[LAU] Can't use HPET by JACK2 under kxstudio (probably, it is ubuntu problem too).

[Peter Nelson]

On Tue, 2011-03-08 at 09:57 +0300, Mike Cookson wrote:
> On attempt to use device (or even just try 'cat /dev/hpet'), it is
> said, that device is busy. But jack has such option :/
> I'm using kernel 2.6.33-rt. I can guess, that under rt kernel i should
> not use HPET directly, because it is used by system,
> and so, may be referred via 'system'. But even if that, such way could
> bring some violations comparing to direct HPET usage.
> 
> May be indeed, HPET should be used directly only on non-rt systems?..
> 
> Btw, how i specified it: i have jack2 and ladish. Clock is specified
> via ladiconf. Clock parameter type is integer; yesterday nedko showed
> me a patch for jack, fixing this problem, where i saw order of
> variants in enumeration (hpet variant is '2').

Jack's HPET support has an issue: /dev/hpet can only be opened a limited
number of times. When you use -c hpet, as well as Jack itself, each
client has /dev/hpet opened twice; once as an fd, and once as mmap. On
my systems /dev/hpet is limited to 3 or 4 opens, so this is practically
unusable for any Jack setup.

However you can tell the kernel to use HPET itself for the system clock:

echo hpet > /sys/devices/system/clocksource/clocksource0/current_clocksource

or put "clocksource=hpet" in your boot up command line.

And then -c system will use HPET, indirectly.

Peter.
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