From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Gleixner Subject: Re: [RFC v5 3/6] Add history to cross timestamp interface supporting slower devices Date: Fri, 8 Jan 2016 15:04:28 +0100 (CET) Message-ID: References: <1451911523-8534-1-git-send-email-christopher.s.hall@intel.com> <1451911523-8534-4-git-send-email-christopher.s.hall@intel.com> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: John Stultz , Richard Cochran , Ingo Molnar , "H. Peter Anvin" , Jeff Kirsher , "x86@kernel.org" , lkml , intel-wired-lan@lists.osuosl.org, netdev@vger.kernel.org, "Stanton, Kevin B" To: Christopher Hall Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Thu, 7 Jan 2016, Christopher Hall wrote: > On Wed, 06 Jan 2016 11:37:23 -0800, John Stultz > wrote: > > I've not done a super close reading here. But is it very likely the > > the history_ref->cs_seq is the same as the captured seq? I thought > > this history_ref was to allow old cross stamps to be used to improve > > the back-calculation of the time at the given cycle value. So throwing > > them out if they are older then the last tick seems strange. > > Maybe this needs more explanation. The clocksource sequence (cs_seq) is > incremented for each change in clocksource. I use this to detect a rare corner > case where the clocksource is changed from (on x86 anyway) TSC and then back. > If the history crosses one of these changes then interpolation shouldn't be > attempted (return error). It's not really enough when using the history to > just check that the current clocksource is equal to the one used at the start > of the history. The clocksource must not have changed at all. To answer your > question, it's not at all likely that this would occur. You can flush the history when a clocksource change happens. No need to add extra data to the core structures. Thanks, tglx