timekeeping: Introduce read_boot_clock

Add the new function read_boot_clock to get the exact time the system
has been started. For architectures without support for exact boot
time a new weak function is added that returns 0.  Use the exact boot
time to initialize wall_to_monotonic, or xtime if the read_boot_clock
returned 0.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Acked-by: John Stultz <johnstul@us.ibm.com>
Cc: Daniel Walker <dwalker@fifo99.com>
LKML-Reference: <20090814134811.296703241@de.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 22 insertions, 2 deletions

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index f1a21ce491e6..15e06defca55 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -501,6 +501,21 @@ void __attribute__((weak)) read_persistent_clock(struct timespec *ts)
         ts->tv_nsec = 0;
 }
 
+/**
+ * read_boot_clock -  Return time of the system start.
+ *
+ * Weak dummy function for arches that do not yet support it.
+ * Function to read the exact time the system has been started.
+ * Returns a timespec with tv_sec=0 and tv_nsec=0 if unsupported.
+ *
+ *  XXX - Do be sure to remove it once all arches implement it.
+ */
+void __attribute__((weak)) read_boot_clock(struct timespec *ts)
+{
+        ts->tv_sec = 0;
+        ts->tv_nsec = 0;
+}
+
 /*
  * timekeeping_init - Initializes the clocksource and common timekeeping values
  */
@@ -508,9 +523,10 @@ void __init timekeeping_init(void)
 {
         struct clocksource *clock;
         unsigned long flags;
-        struct timespec now;
+        struct timespec now, boot;
 
         read_persistent_clock(&now);
+        read_boot_clock(&boot);
 
         write_seqlock_irqsave(&xtime_lock, flags);
 
@@ -525,8 +541,12 @@ void __init timekeeping_init(void)
         xtime.tv_nsec = now.tv_nsec;
         raw_time.tv_sec = 0;
         raw_time.tv_nsec = 0;
+        if (boot.tv_sec == 0 && boot.tv_nsec == 0) {
+                boot.tv_sec = xtime.tv_sec;
+                boot.tv_nsec = xtime.tv_nsec;
+        }
         set_normalized_timespec(&wall_to_monotonic,
-                -xtime.tv_sec, -xtime.tv_nsec);
+                                -boot.tv_sec, -boot.tv_nsec);
         update_xtime_cache(0);
         total_sleep_time.tv_sec = 0;
         total_sleep_time.tv_nsec = 0;