time below 15 mseconds, which should be 
compensated byt the headset side buffering.

So let's say i need this information every ~20 seconds.

Note : of course this is for a2dp only, sco matters are completely 
different.

Cheers,

Fabien

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