ia64: Update fsyscall gettime to use modern vsyscall_update

John Stultz provided the outline for this patch back in May 2014 here:

	http://patches.linaro.org/patch/30501/

but I let this sit on the shelf for too long and in the intervening
years almost every field in "struct timekeeper" was changed. So this
is almost completely different from his original. Though the key change
in arch/ia64/kernel/fsys.S remains the same.

The core logic change with the updated vsyscall method is that we
preserve the base nanosecond value in shifted nanoseconds, which
allows us to avoid truncating and rounding up to the next nanosecond
every tick to avoid inconsistencies.

Thus the logic moved from
nsec = ((cycle_delta * mult)>>shift) + base_nsec;
to
nsec = ((cycle_delta * mult) + base_snsec) >> shift;

Cc: John Stultz <john.stultz@linaro.org>
Cc: linux-ia64@vger.kernel.org
Signed-off-by: Tony Luck <tony.luck@intel.com>

1 files changed, 8 insertions, 2 deletions

diff --git a/arch/ia64/kernel/fsyscall_gtod_data.h b/arch/ia64/kernel/fsyscall_gtod_data.h
index dcc514917731..28363bfc9f57 100644
--- a/arch/ia64/kernel/fsyscall_gtod_data.h
+++ b/arch/ia64/kernel/fsyscall_gtod_data.h
@@ -5,10 +5,16 @@
  * fsyscall gettimeofday data
  */
 
+/* like timespec, but includes "shifted nanoseconds" */
+struct time_sn_spec {
+	u64	sec;
+	u64	snsec;
+};
+
 struct fsyscall_gtod_data_t {
 	seqcount_t	seq;
-	struct timespec	wall_time;
-	struct timespec monotonic_time;
+	struct time_sn_spec wall_time;
+	struct time_sn_spec monotonic_time;
 	u64		clk_mask;
 	u32		clk_mult;
 	u32		clk_shift;