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>

3 files changed, 28 insertions, 14 deletions

diff --git a/arch/s390/kernel/time.c b/arch/s390/kernel/time.c
index a94ec48587b4..6bff1a1d9060 100644
--- a/arch/s390/kernel/time.c
+++ b/arch/s390/kernel/time.c
@@ -187,6 +187,11 @@ void read_persistent_clock(struct timespec *ts)
         tod_to_timeval(get_clock() - TOD_UNIX_EPOCH, ts);
 }
 
+void read_boot_clock(struct timespec *ts)
+{
+        tod_to_timeval(sched_clock_base_cc - TOD_UNIX_EPOCH, ts);
+}
+
 static cycle_t read_tod_clock(struct clocksource *cs)
 {
         return get_clock();
@@ -243,9 +248,6 @@ void update_vsyscall_tz(void)
  */
 void __init time_init(void)
 {
-        struct timespec ts;
-        unsigned long flags;
-
         /* Reset time synchronization interfaces. */
         etr_reset();
         stp_reset();
@@ -261,15 +263,6 @@ void __init time_init(void)
         if (clocksource_register(&clocksource_tod) != 0)
                 panic("Could not register TOD clock source");
 
-        /*
-         * Reset wall_to_monotonic to the initial timestamp created
-         * in head.S to get a precise value in /proc/uptime.
-         */
-        write_seqlock_irqsave(&xtime_lock, flags);
-        tod_to_timeval(sched_clock_base_cc - TOD_UNIX_EPOCH, &ts);
-        set_normalized_timespec(&wall_to_monotonic, -ts.tv_sec, -ts.tv_nsec);
-        write_sequnlock_irqrestore(&xtime_lock, flags);
-
         /* Enable TOD clock interrupts on the boot cpu. */
         init_cpu_timer();
 
diff --git a/include/linux/time.h b/include/linux/time.h
index 53a3216f0d1b..f505988398e6 100644
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -102,6 +102,7 @@ extern struct timespec wall_to_monotonic;
 extern seqlock_t xtime_lock;
 
 extern void read_persistent_clock(struct timespec *ts);
+extern void read_boot_clock(struct timespec *ts);
 extern int update_persistent_clock(struct timespec now);
 extern int no_sync_cmos_clock __read_mostly;
 void timekeeping_init(void);
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;