Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759043Ab3CYXHd (ORCPT ); Mon, 25 Mar 2013 19:07:33 -0400 Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:39380 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753825Ab3CYXHb (ORCPT ); Mon, 25 Mar 2013 19:07:31 -0400 Date: Mon, 25 Mar 2013 23:07:30 +0000 From: Catalin Marinas To: Rob Herring Cc: Russell King - ARM Linux , Mark Rutland , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , Kukjin Kim , Arnd Bergmann , "linux-sh@vger.kernel.org" , Tony Lindgren , Magnus Damm , "rob.herring@calxeda.com" , "linux-samsung-soc@vger.kernel.org" , Simon Horman , John Stultz , Will Deacon , Thomas Gleixner , "linux-omap@vger.kernel.org" Subject: Re: [PATCH] ARM: convert arm/arm64 arch timer to use CLKSRC_OF init Message-ID: <20130325230730.GA5518@MacBook-Pro.local> References: <1363818875-15978-1-git-send-email-robherring2@gmail.com> <20130321110646.GE15279@e106331-lin.cambridge.arm.com> <20130325172633.GP30923@n2100.arm.linux.org.uk> <5150C16A.3040208@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5150C16A.3040208@gmail.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 Content-Length: 2078 Lines: 41 On Mon, Mar 25, 2013 at 09:28:10PM +0000, Rob Herring wrote: > On 03/25/2013 12:26 PM, Russell King - ARM Linux wrote: > > On Thu, Mar 21, 2013 at 11:06:47AM +0000, Mark Rutland wrote: > >> On TC2 this series leads to using the vexpress 24MHz clock as the sched clock > >> in preference to the architected timer: > >> > >> Architected local timer running at 24.00MHz (virt). > >> Switching to timer-based delay loop > >> Registered arch_counter_get_cntvct+0x0/0x14 as sched_clock source > >> sched_clock: 32 bits at 24MHz, resolution 41ns, wraps every 178956ms > >> Registered versatile_read_sched_clock+0x0/0x28 as sched_clock source > >> > >> As they both have the same frequency, neither overrides the other, and > >> whichever gets registered last is used as the sched_clock. As accesses > >> to the architected timer are going to have a much lower overhead, this > >> isn't very nice (and it could be better to use it even if it had a lower > >> frequency). > > > > I'll remind people that sched_clock() is supposed to be functional at > > the point in the boot sequence where the call to sched_init() is called. > > That is after setup_arch() and *before* time_init() is called. > > I count integrator-cp, realview, versatile and non-DT VExpress that do > this (not surprisingly) and 25 platforms or timer implementations plus > arm64 that do sched_clock setup in time_init. Before time_init(), sched_clock() currently returns 0 with the architected timers (though I don't particularly like this for arm64). Marc Rutland has patches to make arch_timer_read_counter() a function which always returns the virtual counter. It requires the CNTVOFF register to be set to 0 on AArch32 during boot. But this way sched_clock() on arm64 would always return meaningful values as we have the architected timers. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/