Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752742Ab3IQL1L (ORCPT ); Tue, 17 Sep 2013 07:27:11 -0400 Received: from www.linutronix.de ([62.245.132.108]:52538 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751907Ab3IQL1J (ORCPT ); Tue, 17 Sep 2013 07:27:09 -0400 Date: Tue, 17 Sep 2013 13:26:56 +0200 (CEST) From: Thomas Gleixner To: Russell King - ARM Linux cc: Ludovic Desroches , Marc Kleine-Budde , nicolas.ferre@atmel.com, linux-kernel@vger.kernel.org, Marc Pignat , john.stultz@linaro.org, kernel@pengutronix.de, Ronald Wahl , linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH] [PATCH] clocksource: tcb: fix min_delta calculation In-Reply-To: <20130917100417.GQ12758@n2100.arm.linux.org.uk> Message-ID: References: <1379077365-18458-1-git-send-email-mkl@pengutronix.de> <20130917095600.GJ26819@ludovic.desroches@atmel.com> <20130917100417.GQ12758@n2100.arm.linux.org.uk> User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2704 Lines: 86 On Tue, 17 Sep 2013, Russell King - ARM Linux wrote: > On Tue, Sep 17, 2013 at 11:56:00AM +0200, Ludovic Desroches wrote: > > Any reason to not do this: > > > > --- a/drivers/clocksource/tcb_clksrc.c > > +++ b/drivers/clocksource/tcb_clksrc.c > > @@ -144,6 +144,9 @@ static void tc_mode(enum clock_event_mode m, struct > > clock_event_device *d) > > > > static int tc_next_event(unsigned long delta, struct clock_event_device > > *d) > > { > > + if (delta < d->min_delta_ticks) > > + delta = d->min_delta_ticks; > > + > > __raw_writel(delta, tcaddr + ATMEL_TC_REG(2, RC)); > > > > /* go go gadget! */ > > > > Then we can keep the same min_delta. > > You really should not play such games in your set_next_event() code - if > the interval is not supported, you should return -ETIME so that the core > code knows about it and can adjust things to suit. If you're getting > deltas which are too small for the hardware, that'll either be because > the bounds are wrong, or there's a bug in the core code. The core code does: int clockevents_program_event(struct clock_event_device *dev, ktime_t expires, bool force) { .... delta = min(delta, (int64_t) dev->max_delta_ns); delta = max(delta, (int64_t) dev->min_delta_ns); clc = ((unsigned long long) delta * dev->mult) >> dev->shift; rc = dev->set_next_event((unsigned long) clc, dev); } So, the min_delta in timer ticks is set to 1 and converted by the core to nsecs for various reasons. This is where the rounding problem comes into play. The patch below should fix that. Thanks, tglx ---- diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c index 38959c8..41b7a30 100644 --- a/kernel/time/clockevents.c +++ b/kernel/time/clockevents.c @@ -42,7 +42,7 @@ struct ce_unbind { */ u64 clockevent_delta2ns(unsigned long latch, struct clock_event_device *evt) { - u64 clc = (u64) latch << evt->shift; + u64 tmp, clc = (u64) latch << evt->shift; if (unlikely(!evt->mult)) { evt->mult = 1; @@ -52,6 +52,14 @@ u64 clockevent_delta2ns(unsigned long latch, struct clock_event_device *evt) do_div(clc, evt->mult); if (clc < 1000) clc = 1000; + + /* Avoid rounding artifacts */ + tmp = (clc * dev->mult) >> dev->shift; + while (tmp < (u64) dev->min_delta_ticks) { + clc += 1000; + tmp = (clc * dev->mult) >> dev->shift; + } + if (clc > KTIME_MAX) clc = KTIME_MAX; -- 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/