pwm: Get rid of pwm->lock

PWM devices are not protected against concurrent accesses. The lock in
struct pwm_device might let PWM users think it is, but it's actually
only protecting the enabled state.

Removing this lock should be fine as long as all PWM users are aware
that accesses to the PWM device have to be serialized, which seems to be
the case for all of them except the sysfs interface. Patch the sysfs
code by adding a lock to the pwm_export struct and making sure it's
taken for all relevant accesses to the exported PWM device.
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/core.c

@@ -269,7 +269,6 @@ int pwmchip_add_with_polarity(struct pwm_chip *chip,
 		pwm->pwm = chip->base + i;
 		pwm->hwpwm = i;
 		pwm->polarity = polarity;
-		mutex_init(&pwm->lock);
 
 		radix_tree_insert(&pwm_tree, pwm->pwm, pwm);
 	}
@@ -474,22 +473,16 @@ int pwm_set_polarity(struct pwm_device *pwm, enum pwm_polarity polarity)
 	if (!pwm->chip->ops->set_polarity)
 		return -ENOSYS;
 
-	mutex_lock(&pwm->lock);
-
-	if (pwm_is_enabled(pwm)) {
-		err = -EBUSY;
-		goto unlock;
-	}
+	if (pwm_is_enabled(pwm))
+		return -EBUSY;
 
 	err = pwm->chip->ops->set_polarity(pwm->chip, pwm, polarity);
 	if (err)
-		goto unlock;
+		return err;
 
 	pwm->polarity = polarity;
 
-unlock:
-	mutex_unlock(&pwm->lock);
-	return err;
+	return 0;
 }
 EXPORT_SYMBOL_GPL(pwm_set_polarity);
 
@@ -506,16 +499,12 @@ int pwm_enable(struct pwm_device *pwm)
 	if (!pwm)
 		return -EINVAL;
 
-	mutex_lock(&pwm->lock);
-
 	if (!test_and_set_bit(PWMF_ENABLED, &pwm->flags)) {
 		err = pwm->chip->ops->enable(pwm->chip, pwm);
 		if (err)
 			clear_bit(PWMF_ENABLED, &pwm->flags);
 	}
 
-	mutex_unlock(&pwm->lock);
-
 	return err;
 }
 EXPORT_SYMBOL_GPL(pwm_enable);

drivers/pwm/sysfs.c

@@ -26,6 +26,7 @@
 struct pwm_export {
 	struct device child;
 	struct pwm_device *pwm;
+	struct mutex lock;
 };
 
 static struct pwm_export *child_to_pwm_export(struct device *child)
@@ -53,7 +54,8 @@ static ssize_t period_store(struct device *child,
 			    struct device_attribute *attr,
 			    const char *buf, size_t size)
 {
-	struct pwm_device *pwm = child_to_pwm_device(child);
+	struct pwm_export *export = child_to_pwm_export(child);
+	struct pwm_device *pwm = export->pwm;
 	unsigned int val;
 	int ret;
 
@@ -61,7 +63,9 @@ static ssize_t period_store(struct device *child,
 	if (ret)
 		return ret;
 
+	mutex_lock(&export->lock);
 	ret = pwm_config(pwm, pwm_get_duty_cycle(pwm), val);
+	mutex_unlock(&export->lock);
 
 	return ret ? : size;
 }
@@ -79,7 +83,8 @@ static ssize_t duty_cycle_store(struct device *child,
 				struct device_attribute *attr,
 				const char *buf, size_t size)
 {
-	struct pwm_device *pwm = child_to_pwm_device(child);
+	struct pwm_export *export = child_to_pwm_export(child);
+	struct pwm_device *pwm = export->pwm;
 	unsigned int val;
 	int ret;
 
@@ -87,7 +92,9 @@ static ssize_t duty_cycle_store(struct device *child,
 	if (ret)
 		return ret;
 
+	mutex_lock(&export->lock);
 	ret = pwm_config(pwm, val, pwm_get_period(pwm));
+	mutex_unlock(&export->lock);
 
 	return ret ? : size;
 }
@@ -105,13 +112,16 @@ static ssize_t enable_store(struct device *child,
 			    struct device_attribute *attr,
 			    const char *buf, size_t size)
 {
-	struct pwm_device *pwm = child_to_pwm_device(child);
+	struct pwm_export *export = child_to_pwm_export(child);
+	struct pwm_device *pwm = export->pwm;
 	int val, ret;
 
 	ret = kstrtoint(buf, 0, &val);
 	if (ret)
 		return ret;
 
+	mutex_lock(&export->lock);
+
 	switch (val) {
 	case 0:
 		pwm_disable(pwm);
@@ -124,6 +134,8 @@ static ssize_t enable_store(struct device *child,
 		break;
 	}
 
+	mutex_unlock(&export->lock);
+
 	return ret ? : size;
 }
 
@@ -151,7 +163,8 @@ static ssize_t polarity_store(struct device *child,
 			      struct device_attribute *attr,
 			      const char *buf, size_t size)
 {
-	struct pwm_device *pwm = child_to_pwm_device(child);
+	struct pwm_export *export = child_to_pwm_export(child);
+	struct pwm_device *pwm = export->pwm;
 	enum pwm_polarity polarity;
 	int ret;
 
@@ -162,7 +175,9 @@ static ssize_t polarity_store(struct device *child,
 	else
 		return -EINVAL;
 
+	mutex_lock(&export->lock);
 	ret = pwm_set_polarity(pwm, polarity);
+	mutex_unlock(&export->lock);
 
 	return ret ? : size;
 }
@@ -203,6 +218,7 @@ static int pwm_export_child(struct device *parent, struct pwm_device *pwm)
 	}
 
 	export->pwm = pwm;
+	mutex_init(&export->lock);
 
 	export->child.release = pwm_export_release;
 	export->child.parent = parent;

include/linux/pwm.h

@@ -106,7 +106,6 @@ enum {
  * @pwm: global index of the PWM device
  * @chip: PWM chip providing this PWM device
  * @chip_data: chip-private data associated with the PWM device
- * @lock: used to serialize accesses to the PWM device where necessary
  * @period: period of the PWM signal (in nanoseconds)
  * @duty_cycle: duty cycle of the PWM signal (in nanoseconds)
  * @polarity: polarity of the PWM signal
@@ -119,7 +118,6 @@ struct pwm_device {
 	unsigned int pwm;
 	struct pwm_chip *chip;
 	void *chip_data;
-	struct mutex lock;
 
 	unsigned int period;
 	unsigned int duty_cycle;