Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752244AbaFDRbB (ORCPT ); Wed, 4 Jun 2014 13:31:01 -0400 Received: from mail-yh0-f73.google.com ([209.85.213.73]:58005 "EHLO mail-yh0-f73.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752092AbaFDRa6 (ORCPT ); Wed, 4 Jun 2014 13:30:58 -0400 From: Doug Anderson To: Kukjin Kim , Tomasz Figa Cc: Daniel Lezcano , Vincent Guittot , Chirantan Ekbote , David Riley , olof@lixom.net, linux-samsung-soc@vger.kernel.org, Doug Anderson , tglx@linutronix.de, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: [PATCH 3/3] clocksource: exynos_mct: Optimize register reads with ldmia Date: Wed, 4 Jun 2014 10:30:34 -0700 Message-Id: <1401903034-20074-3-git-send-email-dianders@chromium.org> X-Mailer: git-send-email 2.0.0.526.g5318336 In-Reply-To: <1401903034-20074-1-git-send-email-dianders@chromium.org> References: <1401903034-20074-1-git-send-email-dianders@chromium.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org As we saw in (clocksource: exynos_mct: cache mct upper count), the time spent reading the MCT shows up fairly high in real-world profiles. That means that it's worth some optimization. We get a roughly 10% speedup in userspace gettimeofday() by using an ldmia to read the two halfs of the MCT. That seems like a worthwhile thing to do. Before: 1173084 us for 1000000 gettimeofday in userspace After: 1045674 us for 1000000 gettimeofday in userspace NOTE: we could actually do better than this if we really wanted to. Technically we could register the clocksource as a 32-bit timer and only use the "lower" half. Doing so brings us down to 1014429 us for 1000000 gettimeofday in userspace (and doesn't even require assembly code). That would be an alternative to this change. Signed-off-by: Doug Anderson --- drivers/clocksource/exynos_mct.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/drivers/clocksource/exynos_mct.c b/drivers/clocksource/exynos_mct.c index 7cbe4aa..6e3017b 100644 --- a/drivers/clocksource/exynos_mct.c +++ b/drivers/clocksource/exynos_mct.c @@ -172,8 +172,9 @@ static inline cycle_t notrace _exynos4_frc_read(void) do { hi = hi2; - lo = __raw_readl(reg_base + EXYNOS4_MCT_G_CNT_L); - hi2 = __raw_readl(reg_base + EXYNOS4_MCT_G_CNT_U); + asm volatile("ldmia %2, {%0, %1}" + : "=r" (lo), "=r" (hi2) + : "r" (reg_base + EXYNOS4_MCT_G_CNT_L)); } while (hi != hi2); return ((cycle_t)hi << 32) | lo; -- 2.0.0.526.g5318336 -- 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/