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>

1 files changed, 2 insertions, 2 deletions

diff --git a/drivers/net/ethernet/stmicro/stmmac/dwmac-intel.c b/drivers/net/ethernet/stmicro/stmmac/dwmac-intel.c
index 8e8778cfbbad..5943ff9f21c2 100644
--- a/drivers/net/ethernet/stmicro/stmmac/dwmac-intel.c
+++ b/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);
 	}