leds: ledtrig-tty: Replace mutex with completion

With this commit, the mutex handling is replaced by the completion
handling. When handling mutex, it must always be ensured that the held
mutex is also released again. This is more error-prone should the number
of code paths increase.

This is a preparatory commit to make the trigger more configurable via
additional sysfs parameters. With this change, the worker always runs and
is no longer stopped if no ttyname is set.
Signed-off-by: Florian Eckert <fe@dev.tdt.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20231127110311.3583957-3-fe@dev.tdt.deSigned-off-by: Lee Jones <lee@kernel.org>

drivers/leds/trigger/ledtrig-tty.c

 // SPDX-License-Identifier: GPL-2.0
 
+#include <linux/completion.h>
 #include <linux/delay.h>
 #include <linux/leds.h>
 #include <linux/module.h>
@@ -12,15 +13,23 @@
 struct ledtrig_tty_data {
 	struct led_classdev *led_cdev;
 	struct delayed_work dwork;
-	struct mutex mutex;
+	struct completion sysfs;
 	const char *ttyname;
 	struct tty_struct *tty;
 	int rx, tx;
 };
 
-static void ledtrig_tty_restart(struct ledtrig_tty_data *trigger_data)
+static int ledtrig_tty_wait_for_completion(struct device *dev)
 {
-	schedule_delayed_work(&trigger_data->dwork, 0);
+	struct ledtrig_tty_data *trigger_data = led_trigger_get_drvdata(dev);
+	int ret;
+
+	ret = wait_for_completion_timeout(&trigger_data->sysfs,
+					  msecs_to_jiffies(LEDTRIG_TTY_INTERVAL * 20));
+	if (ret == 0)
+		return -ETIMEDOUT;
+
+	return ret;
 }
 
 static ssize_t ttyname_show(struct device *dev,
@@ -28,14 +37,16 @@ static ssize_t ttyname_show(struct device *dev,
 {
 	struct ledtrig_tty_data *trigger_data = led_trigger_get_drvdata(dev);
 	ssize_t len = 0;
+	int completion;
 
-	mutex_lock(&trigger_data->mutex);
+	reinit_completion(&trigger_data->sysfs);
+	completion = ledtrig_tty_wait_for_completion(dev);
+	if (completion < 0)
+		return completion;
 
 	if (trigger_data->ttyname)
 		len = sprintf(buf, "%s\n", trigger_data->ttyname);
 
-	mutex_unlock(&trigger_data->mutex);
-
 	return len;
 }
 
@@ -46,7 +57,7 @@ static ssize_t ttyname_store(struct device *dev,
 	struct ledtrig_tty_data *trigger_data = led_trigger_get_drvdata(dev);
 	char *ttyname;
 	ssize_t ret = size;
-	bool running;
+	int completion;
 
 	if (size > 0 && buf[size - 1] == '\n')
 		size -= 1;
@@ -59,9 +70,10 @@ static ssize_t ttyname_store(struct device *dev,
 		ttyname = NULL;
 	}
 
-	mutex_lock(&trigger_data->mutex);
-
-	running = trigger_data->ttyname != NULL;
+	reinit_completion(&trigger_data->sysfs);
+	completion = ledtrig_tty_wait_for_completion(dev);
+	if (completion < 0)
+		return completion;
 
 	kfree(trigger_data->ttyname);
 	tty_kref_put(trigger_data->tty);
@@ -69,11 +81,6 @@ static ssize_t ttyname_store(struct device *dev,
 
 	trigger_data->ttyname = ttyname;
 
-	mutex_unlock(&trigger_data->mutex);
-
-	if (ttyname && !running)
-		ledtrig_tty_restart(trigger_data);
-
 	return ret;
 }
 static DEVICE_ATTR_RW(ttyname);
@@ -85,13 +92,8 @@ static void ledtrig_tty_work(struct work_struct *work)
 	struct serial_icounter_struct icount;
 	int ret;
 
-	mutex_lock(&trigger_data->mutex);
-
-	if (!trigger_data->ttyname) {
-		/* exit without rescheduling */
-		mutex_unlock(&trigger_data->mutex);
-		return;
-	}
+	if (!trigger_data->ttyname)
+		goto out;
 
 	/* try to get the tty corresponding to $ttyname */
 	if (!trigger_data->tty) {
@@ -116,11 +118,8 @@ static void ledtrig_tty_work(struct work_struct *work)
 	}
 
 	ret = tty_get_icount(trigger_data->tty, &icount);
-	if (ret) {
-		dev_info(trigger_data->tty->dev, "Failed to get icount, stopped polling\n");
-		mutex_unlock(&trigger_data->mutex);
-		return;
-	}
+	if (ret)
+		goto out;
 
 	if (icount.rx != trigger_data->rx ||
 	    icount.tx != trigger_data->tx) {
@@ -134,7 +133,7 @@ static void ledtrig_tty_work(struct work_struct *work)
 	}
 
 out:
-	mutex_unlock(&trigger_data->mutex);
+	complete_all(&trigger_data->sysfs);
 	schedule_delayed_work(&trigger_data->dwork,
 			      msecs_to_jiffies(LEDTRIG_TTY_INTERVAL * 2));
 }
@@ -157,7 +156,9 @@ static int ledtrig_tty_activate(struct led_classdev *led_cdev)
 
 	INIT_DELAYED_WORK(&trigger_data->dwork, ledtrig_tty_work);
 	trigger_data->led_cdev = led_cdev;
-	mutex_init(&trigger_data->mutex);
+	init_completion(&trigger_data->sysfs);
+
+	schedule_delayed_work(&trigger_data->dwork, 0);
 
 	return 0;
 }