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Date: Thu, 7 Jan 2016 17:12:16 -0800 From: John Stultz <john.stultz@...aro.org> To: Christopher Hall <christopher.s.hall@...el.com> Cc: Thomas Gleixner <tglx@...utronix.de>, Richard Cochran <richardcochran@...il.com>, Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>, Jeff Kirsher <jeffrey.t.kirsher@...el.com>, "x86@...nel.org" <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>, intel-wired-lan@...ts.osuosl.org, netdev@...r.kernel.org, "Stanton, Kevin B" <kevin.b.stanton@...el.com> Subject: Re: [RFC v5 3/6] Add history to cross timestamp interface supporting slower devices On Thu, Jan 7, 2016 at 5:07 PM, Christopher Hall <christopher.s.hall@...el.com> wrote: > On Wed, 06 Jan 2016 11:37:23 -0800, John Stultz <john.stultz@...aro.org> > wrote: >> On Mon, Jan 4, 2016 at 4:45 AM, Christopher S. Hall >>> >>> + if (!history_ref || history_ref->cs_seq != cs_seq || >> >> >> 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. Yes. Apologies. I had mis-skimmed the cs_seq increment as happening in the update_wall_time not setup_internals. Instead of cs_seq, which is easily confused as being related to the seqcount lock, could you instead call it something more explicit like clocksource_changed_count? And yea, having it as part of the timekeeper structure would be the better place for it. thanks -john
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