linux-kernel - [PATCH v2] clockevents/drivers/arm_global_timer: Use writel_relaxed in gt_compare

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Date:	Thu, 26 Nov 2015 20:20:38 +0800
From:	Jisheng Zhang <jszhang@...vell.com>
To:	<srinivas.kandagatla@...il.com>, <maxime.coquelin@...com>,
	<patrice.chotard@...com>, <daniel.lezcano@...aro.org>,
	<tglx@...utronix.de>, <arnd@...db.de>
CC:	<linux-arm-kernel@...ts.infradead.org>, <kernel@...inux.com>,
	<linux-kernel@...r.kernel.org>, Jisheng Zhang <jszhang@...vell.com>
Subject: [PATCH v2] clockevents/drivers/arm_global_timer: Use writel_relaxed in gt_compare_set

Use the relaxed version to improve performance. we measured time of
4096 rounds of gt_compare_set() spent on Marvell BG2Q:

before the patch: 3690648ns on average
after the patch: 1083023ns on average

improved by 70%!

Signed-off-by: Jisheng Zhang <jszhang@...vell.com>
---
 since v1:
 - only use relaxed io in gt_compare_set(), this function is the
   remaining hot code path.
 - Add performance numbers in commit msg.

 drivers/clocksource/arm_global_timer.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/clocksource/arm_global_timer.c b/drivers/clocksource/arm_global_timer.c
index a2cb6fa..f99be6b 100644
--- a/drivers/clocksource/arm_global_timer.c
+++ b/drivers/clocksource/arm_global_timer.c
@@ -99,17 +99,17 @@ static void gt_compare_set(unsigned long delta, int periodic)
 
 	counter += delta;
 	ctrl = GT_CONTROL_TIMER_ENABLE;
-	writel(ctrl, gt_base + GT_CONTROL);
-	writel(lower_32_bits(counter), gt_base + GT_COMP0);
-	writel(upper_32_bits(counter), gt_base + GT_COMP1);
+	writel_relaxed(ctrl, gt_base + GT_CONTROL);
+	writel_relaxed(lower_32_bits(counter), gt_base + GT_COMP0);
+	writel_relaxed(upper_32_bits(counter), gt_base + GT_COMP1);
 
 	if (periodic) {
-		writel(delta, gt_base + GT_AUTO_INC);
+		writel_relaxed(delta, gt_base + GT_AUTO_INC);
 		ctrl |= GT_CONTROL_AUTO_INC;
 	}
 
 	ctrl |= GT_CONTROL_COMP_ENABLE | GT_CONTROL_IRQ_ENABLE;
-	writel(ctrl, gt_base + GT_CONTROL);
+	writel_relaxed(ctrl, gt_base + GT_CONTROL);
 }
 
 static int gt_clockevent_shutdown(struct clock_event_device *evt)
-- 
2.6.2

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