net: stmmac: optimize locking around PTP clock reads

Reading the PTP clock is a simple operation requiring only 3 register
reads. Under a PREEMPT_RT kernel, protecting those reads by a spin_lock is
counter-productive: if the 2nd task preempting the 1st has a higher prio
but needs to read time as well, it will require 2 context switches, which
will pretty much always be more costly than just disabling preemption for
the duration of the reads. Moreover, with the code logic recently added
to get_systime(), disabling preemption is not even required anymore:
reads and writes just need to be protected from each other, to prevent a
clock read while the clock is being updated.

Improve the above situation by replacing the PTP spinlock by a rwlock, and
using read_lock for PTP clock reads so simultaneous reads do not block
each other.

Signed-off-by: Yannick Vignon <yannick.vignon@nxp.com>
Link: https://lore.kernel.org/r/20220204135545.2770625-1-yannick.vignon@oss.nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/stmicro/stmmac/dwmac-intel.c

@@ -383,10 +383,10 @@ static int intel_crosststamp(ktime_t *device,
 
 	/* Repeat until the timestamps are from the FIFO last segment */
 	for (i = 0; i < num_snapshot; i++) {
-		spin_lock_irqsave(&priv->ptp_lock, flags);
+		read_lock_irqsave(&priv->ptp_lock, flags);
 		stmmac_get_ptptime(priv, ptpaddr, &ptp_time);
 		*device = ns_to_ktime(ptp_time);
-		spin_unlock_irqrestore(&priv->ptp_lock, flags);
+		read_unlock_irqrestore(&priv->ptp_lock, flags);
 		get_arttime(priv->mii, intel_priv->mdio_adhoc_addr, &art_time);
 		*system = convert_art_to_tsc(art_time);
 	}

drivers/net/ethernet/stmicro/stmmac/stmmac.h

@@ -263,7 +263,7 @@ struct stmmac_priv {
 	u32 adv_ts;
 	int use_riwt;
 	int irq_wake;
-	spinlock_t ptp_lock;
+	rwlock_t ptp_lock;
 	/* Protects auxiliary snapshot registers from concurrent access. */
 	struct mutex aux_ts_lock;
 

drivers/net/ethernet/stmicro/stmmac/stmmac_hwtstamp.c

@@ -196,9 +196,9 @@ static void timestamp_interrupt(struct stmmac_priv *priv)
 		       GMAC_TIMESTAMP_ATSNS_SHIFT;
 
 	for (i = 0; i < num_snapshot; i++) {
-		spin_lock_irqsave(&priv->ptp_lock, flags);
+		read_lock_irqsave(&priv->ptp_lock, flags);
 		get_ptptime(priv->ptpaddr, &ptp_time);
-		spin_unlock_irqrestore(&priv->ptp_lock, flags);
+		read_unlock_irqrestore(&priv->ptp_lock, flags);
 		event.type = PTP_CLOCK_EXTTS;
 		event.index = 0;
 		event.timestamp = ptp_time;

drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c

@@ -39,9 +39,9 @@ static int stmmac_adjust_freq(struct ptp_clock_info *ptp, s32 ppb)
 	diff = div_u64(adj, 1000000000ULL);
 	addend = neg_adj ? (addend - diff) : (addend + diff);
 
-	spin_lock_irqsave(&priv->ptp_lock, flags);
+	write_lock_irqsave(&priv->ptp_lock, flags);
 	stmmac_config_addend(priv, priv->ptpaddr, addend);
-	spin_unlock_irqrestore(&priv->ptp_lock, flags);
+	write_unlock_irqrestore(&priv->ptp_lock, flags);
 
 	return 0;
 }
@@ -86,9 +86,9 @@ static int stmmac_adjust_time(struct ptp_clock_info *ptp, s64 delta)
 		mutex_unlock(&priv->plat->est->lock);
 	}
 
-	spin_lock_irqsave(&priv->ptp_lock, flags);
+	write_lock_irqsave(&priv->ptp_lock, flags);
 	stmmac_adjust_systime(priv, priv->ptpaddr, sec, nsec, neg_adj, xmac);
-	spin_unlock_irqrestore(&priv->ptp_lock, flags);
+	write_unlock_irqrestore(&priv->ptp_lock, flags);
 
