Re: [Xenomai-core] Clock drift.

[Gilles Chanteperdrix <gilles.chanteperdrix@xenomai.org>]

Gilles Chanteperdrix wrote:
> Philippe Gerum wrote:
>> Gilles Chanteperdrix wrote:
>>> Philippe Gerum wrote:
>>>> Gilles Chanteperdrix wrote:
>>>>> Hi,
>>>>>
>>>>> I run clocktest on a system here, not running NTP, using
>>>>> CONFIG_GENERIC_CLOCKEVENT, with a systematic drift of 3us/s. How can
>>>>> this happen? Does it come from the error due to the multiply and shift
>>>>> used for tsc to/from ns conversions ?
>>>>>
>>>> - ipipe_request_tickdev() returning a timer freq we don't like?
>>> Well, I would expect ipipe_request_tickdev() to return the same
>>> frequency as the one used by Linux.
>>>
>> Yes it does. But this value is used by Xenomai code.
> 
> It turns out that the frequency used to convert tsc to ns is the cpu
> frequency (which is kind of a misnomer, since your emulated tsc on arm
> does not run at cpu frequency), which is computed though it should have
> the same value as the timer frequency but is suject to rounding errors.

The timer frequency is as approximate as the cpu frequency,
ipipe_request_tickdev does this:

	freq = (1000000000ULL * evtdev->mult) >> evtdev->shift;
	*tmfreq = (unsigned long)freq;

-- 
                                                 Gilles.