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From:   Tao Ren <taoren@fb.com>
Smtp-Origin-Hostname: devvm24792.prn1.facebook.com
To:     Daniel Lezcano <daniel.lezcano@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        <linux-kernel@vger.kernel.org>
CC:     Tao Ren <taoren@fb.com>, <openbmc@lists.ozlabs.org>, <cov@fb.com>,
        <tfang@fb.com>
Smtp-Origin-Cluster: prn1c35
Subject: [PATCH] clocksource/drivers/fttmr010: fix set_next_event handler
Date:   Wed, 19 Sep 2018 15:13:31 -0700
Message-ID: <20180919221331.2511558-1-taoren@fb.com>
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

Currently, the aspeed MATCH1 register is updated to <current_count -
cycles> in set_next_event handler, with the assumption that COUNT
register value is preserved when the timer is disabled and it continues
decrementing after the timer is enabled. But the assumption is wrong:
RELOAD register is loaded into COUNT register when the aspeed timer is
enabled, which means the next event may be delayed because timer
interrupt won't be generated until <0xFFFFFFFF - current_count +
cycles>.

The problem can be fixed by updating RELOAD register to <cycles>, and
COUNT register will be re-loaded when the timer is enabled and interrupt
is generated when COUNT register overflows.

The test result on Facebook Backpack-CMM BMC hardware (AST2500) shows
the issue is fixed: without the patch, usleep(100) suspends the process
for several milliseconds (and sometimes even over 40 milliseconds);
after applying the fix, usleep(100) takes averagely 240 microseconds to
return under the same workload level.

Signed-off-by: Tao Ren <taoren@fb.com>
---
 drivers/clocksource/timer-fttmr010.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/drivers/clocksource/timer-fttmr010.c b/drivers/clocksource/timer-fttmr010.c
index c020038ebfab..cf93f6419b51 100644
--- a/drivers/clocksource/timer-fttmr010.c
+++ b/drivers/clocksource/timer-fttmr010.c
@@ -130,13 +130,17 @@ static int fttmr010_timer_set_next_event(unsigned long cycles,
 	cr &= ~fttmr010->t1_enable_val;
 	writel(cr, fttmr010->base + TIMER_CR);
 
-	/* Setup the match register forward/backward in time */
-	cr = readl(fttmr010->base + TIMER1_COUNT);
-	if (fttmr010->count_down)
-		cr -= cycles;
-	else
-		cr += cycles;
-	writel(cr, fttmr010->base + TIMER1_MATCH1);
+	if (fttmr010->count_down) {
+		/*
+		 * ASPEED Timer Controller will load TIMER1_LOAD register
+		 * into TIMER1_COUNT register when the timer is re-enabled.
+		 */
+		writel(cycles, fttmr010->base + TIMER1_LOAD);
+	} else {
+		/* Setup the match register forward in time */
+		cr = readl(fttmr010->base + TIMER1_COUNT);
+		writel(cr + cycles, fttmr010->base + TIMER1_MATCH1);
+	}
 
 	/* Start */
 	cr = readl(fttmr010->base + TIMER_CR);
-- 
2.17.1