From mboxrd@z Thu Jan 1 00:00:00 1970 From: Russell King - ARM Linux Subject: Re: [PATCH v9 4/9] clocksource/drivers/arm_arch_timer: use readq to get 64-bit CNTVCT Date: Mon, 25 Jul 2016 23:49:57 +0100 Message-ID: <20160725224957.GU1041@n2100.armlinux.org.uk> References: <1469460427-8643-1-git-send-email-fu.wei@linaro.org> <1469460427-8643-5-git-send-email-fu.wei@linaro.org> <20160725153118.GD19209@arm.com> <20160725163144.GE19209@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20160725163144.GE19209-5wv7dgnIgG8@public.gmane.org> Sender: linux-watchdog-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Will Deacon Cc: Fu Wei , Linaro ACPI Mailman List , Catalin Marinas , rruigrok-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org, Wim Van Sebroeck , wei-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, Lorenzo Pieralisi , Al Stone , Timur Tabi , Daniel Lezcano , ACPI Devel Maling List , Guenter Roeck , Len Brown , harba-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org, linux-watchdog-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Arnd Bergmann , Marc Zyngier , Jon Masters , Christopher Covington , Thomas Gleixner , linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org, G Gregory , "Rafael J. Wysocki" , Linux Kernel Mailing List List-Id: linux-acpi@vger.kernel.org On Mon, Jul 25, 2016 at 05:31:45PM +0100, Will Deacon wrote: > On Mon, Jul 25, 2016 at 11:55:49PM +0800, Fu Wei wrote: > > On 25 July 2016 at 23:31, Will Deacon wrote: > > > On Mon, Jul 25, 2016 at 11:27:02PM +0800, fu.wei-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org wrote: > > >> From: Fu Wei > > >> > > >> This patch simplify arch_counter_get_cntvct_mem function by > > >> using readq to get 64-bit CNTVCT value instead of readl_relaxed. > > >> > > >> Signed-off-by: Fu Wei > > >> --- > > >> drivers/clocksource/arm_arch_timer.c | 10 +--------- > > >> 1 file changed, 1 insertion(+), 9 deletions(-) > > >> > > >> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c > > >> index e6fd42d..483d2f9 100644 > > >> --- a/drivers/clocksource/arm_arch_timer.c > > >> +++ b/drivers/clocksource/arm_arch_timer.c > > >> @@ -418,15 +418,7 @@ u32 arch_timer_get_rate(void) > > >> > > >> static u64 arch_counter_get_cntvct_mem(void) > > >> { > > >> - u32 vct_lo, vct_hi, tmp_hi; > > >> - > > >> - do { > > >> - vct_hi = readl_relaxed(arch_counter_base + CNTVCT_HI); > > >> - vct_lo = readl_relaxed(arch_counter_base + CNTVCT_LO); > > >> - tmp_hi = readl_relaxed(arch_counter_base + CNTVCT_HI); > > >> - } while (vct_hi != tmp_hi); > > >> - > > >> - return ((u64) vct_hi << 32) | vct_lo; > > >> + return readq(arch_counter_base + CNTVCT_LO); > > > > > > Please drop this patch. It doesn't work. > > > > I am OK to drop this, but could you let me know why it doesn't work? > > > > I did get some problem on Foundation model about readq, but it works on Seattle. > > I guess that is a problem of model, but not a code problem. > > So I just got confused, why readq doesn't work, :-) > > The kernel really needs to support both of those platforms :/ > > For the memory-mapped counter registers, the architecture says: > > `If the implementation supports 64-bit atomic accesses, then the > CNTV_CVAL register must be accessible as an atomic 64-bit value.' > > which is borderline tautological. If we take the generous reading that > this means AArch64 CPUs can use readq (and I'm not completely > comfortable with that assertion, particularly as you say that it breaks > the model), then you still need to use readq_relaxed here to avoid a > DSB. Furthermore, what are you going to do for AArch32? readq doesn't > exist over there, and if you use the generic implementation then it's > not atomic. In which case, we end up with the current code, as well as a > readq_relaxed guarded by a questionable #ifdef that is known to break a > supported platform for an unknown performance improvement. Hardly a big > win. > > Did you see any performance advantage from this? Given that you've added > a DSB, this looks to be extremely premature. +1, absolutely agreed on the 32-bit ARM bits. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net. -- To unsubscribe from this list: send the line "unsubscribe linux-watchdog" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html