counter: rz-mtu3-cnt: prevent counter from being toggled multiple times

Runtime PM counter is incremented / decremented each time the sysfs
enable file is written to.

If user writes 0 to the sysfs enable file multiple times, runtime PM
usage count underflows, generating the following message.

rz-mtu3-counter rz-mtu3-counter.0: Runtime PM usage count underflow!

At the same time, hardware registers end up being accessed with clocks
off in rz_mtu3_terminate_counter() to disable an already disabled
channel.

If user writes 1 to the sysfs enable file multiple times, runtime PM
usage count will be incremented each time, requiring the same number of
0 writes to get it back to 0.

If user writes 0 to the sysfs enable file while PWM is in progress, PWM
is stopped without counter being the owner of the underlying MTU3
channel.

Check against the cached count_is_enabled value and exit if the user
is trying to set the same enable value.

Cc: stable@vger.kernel.org
Fixes: 0be8907 ("counter: Add Renesas RZ/G2L MTU3a counter driver")
Signed-off-by: Cosmin Tanislav <cosmin-gabriel.tanislav.xa@renesas.com>
Link: https://lore.kernel.org/r/20260130122353.2263273-5-cosmin-gabriel.tanislav.xa@renesas.com
Signed-off-by: William Breathitt Gray <wbg@kernel.org>

Author: Demon000

drivers/counter/rz-mtu3-cnt.c

@@ -499,21 +499,25 @@ static int rz_mtu3_count_enable_write(struct counter_device *counter,
 	struct rz_mtu3_cnt *const priv = counter_priv(counter);
 	int ret = 0;
 
+	mutex_lock(&priv->lock);
+
+	if (priv->count_is_enabled[count->id] == enable)
+		goto exit;
+
 	if (enable) {
-		mutex_lock(&priv->lock);
 		pm_runtime_get_sync(ch->dev);
 		ret = rz_mtu3_initialize_counter(counter, count->id);
 		if (ret == 0)
 			priv->count_is_enabled[count->id] = true;
-		mutex_unlock(&priv->lock);
 	} else {
-		mutex_lock(&priv->lock);
 		rz_mtu3_terminate_counter(counter, count->id);
 		priv->count_is_enabled[count->id] = false;
 		pm_runtime_put(ch->dev);
-		mutex_unlock(&priv->lock);
 	}
 
+exit:
+	mutex_unlock(&priv->lock);
+
 	return ret;
 }