[PATCH] arm64: perf_event: Fix time_offset for arch timer

[Leo Yan]

Between the system powering on and kernel's sched clock registration,
the arch timer usually has been enabled at the early time and its
counter is incremented during the period of the booting up.  Thus the
arch timer's counter is not completely accounted into the sched clock,
and has a delta between the arch timer's counter and sched clock.  This
delta value should be stored into userpg->time_offset, which later can
be retrieved by Perf tool in the user space for sample timestamp
calculation.

Now userpg->time_offset is assigned to the negative sched clock with
'-now', this value cannot reflect the delta between arch timer's counter
and sched clock, so Perf cannot use it to calculate the sample time.

To fix this issue, this patch calculate the delta between the arch
timer's and sched clock and assign the delta to userpg->time_offset.
The detailed steps are firstly to convert counter to nanoseconds 'ns',
then the offset is calculated as 'now' minus 'ns'.

        |<------------------- 'ns' ---------------------->|
                                |<-------- 'now' -------->|
        |<---- time_offset ---->|
        |-----------------------|-------------------------|
        ^                       ^                         ^
  Power on system     sched clock registration      Perf starts

Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
---
 arch/arm64/kernel/perf_event.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
index e40b65645c86..226d25d77072 100644
--- a/arch/arm64/kernel/perf_event.c
+++ b/arch/arm64/kernel/perf_event.c
@@ -1143,6 +1143,7 @@ void arch_perf_update_userpage(struct perf_event *event,
 {
 	u32 freq;
 	u32 shift;
+	u64 count, ns, quot, rem;
 
 	/*
 	 * Internal timekeeping for enabled/running/stopped times
@@ -1164,5 +1165,21 @@ void arch_perf_update_userpage(struct perf_event *event,
 		userpg->time_mult >>= 1;
 	}
 	userpg->time_shift = (u16)shift;
-	userpg->time_offset = -now;
+
+	/*
+	 * Since arch timer is enabled ealier than sched clock registration,
+	 * compuate the delta (in nanosecond unit) between the arch timer
+	 * counter and sched clock, assign the delta to time_offset and
+	 * perf tool can use it for timestamp calculation.
+	 *
+	 * The formula for conversion arch timer cycle to ns is:
+	 *   quot = (cyc >> time_shift);
+	 *   rem  = cyc & ((1 << time_shift) - 1);
+	 *   ns   = quot * time_mult + ((rem * time_mult) >> time_shift);
+	 */
+	count = arch_timer_read_counter();
+	quot = count >> shift;
+	rem = count & ((1 << shift) - 1);
+	ns = quot * userpg->time_mult + ((rem * userpg->time_mult) >> shift);
+	userpg->time_offset = now - ns;
 }
-- 
2.17.1