From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH] x86: Calculate MHz using APERF/MPERF for cpuinfo and scaling_cur_freq Date: Fri, 1 Apr 2016 10:29:23 +0200 Message-ID: <20160401082923.GH11035@twins.programming.kicks-ass.net> References: <6e0c25e64e0fb65a42dfc63ad5f660302e07cd87.1459485198.git.len.brown@intel.com> <52f711be59539723358bea1aa3c368910a68b46d.1459485198.git.len.brown@intel.com> <20160401080328.GC3448@twins.programming.kicks-ass.net> <9222CD56-C603-449C-A049-E518DAFA6883@linux.intel.com> <20160401082323.GE3448@twins.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20160401082323.GE3448@twins.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org To: Stephane Gasparini Cc: Len Brown , x86@kernel.org, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org, Len Brown List-Id: linux-pm@vger.kernel.org On Fri, Apr 01, 2016 at 10:23:23AM +0200, Peter Zijlstra wrote: > > Trim your emails > > On Fri, Apr 01, 2016 at 10:16:42AM +0200, Stephane Gasparini wrote: > > > > That means these delta's can be arbitrarily large, in fact the MSRs can > > > have wrapped however many times. > > > > 64 bits is 18 446 744 073 709 551 615 > > > > so even assuming a 10 GHz frequency if my math are good this is more than > > 58 years before the MSR wrap around, assuming the device ran always at max > > freq. > > fair enough.. but going with 10Ghz, cpu_khz would be 10e6 ~ 33 bits, I can't do maths this morning; 23 bits > which effectively reduces the wrap/overflow time to just 31 bits, which > per that frequency is just ~1/4th of a second. 41 giving lots more, but a reasonable time to wrap/overflow.