net: dsa: felix: fix race between reading PSFP stats and port stats

Both PSFP stats and the port stats read by ocelot_check_stats_work() are
indirectly read through the same mechanism - write to STAT_CFG:STAT_VIEW,
read from SYS:STAT:CNT[n].

It's just that for port stats, we write STAT_VIEW with the index of the
port, and for PSFP stats, we write STAT_VIEW with the filter index.

So if we allow them to run concurrently, ocelot_check_stats_work() may
change the view from vsc9959_psfp_counters_get(), and vice versa.

Fixes: 7d4b564 ("net: dsa: felix: support psfp filter on vsc9959")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20220629183007.3808130-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/ocelot/felix_vsc9959.c

@@ -1886,6 +1886,8 @@ static void vsc9959_psfp_sgi_table_del(struct ocelot *ocelot,
 static void vsc9959_psfp_counters_get(struct ocelot *ocelot, u32 index,
 				      struct felix_stream_filter_counters *counters)
 {
+	mutex_lock(&ocelot->stats_lock);
+
 	ocelot_rmw(ocelot, SYS_STAT_CFG_STAT_VIEW(index),
 		   SYS_STAT_CFG_STAT_VIEW_M,
 		   SYS_STAT_CFG);
@@ -1900,6 +1902,8 @@ static void vsc9959_psfp_counters_get(struct ocelot *ocelot, u32 index,
 		     SYS_STAT_CFG_STAT_VIEW(index) |
 		     SYS_STAT_CFG_STAT_CLEAR_SHOT(0x10),
 		     SYS_STAT_CFG);
+
+	mutex_unlock(&ocelot->stats_lock);
 }
 
 static int vsc9959_psfp_filter_add(struct ocelot *ocelot, int port,