From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755986Ab2LNDNn (ORCPT ); Thu, 13 Dec 2012 22:13:43 -0500 Received: from mga02.intel.com ([134.134.136.20]:12251 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755609Ab2LNDNm (ORCPT ); Thu, 13 Dec 2012 22:13:42 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.84,277,1355126400"; d="scan'208";a="233851002" Date: Fri, 14 Dec 2012 11:13:30 +0800 From: Feng Tang To: Jason Gunthorpe Cc: John Stultz , Thomas Gleixner , Alessandro Zummo , linux-kernel@vger.kernel.org, alek.du@intel.com Subject: Re: [PATCH 1/3] timekeeping: Add persistent_clock_exist flag Message-ID: <20121214031330.GC11276@feng-snb> References: <1355364328-19550-1-git-send-email-feng.tang@intel.com> <50CA7EE4.3000306@linaro.org> <20121214013725.GA11276@feng-snb> <50CA8837.5010800@linaro.org> <20121214023826.GA31613@obsidianresearch.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20121214023826.GA31613@obsidianresearch.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 13, 2012 at 07:38:26PM -0700, Jason Gunthorpe wrote: > On Thu, Dec 13, 2012 at 06:00:23PM -0800, John Stultz wrote: > > > So per Jason's related patch, he's made the point that the > > persistent_clock and RTC class functionality are basically exclusive > > (well, in his case, he said this with respect to updating the RTC, > > not reading it - I don't mean to put words in his mouth - Please do > > correct me here Jason. :). In other words, we probably should avoid > > configurations where both the rtc hctosys and persistent_clock > > interfaces are both active. > > I only studied update_persistent_clock, read_persistent_clock is > very much different. > > Looking at it, I don't think that update_persistent_clock is in any > way related to read_persistent_clock.. update_persistent_clock is > *only* called by NTP, and its *only* purpose is to update the RTC with > NTP synchronized time. In many configurations it will never even be > called. > > I think update_persistent_clock is badly named, it should be called > platform_save_ntp_time_to_rtc(), keep it divorced from > read_presistent_clock :) > > > make the HCTOSYS option be dependent on !HAS_PERSISTENT_CLOCK. This > > way we avoid having configs where there are conflicting paths that > > we chose from. > > On ARM the read_presistent_clock is used to access a true monotonic > counter that is divorced from the system RTC - look at > arch/arm/plat-omap/counter_32k.c for instance. > > This seems like a great use of that hardware resource, and no doubt > those mach's also have a class RTC driver available talking to > different hardware. Interesting to know this, thanks for the info. For the x86 desktop and mobile processors I've used, the read_persistent_clock and rtc are the same on-board device (always power on), so I see many time related code are execuated twice, like init/suspend/resume if HCTOSYS config is enabled, that's why I came up with the patches. > > For mach's without that functionality ARM returns a fixed 0 value > from read_persistent_clock, persumably the kernel detects this and > falls back to using class rtc functions? > > Maybe Feng would be better off adjusting read_persistent_clock to > return ENODEV in such cases?? For mach's without read_persistent_clock capability, there is already a weakly defined void __attribute__((weak)) read_persistent_clock(struct timespec *ts) { ts->tv_sec = 0; ts->tv_nsec = 0; } so those machs can simply do nothing, and let time core code to judge it. > > So, I think you have to keep your test as a run time test. To support > the single image ARM boot you can't make the distinction with kconfig. Good point. Figuring out the kconfig for all arm platforms is very challenging. Thanks, Feng