[PATCH v6 3/4] x86/time: read_boot_clock64() implementation

Pavel Tatashin
2017-08-30T18:05:39Z

read_boot_clock64() returns time of when system started. Now, that
sched_clock_early() is available on systems with unstable clocks it is
possible to implement x86 specific version of read_boot_clock64() that
takes advantage of this new interface.

Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
---
arch/x86/kernel/time.c | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)

diff --git a/arch/x86/kernel/time.c b/arch/x86/kernel/time.c
index e0754cdbad37..fbad8bf2fa24 100644
--- a/arch/x86/kernel/time.c
+++ b/arch/x86/kernel/time.c
@@ -14,6 +14,7 @@
#include <linux/i8253.h>
#include <linux/time.h>
#include <linux/export.h>
+#include <linux/sched/clock.h>

#include <asm/vsyscall.h>
#include <asm/x86_init.h>
@@ -95,3 +96,32 @@ void __init time_init(void)
{
late_time_init = x86_late_time_init;
}
+
+/*
+ * Called once during to boot to initialize boot time.
+ * This function returns timestamp in timespec format which is sec/nsec from
+ * epoch of when boot started.
+ * We use sched_clock_early() that gives us nanoseconds from when this clock has
+ * been started and it happens quiet early during boot process. To calculate
+ * offset from epoch we use information provided in 'now' by the caller
+ *
+ * If sched_clock_early() is not available or if there is any kind of error
+ * i.e. time from epoch is smaller than boot time, we must return zeros in ts,
+ * and the caller will take care of the error: by assuming that the time when
+ * this function was called is the beginning of boot time.
+ */
+void __init read_boot_clock64(struct timespec64 *now, struct timespec64 *ts)
+{
+ u64 ns_boot = sched_clock_early();
+ bool valid_clock;
+ u64 ns_now;
+
+ ns_now = timespec64_to_ns(now);
+ valid_clock = ns_boot && timespec64_valid_strict(now) &&
+ (ns_now > ns_boot);
+
+ if (!valid_clock)
+ *ts = (struct timespec64){0, 0};
+ else
+ *ts = ns_to_timespec64(ns_now - ns_boot);
+}
--
2.14.1