Xen devel - May 2005 - RE: asterisk, ztdummy, and usb (and HZ = 100 under xen ???)

> > The problem is that the ztdummy driver assumes 1000HZ,
> 
> Do you mean it has a hardcoded value instead of using the "HZ"
macro???
> I''d consider that a bug - have you tried contacting the codes
> maintainers?
It''s not as straight forward as that. Asterisk needs a 1000hz timer for
certain functions to work. For certain things to work really properly
(eg fax), this timer needs to be synchronised exactly to the exchange
(eg from the isdn/e1 card) otherwise frames might be dropped.

Under 2.4, the ztdummy driver uses the UHCI USB hardware to get a 1000hz
interval. Under 2.6, HZ=1000 (on the x86 anyway, except under xen :) so
it counts jiffies instead. If you have HZ=100 then you can''t get 1000
ticks from it no matter what macro you use, so the best you could do
would be to fall back to UHCI timing when HZ is not evenly divisible by
1000, or fail if no UHCI controller is present.

I''m surprised my patch (eg generate 10 ticks ever 1/100 seconds) works
as well as it does.

James

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On 21 May 2005, at 08:53, James Harper wrote:
> It''s not as straight forward as that. Asterisk needs a 1000hz
timer for
> certain functions to work. For certain things to work really properly
> (eg fax), this timer needs to be synchronised exactly to the exchange
> (eg from the isdn/e1 card) otherwise frames might be dropped.
>
> Under 2.4, the ztdummy driver uses the UHCI USB hardware to get a 
> 1000hz
> interval. Under 2.6, HZ=1000 (on the x86 anyway, except under xen :) so
> it counts jiffies instead. If you have HZ=100 then you can''t get
1000
> ticks from it no matter what macro you use, so the best you could do
> would be to fall back to UHCI timing when HZ is not evenly divisible by
> 1000, or fail if no UHCI controller is present.
>
> I''m surprised my patch (eg generate 10 ticks ever 1/100 seconds)
works
> as well as it does.
We''re going to rip the fixed ticker out of Xen and allow domains to 
specify a background ticker at a rate of their choice, which ought to 
fix this for you. OF course, you''re pretty screwed running this kind of
thing on Xen unless you dedicate a whole CPU to the relevant domain.

  -- Keir


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