memory: tegra20: Protect debug code with a lock

Simultaneous accesses to MC_STAT h/w shouldn't be allowed since one
collection process stomps on another. There is no good reason for
polling stats in parallel in practice, nevertheless let's add a
protection lock, just for consistency.
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20210323210446.24867-2-digetx@gmail.comSigned-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>

drivers/memory/tegra/tegra20.c

@@ -5,6 +5,7 @@
 
 #include <linux/bitfield.h>
 #include <linux/delay.h>
+#include <linux/mutex.h>
 #include <linux/of_device.h>
 #include <linux/slab.h>
 #include <linux/string.h>
@@ -55,6 +56,8 @@
 /* we store collected statistics as a fixed point values */
 #define MC_FX_FRAC_SCALE			100
 
+static DEFINE_MUTEX(tegra20_mc_stat_lock);
+
 struct tegra20_mc_stat_gather {
 	unsigned int pri_filter;
 	unsigned int pri_event;
@@ -615,8 +618,12 @@ static int tegra20_mc_stats_show(struct seq_file *s, void *unused)
 	if (!stats)
 		return -ENOMEM;
 
+	mutex_lock(&tegra20_mc_stat_lock);
+
 	tegra20_mc_collect_stats(mc, stats);
 
+	mutex_unlock(&tegra20_mc_stat_lock);
+
 	seq_puts(s, "Memory client   Events   Timeout   High priority   Bandwidth ARB   RW change   Successive   Page miss\n");
 	seq_puts(s, "-----------------------------------------------------------------------------------------------------\n");