From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paul Walmsley Subject: Re: [PATCH] timekeeping: Move persistent clock registration code from ARM to kernel Date: Fri, 9 Jan 2015 19:48:42 +0000 (UTC) Message-ID: References: <20141110095325.GC12126@ulmo> <20150109094913.GB27845@ulmo> <20150109135907.GF12942@leverpostej> <20150109140902.GA7526@ulmo.nvidia.com> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Return-path: In-Reply-To: <20150109140902.GA7526-AwZRO8vwLAwmlAP/+Wk3EA@public.gmane.org> Sender: linux-tegra-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Thierry Reding Cc: Mark Rutland , Thomas Gleixner , Anatol Pomozov , Stephen Warren , Daniel Lezcano , Russell King , LKML , "linux-tegra-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , Tony Lindgren , John Stultz , marc.zyngier-5wv7dgnIgG8@public.gmane.org, pdeschrijver-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org List-Id: linux-tegra@vger.kernel.org Hi On Fri, 9 Jan 2015, Thierry Reding wrote: > On Fri, Jan 09, 2015 at 01:59:07PM +0000, Mark Rutland wrote: > > On Fri, Jan 09, 2015 at 09:49:14AM +0000, Thierry Reding wrote: > > > > > As I understand it the architected timer will be turned off along with > > > the rest of the CPU complex on Tegra. I'm not sure if that's specific to > > > Tegra or something that other SoCs may do as well. > > > > That doesn't sound right to me: the architecture specifies that the > > system counter must be implemented in an always-on power domain. > > > > Note that that only applies to the counter, not the timers (as the > > comparators can be turned off with the CPUs). > > I'll let Paul comment on this, since I don't know the intimate details. I believe Mark's comments are accurate. In Linux terms, when deep CPU power management states are in use, the ARM architected "timer" is useful as a clocksource, but useless as a wakeup source (a clockevent device). Regarding clocksources, my personal view is that the SoC timer IP blocks should be exposed as a clocksource option if the timer IP blocks are present in the hardware. That written, I would expect that most production implementations will use the ARM architected "timer" as the clocksource. - Paul