[PATCH AUTOSEL 4.20 236/304] mlx5: update timecounter at least twice per counter overflow

Sasha Levin
2019-01-28T17:35:24Z

From: Miroslav Lichvar <mlichvar@redhat.com>

[ Upstream commit 5d8678365c90b9ce1fd2243ff5ea562609f6cec1 ]

The timecounter needs to be updated at least once in half of the
cyclecounter interval to prevent timecounter_cyc2time() interpreting a
new timestamp as an old value and causing a backward jump.

This would be an issue if the timecounter multiplier was so small that
the update interval would not be limited by the 64-bit overflow in
multiplication.

Shorten the calculated interval to make sure the timecounter is updated
in time even when the system clock is slowed down by up to 10%, the
multiplier is increased by up to 10%, and the scheduled overflow check
is late by 15%.

Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Ariel Levkovich <lariel@mellanox.com>
Cc: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/net/ethernet/mellanox/mlx5/core/lib/clock.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/lib/clock.c b/drivers/net/ethernet/mellanox/mlx5/core/lib/clock.c
index 0d90b1b4a3d3..2d6168ee99e8 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/lib/clock.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/lib/clock.c
@@ -511,14 +511,14 @@ void mlx5_init_clock(struct mlx5_core_dev *mdev)
ktime_to_ns(ktime_get_real()));

/* Calculate period in seconds to call the overflow watchdog - to make
- * sure counter is checked at least once every wrap around.
+ * sure counter is checked at least twice every wrap around.
* The period is calculated as the minimum between max HW cycles count
* (The clock source mask) and max amount of cycles that can be
* multiplied by clock multiplier where the result doesn't exceed
* 64bits.
*/
overflow_cycles = div64_u64(~0ULL >> 1, clock->cycles.mult);
- overflow_cycles = min(overflow_cycles, clock->cycles.mask >> 1);
+ overflow_cycles = min(overflow_cycles, div_u64(clock->cycles.mask, 3));

ns = cyclecounter_cyc2ns(&clock->cycles, overflow_cycles,
frac, &frac);
--
2.19.1