From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <49F70883.4080005@domain.hid> Date: Tue, 28 Apr 2009 15:45:39 +0200 From: Gilles Chanteperdrix MIME-Version: 1.0 References: <49F69924.8070001@domain.hid> <1240913240.6990.347.camel@domain.hid> <49F6F4DD.6000902@domain.hid> <1240926119.6990.368.camel@domain.hid> In-Reply-To: <1240926119.6990.368.camel@domain.hid> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] xenomai 2.5 rc1 timing computations. List-Id: Xenomai life and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Philippe Gerum Cc: Xenomai core Philippe Gerum wrote: > On Tue, 2009-04-28 at 14:21 +0200, Gilles Chanteperdrix wrote: >> Philippe Gerum wrote: >>> On Tue, 2009-04-28 at 07:50 +0200, Gilles Chanteperdrix wrote: >>>> Hi, >>>> >>>> currently, the situation is this: >>>> - the timing core uses an approximate value of the cpu frequency (using >>>> xnarch_llmulshft) to do conversions between tsc and ns; >>>> - the APIC timer reprogrammation still uses imuldiv, that is a more >>>> exact cpu frequency, coupled with an approximate APIC frequency >>>> (obtained from ipipe_request_tickdev), to do the conversion between tsc >>>> delays and APIC delays; >>>> - the posix skin uses llimd to do conversions between timespec and >>>> timeval structures and tsc values >>>> - the posix and native skins in user-space still use llimd to do >>>> conversions between ns and tsc (only if using tsc in user-space). >>>> >>>> These are causes of unprecisions, I think we should fix this, though it >>>> is unlikely to be the cause of the high jitters observed by Vikesh with >>>> 2.5-rc1. >>>> >>> Ack, we definitely should. Working on allowing userland to use >>> llmulshift for timing computations for the native skin here, as a first >>> step. For that purpose, xnsysinfo_t has just been extended to carry the >>> pre-scaled values needed for llmulshft. I just don't want to have to >>> issue a syscall for those conversions to take place. >> Actually, the pre-scaled values could be re-computed in user-space >> without changing the ABI. > > We don't care that much about changing the ABI between -rc1 and -rc2 > actually, so it mostly depends whether we want to backport the fix to > 2.4 regarding the tsc->ns conversion, since we do not have any nodiv > implementation for the ns->tsc one there yet. But we have llmulshft, do we ? which means that we do have the problem of approximate cpu frequency. -- Gilles.