net/mlx4_en: fix overflow in mlx4_en_init_timestamp()

The cited commit makes a great job of finding optimal shift/multiplier
values assuming a 10 seconds wrap around, but forgot to change the
overflow_period computation.

It overflows in cyclecounter_cyc2ns(), and the final result is 804 ms,
which is silly.

Lets simply use 5 seconds, no need to recompute this, given how it is
supposed to work.

Later, we will use a timer instead of a work queue, since the new RX
allocation schem will no longer need mlx4_en_recover_from_oom() and the
service_task firing every 250 ms.

Fixes: 31c128b66e5b ("net/mlx4_en: Choose time-stamping shift value according to HW frequency")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tariq Toukan <tariqt@mellanox.com>
Cc: Eugenia Emantayev <eugenia@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_clock.c

@@ -89,10 +89,17 @@ void mlx4_en_remove_timestamp(struct mlx4_en_dev *mdev)
 	}
 }
 
+#define MLX4_EN_WRAP_AROUND_SEC	10UL
+/* By scheduling the overflow check every 5 seconds, we have a reasonably
+ * good chance we wont miss a wrap around.
+ * TOTO: Use a timer instead of a work queue to increase the guarantee.
+ */
+#define MLX4_EN_OVERFLOW_PERIOD (MLX4_EN_WRAP_AROUND_SEC * HZ / 2)
+
 void mlx4_en_ptp_overflow_check(struct mlx4_en_dev *mdev)
 {
 	bool timeout = time_is_before_jiffies(mdev->last_overflow_check +
-					      mdev->overflow_period);
+					      MLX4_EN_OVERFLOW_PERIOD);
 	unsigned long flags;
 
 	if (timeout) {
@@ -237,7 +244,6 @@ static const struct ptp_clock_info mlx4_en_ptp_clock_info = {
 	.enable		= mlx4_en_phc_enable,
 };
 
-#define MLX4_EN_WRAP_AROUND_SEC	10ULL
 
 /* This function calculates the max shift that enables the user range
  * of MLX4_EN_WRAP_AROUND_SEC values in the cycles register.
@@ -258,7 +264,6 @@ void mlx4_en_init_timestamp(struct mlx4_en_dev *mdev)
 {
 	struct mlx4_dev *dev = mdev->dev;
 	unsigned long flags;
-	u64 ns, zero = 0;
 
 	/* mlx4_en_init_timestamp is called for each netdev.
 	 * mdev->ptp_clock is common for all ports, skip initialization if
@@ -282,13 +287,6 @@ void mlx4_en_init_timestamp(struct mlx4_en_dev *mdev)
 			 ktime_to_ns(ktime_get_real()));
 	write_sequnlock_irqrestore(&mdev->clock_lock, flags);
 
-	/* Calculate period in seconds to call the overflow watchdog - to make
-	 * sure counter is checked at least once every wrap around.
-	 */
-	ns = cyclecounter_cyc2ns(&mdev->cycles, mdev->cycles.mask, zero, &zero);
-	do_div(ns, NSEC_PER_SEC / 2 / HZ);
-	mdev->overflow_period = ns;
-
 	/* Configure the PHC */
 	mdev->ptp_clock_info = mlx4_en_ptp_clock_info;
 	snprintf(mdev->ptp_clock_info.name, 16, "mlx4 ptp");

drivers/net/ethernet/mellanox/mlx4/mlx4_en.h

@@ -430,7 +430,6 @@ struct mlx4_en_dev {
 	seqlock_t		clock_lock;
 	struct timecounter	clock;
 	unsigned long		last_overflow_check;
-	unsigned long		overflow_period;
 	struct ptp_clock	*ptp_clock;
 	struct ptp_clock_info	ptp_clock_info;
 	struct notifier_block	nb;