 	/* Caculate new basetime and re-configured EST after PTP time adjust. */
 	if (est_rst) {
@@ -137,9 +137,9 @@ static int stmmac_get_time(struct ptp_clock_info *ptp, struct timespec64 *ts)
 	unsigned long flags;
 	u64 ns = 0;
 
-	spin_lock_irqsave(&priv->ptp_lock, flags);
+	read_lock_irqsave(&priv->ptp_lock, flags);
 	stmmac_get_systime(priv, priv->ptpaddr, &ns);
-	spin_unlock_irqrestore(&priv->ptp_lock, flags);
+	read_unlock_irqrestore(&priv->ptp_lock, flags);
 
 	*ts = ns_to_timespec64(ns);
 
@@ -162,9 +162,9 @@ static int stmmac_set_time(struct ptp_clock_info *ptp,
 	    container_of(ptp, struct stmmac_priv, ptp_clock_ops);
 	unsigned long flags;
 
-	spin_lock_irqsave(&priv->ptp_lock, flags);
+	write_lock_irqsave(&priv->ptp_lock, flags);
 	stmmac_init_systime(priv, priv->ptpaddr, ts->tv_sec, ts->tv_nsec);
-	spin_unlock_irqrestore(&priv->ptp_lock, flags);
+	write_unlock_irqrestore(&priv->ptp_lock, flags);
 
 	return 0;
 }
@@ -194,12 +194,12 @@ static int stmmac_enable(struct ptp_clock_info *ptp,
 		cfg->period.tv_sec = rq->perout.period.sec;
 		cfg->period.tv_nsec = rq->perout.period.nsec;
 
-		spin_lock_irqsave(&priv->ptp_lock, flags);
+		write_lock_irqsave(&priv->ptp_lock, flags);
 		ret = stmmac_flex_pps_config(priv, priv->ioaddr,
 					     rq->perout.index, cfg, on,
 					     priv->sub_second_inc,
 					     priv->systime_flags);
-		spin_unlock_irqrestore(&priv->ptp_lock, flags);
+		write_unlock_irqrestore(&priv->ptp_lock, flags);
 		break;
 	case PTP_CLK_REQ_EXTTS:
 		priv->plat->ext_snapshot_en = on;
@@ -314,7 +314,7 @@ void stmmac_ptp_register(struct stmmac_priv *priv)
 	stmmac_ptp_clock_ops.n_per_out = priv->dma_cap.pps_out_num;
 	stmmac_ptp_clock_ops.n_ext_ts = priv->dma_cap.aux_snapshot_n;
 
-	spin_lock_init(&priv->ptp_lock);
+	rwlock_init(&priv->ptp_lock);
 	mutex_init(&priv->aux_ts_lock);
 	priv->ptp_clock_ops = stmmac_ptp_clock_ops;
 

drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.c

@@ -1777,9 +1777,9 @@ static int stmmac_test_tbs(struct stmmac_priv *priv)
 	if (ret)
 		return ret;
 
-	spin_lock_irqsave(&priv->ptp_lock, flags);
+	read_lock_irqsave(&priv->ptp_lock, flags);
 	stmmac_get_systime(priv, priv->ptpaddr, &curr_time);
-	spin_unlock_irqrestore(&priv->ptp_lock, flags);
+	read_unlock_irqrestore(&priv->ptp_lock, flags);
 
 	if (!curr_time) {
 		ret = -EOPNOTSUPP;
@@ -1799,9 +1799,9 @@ static int stmmac_test_tbs(struct stmmac_priv *priv)
 		goto fail_disable;
 
 	/* Check if expected time has elapsed */
-	spin_lock_irqsave(&priv->ptp_lock, flags);
+	read_lock_irqsave(&priv->ptp_lock, flags);
 	stmmac_get_systime(priv, priv->ptpaddr, &curr_time);
-	spin_unlock_irqrestore(&priv->ptp_lock, flags);
+	read_unlock_irqrestore(&priv->ptp_lock, flags);
 
 	if ((curr_time - start_time) < STMMAC_TBS_LT_OFFSET)
 		ret = -EINVAL;