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Date:   Thu, 19 Jul 2018 15:29:46 -0700
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To:     linux-tip-commits@vger.kernel.org
Subject: [tip:x86/timers] timekeeping: Default boot time offset to
 local_clock()
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Commit-ID:  4b1b7f8054896cee25669f6cea7cb6dd17f508f7
Gitweb:     https://git.kernel.org/tip/4b1b7f8054896cee25669f6cea7cb6dd17f508f7
Author:     Pavel Tatashin <pasha.tatashin@oracle.com>
AuthorDate: Thu, 19 Jul 2018 16:55:35 -0400
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Fri, 20 Jul 2018 00:02:41 +0200

timekeeping: Default boot time offset to local_clock()

read_persistent_wall_and_boot_offset() is called during boot to read
both the persistent clock and also return the offset between the boot time
and the value of persistent clock.

Change the default boot_offset from zero to local_clock() so architectures,
that do not have a dedicated boot_clock but have early sched_clock(), such
as SPARCv9, x86, and possibly more will benefit from this change by getting
a better and more consistent estimate of the boot time without need for an
arch specific implementation.

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: steven.sistare@oracle.com
Cc: daniel.m.jordan@oracle.com
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Cc: linux-s390@vger.kernel.org
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Link: https://lkml.kernel.org/r/20180719205545.16512-17-pasha.tatashin@oracle.com

---
 kernel/time/timekeeping.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index cb738f825c12..30d7f64ffc87 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1503,14 +1503,17 @@ void __weak read_persistent_clock64(struct timespec64 *ts64)
  * Weak dummy function for arches that do not yet support it.
  * wall_time	- current time as returned by persistent clock
  * boot_offset	- offset that is defined as wall_time - boot_time
- *		  default to 0.
+ * The default function calculates offset based on the current value of
+ * local_clock(). This way architectures that support sched_clock() but don't
+ * support dedicated boot time clock will provide the best estimate of the
+ * boot time.
  */
 void __weak __init
 read_persistent_wall_and_boot_offset(struct timespec64 *wall_time,
 				     struct timespec64 *boot_offset)
 {
 	read_persistent_clock64(wall_time);
-	*boot_offset = (struct timespec64){0};
+	*boot_offset = ns_to_timespec64(local_clock());
 }
 
 /* Flag for if timekeeping_resume() has injected sleeptime */