linux-kernel - [PATCH] kernel: time: Modify test_udelay to allow for 1% tolerance.

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Date:   Mon, 20 Feb 2017 15:59:08 -0800
From:   David Riley <davidriley@...omium.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <john.stultz@...aro.org>
Cc:     Russell King <rmk+kernel@...linux.org.uk>,
        LKML <linux-kernel@...r.kernel.org>,
        David Riley <davidriley@...omium.org>
Subject: [PATCH] kernel: time: Modify test_udelay to allow for 1% tolerance.

test_udelay had a tolerance of udelay() being up to 0.5% fast but
that tolerance is insufficient for some platforms.  For ARM, the error
is around 0.7% so increase the test to allow for up to 1% which was
previously described as being acceptable for udelay().

See the following thread for more details:
http://lists.openwall.net/linux-kernel/2011/01/12/372

Cc: John Stultz <john.stultz@...aro.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Russell King <rmk+kernel@...linux.org.uk>
Signed-off-by: David Riley <davidriley@...omium.org>
---
 kernel/time/test_udelay.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/time/test_udelay.c b/kernel/time/test_udelay.c
index b0928ab..5212925 100644
--- a/kernel/time/test_udelay.c
+++ b/kernel/time/test_udelay.c
@@ -39,8 +39,8 @@ static int udelay_test_single(struct seq_file *s, int usecs, uint32_t iters)
 	uint64_t sum = 0;
 	uint64_t avg;
 	int i;
-	/* Allow udelay to be up to 0.5% fast */
-	int allowed_error_ns = usecs * 5;
+	/* Allow udelay to be up to 1% fast */
+	int allowed_error_ns = usecs * 10;
 
 	for (i = 0; i < iters; ++i) {
 		s64 kt1, kt2;
-- 
2.2.1