[RFC PATCH 3/7] timekeeping: Fix cross-timestamp interpolation for non-x86

[Peter Hilber]

So far, get_device_system_crosststamp() unconditionally passes
system_counterval.cycles to timekeeping_cycles_to_ns(). But when
interpolating system time (do_interp == true), system_counterval.cycles is
before tkr_mono.cycle_last, contrary to the timekeeping_cycles_to_ns()
expectations.

On x86, CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE will mitigate on
interpolating, setting delta to 0. With delta == 0, xtstamp->sys_monoraw
and xtstamp->sys_realtime are then set to the last update time, as
implicitly expected by adjust_historical_crosststamp(). On other
architectures, the resulting nonsense xtstamp->sys_monoraw and
xtstamp->sys_realtime corrupt the xtstamp (ts) adjustment in
adjust_historical_crosststamp().

Fix this by always setting the delta to 0 when interpolating.

Fixes: 2c756feb18d9 ("time: Add history to cross timestamp interface supporting slower devices")
Signed-off-by: Peter Hilber <peter.hilber@xxxxxxxxxxxxxxx>
---
 kernel/time/timekeeping.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 7e86d5cd784d..7ccc2377c319 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -1259,10 +1259,15 @@ int get_device_system_crosststamp(int (*get_time_fn)
 				      tk_core.timekeeper.offs_real);
 		base_raw = tk->tkr_raw.base;
 
-		nsec_real = timekeeping_cycles_to_ns(&tk->tkr_mono,
-						     system_counterval.cycles);
-		nsec_raw = timekeeping_cycles_to_ns(&tk->tkr_raw,
-						    system_counterval.cycles);
+		if (do_interp) {
+			nsec_real = timekeeping_delta_to_ns(&tk->tkr_mono, 0);
+			nsec_raw = timekeeping_delta_to_ns(&tk->tkr_raw, 0);
+		} else {
+			nsec_real = timekeeping_cycles_to_ns(
+				&tk->tkr_mono, system_counterval.cycles);
+			nsec_raw = timekeeping_cycles_to_ns(
+				&tk->tkr_raw, system_counterval.cycles);
+		}
 	} while (read_seqcount_retry(&tk_core.seq, seq));
 
 	xtstamp->sys_realtime = ktime_add_ns(base_real, nsec_real);
-- 
2.39.2