Revert "mlx5e: Ensure rx timestamps are monotonically increasing"

This reverts commit ce33f96fcf.

It turns out that doing it this way did indeed prevent backwards
movement of timestamps, however it also lead to an ever increasing
error, eventually yielding timestamps hundreds or thousands of
seconds in the future.

Back this out until we can come up with a solution that prevents
backards timestamps and also avoids accumulating error.

Sponsored by: Netflix

sys/dev/mlx5/mlx5_en/mlx5_en_main.c

@@ -1134,25 +1134,6 @@ mlx5e_hw_clock(struct mlx5e_priv *priv)
 	return (((uint64_t)hw_h << 32) | hw_l);
 }
 
-/*
- * Seed the first calibration point so that base_prev and clbr_hw_prev
- * are always valid.  Called once during attach before the first
- * calibration callout fires.
- */
-static void
-mlx5e_seed_calibration(struct mlx5e_priv *priv)
-{
-	struct mlx5e_clbr_point *cp;
-	struct timespec ts;
-
-	cp = &priv->clbr_points[0];
-	cp->clbr_hw_curr = mlx5e_hw_clock(priv);
-	nanouptime(&ts);
-	cp->base_curr = mlx5e_timespec2usec(&ts);
-	cp->clbr_hw_prev = cp->clbr_hw_curr - 1;
-	cp->base_prev = cp->base_curr - 1;
-}
-
 /*
  * The calibration callout, it runs either in the context of the
  * thread which enables calibration, or in callout.  It takes the
@@ -1166,9 +1147,6 @@ mlx5e_calibration_callout(void *arg)
 	struct mlx5e_priv *priv;
 	struct mlx5e_clbr_point *next, *curr;
 	struct timespec ts;
-	uint64_t hw_delta_new, hw_delta_old;
-	uint64_t old_nsec, old_projected, old_sec;
-	uint64_t res_n, res_s, res_s_mod, rt_delta_old;
 	int clbr_curr_next;
 
 	priv = arg;
@@ -1197,33 +1175,6 @@ mlx5e_calibration_callout(void *arg)
 	nanouptime(&ts);
 	next->base_curr = mlx5e_timespec2usec(&ts);
 
-	/*
-	 * Ensure monotonicity across calibration transitions.  Compute
-	 * what the old calibration would extrapolate to at the new
-	 * hw_curr.  If the new base_curr is less, clamp it so the new
-	 * slope is at least as steep as the old one.  This prevents
-	 * packets from seeing time go backwards when the slope drops.
-	 *
-	 * Use the same split-seconds technique as mlx5e_mbuf_tstmp()
-	 * to avoid overflowing uint64_t in the multiplication.
-	 */
-	hw_delta_new = next->clbr_hw_curr - curr->clbr_hw_curr;
-	rt_delta_old = curr->base_curr - curr->base_prev;
-	hw_delta_old = curr->clbr_hw_curr - curr->clbr_hw_prev;
-	old_sec = hw_delta_new / priv->cclk;
-	old_nsec = hw_delta_new % priv->cclk;
-	res_s = old_sec * rt_delta_old;
-	res_n = old_nsec * rt_delta_old;
-	res_s_mod = res_s % hw_delta_old;
-	res_s /= hw_delta_old;
-	res_s_mod *= priv->cclk;
-	res_n += res_s_mod;
-	res_n /= hw_delta_old;
-	res_s *= priv->cclk;
-	old_projected = curr->base_curr + res_s + res_n;
-	if (next->base_curr < old_projected)
-		next->base_curr = old_projected;
-
 	curr->clbr_gen = 0;
 	atomic_thread_fence_rel();
 	priv->clbr_curr = clbr_curr_next;
@@ -4936,7 +4887,6 @@ mlx5e_create_ifp(struct mlx5_core_dev *mdev)
 	callout_init(&priv->tstmp_clbr, 1);
 	/* Pull out the frequency of the clock in hz */
 	priv->cclk = (uint64_t)MLX5_CAP_GEN(mdev, device_frequency_khz) * 1000ULL;
-	mlx5e_seed_calibration(priv);
 	mlx5e_reset_calibration_callout(priv);
 
 	pa.pa_version = PFIL_VERSION;