From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <496F3AF6.7070509@domain.hid> Date: Thu, 15 Jan 2009 14:32:38 +0100 From: Philippe Gerum MIME-Version: 1.0 References: <496DEB16.5060803@domain.hid> <496DEE61.80706@domain.hid> <496DEFEA.5000709@domain.hid> <496DF150.8050606@domain.hid> <496F36EE.1070305@domain.hid> In-Reply-To: <496F36EE.1070305@domain.hid> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] Clock drift. Reply-To: rpm@xenomai.org List-Id: "Xenomai life and development \(bug reports, patches, discussions\)" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gilles Chanteperdrix Cc: Xenomai core 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), Yes, that's bad. We have a timebase frequency that applies to our clock readings for calculating delays, and a timer frequency used to rescale those delays for programming timer shots. The CPU frequency has no relevance here. The fact that x86 once assumed CPU freq == TSC freq is an unfortunate legacy that goes down deep to the I-pipe as well. which is computed though it should have > the same value as the timer frequency but is suject to rounding errors. > -- Philippe.