netdev - [RFC net-next 4/4] net: stmmac: remove unnecessary locking around PTP clock reads

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Message-Id: <20211210193556.1349090-5-yannick.vignon@oss.nxp.com>
Date:   Fri, 10 Dec 2021 20:35:56 +0100
From:   Yannick Vignon <yannick.vignon@....nxp.com>
To:     Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        netdev@...r.kernel.org, Ong Boon Leong <boon.leong.ong@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jose Abreu <joabreu@...opsys.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Wei Wang <weiwan@...gle.com>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Vladimir Oltean <olteanv@...il.com>,
        Xiaoliang Yang <xiaoliang.yang_1@....com>, mingkai.hu@....com,
        Joakim Zhang <qiangqing.zhang@....com>,
        sebastien.laveze@....com
Cc:     Yannick Vignon <yannick.vignon@....com>
Subject: [RFC net-next 4/4] net: stmmac: remove unnecessary locking around PTP clock reads

From: Yannick Vignon <yannick.vignon@....com>

Reading the PTP clock is a simple operation requiring only 2 register reads,
while the time can be set atomically through another set of registers. Under
a PREEMPT_RT kernel, protecting the reads by a spin_lock is
counter-productive:
 * if the task is preempted in-between the 2 reads, the return time value
could become inconsistent,
 * if the 2nd task preempting the 1st has a higher prio but needs to
read time as well, it will require 2 costly context switches, which
will pretty much always be more costly than disabling preemption with a real
spin_lock.

Remove the unneeded locking around the gettime call.

Signed-off-by: Yannick Vignon <yannick.vignon@....com>
---
 drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c
index 580cc035536b..8f0dcac23b1e 100644
--- a/drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c
+++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_ptp.c
@@ -137,9 +137,7 @@ 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);
 	stmmac_get_systime(priv, priv->ptpaddr, &ns);
-	spin_unlock_irqrestore(&priv->ptp_lock, flags);
 
 	*ts = ns_to_timespec64(ns);
 
-- 
2.25.1