pwm: Add more locking

This ensures that a pwm_chip that has no corresponding driver isn't used
and that a driver doesn't go away while a callback is still running.

In the presence of device links this isn't necessary yet (so this is no
fix) but for pwm character device support this is needed.

To not serialize all pwm_apply_state() calls, this introduces a per chip
lock. An additional complication is that for atomic chips a mutex cannot
be used (as pwm_apply_atomic() must not sleep) and a spinlock cannot be
held while calling an operation for a sleeping chip. So depending on the
chip being atomic or not a spinlock or a mutex is used.

An additional change implemented here is that on driver remove the
.free() callback is called for each requested pwm_device. This is the
right time because later (e.g. when the consumer calls pwm_put()) the
free function is (maybe) not available any more.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/026aa891c8270a11723a1ba7e4256f456f7e1e86.1726819463.git.u.kleine-koenig@baylibre.com
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>

drivers/pwm/core.c

@@ -31,6 +31,24 @@ static DEFINE_MUTEX(pwm_lock);
 
 static DEFINE_IDR(pwm_chips);
 
+static void pwmchip_lock(struct pwm_chip *chip)
+{
+	if (chip->atomic)
+		spin_lock(&chip->atomic_lock);
+	else
+		mutex_lock(&chip->nonatomic_lock);
+}
+
+static void pwmchip_unlock(struct pwm_chip *chip)
+{
+	if (chip->atomic)
+		spin_unlock(&chip->atomic_lock);
+	else
+		mutex_unlock(&chip->nonatomic_lock);
+}
+
+DEFINE_GUARD(pwmchip, struct pwm_chip *, pwmchip_lock(_T), pwmchip_unlock(_T))
+
 static void pwm_apply_debug(struct pwm_device *pwm,
 			    const struct pwm_state *state)
 {
@@ -220,6 +238,7 @@ static int __pwm_apply(struct pwm_device *pwm, const struct pwm_state *state)
 int pwm_apply_might_sleep(struct pwm_device *pwm, const struct pwm_state *state)
 {
 	int err;
+	struct pwm_chip *chip = pwm->chip;
 
 	/*
 	 * Some lowlevel driver's implementations of .apply() make use of
@@ -230,7 +249,12 @@ int pwm_apply_might_sleep(struct pwm_device *pwm, const struct pwm_state *state)
 	 */
 	might_sleep();
 
-	if (IS_ENABLED(CONFIG_PWM_DEBUG) && pwm->chip->atomic) {
+	guard(pwmchip)(chip);
+
+	if (!chip->operational)
+		return -ENODEV;
+
+	if (IS_ENABLED(CONFIG_PWM_DEBUG) && chip->atomic) {
 		/*
 		 * Catch any drivers that have been marked as atomic but
 		 * that will sleep anyway.
@@ -254,9 +278,16 @@ EXPORT_SYMBOL_GPL(pwm_apply_might_sleep);
  */
 int pwm_apply_atomic(struct pwm_device *pwm, const struct pwm_state *state)
 {
-	WARN_ONCE(!pwm->chip->atomic,
+	struct pwm_chip *chip = pwm->chip;
+
+	WARN_ONCE(!chip->atomic,
 		  "sleeping PWM driver used in atomic context\n");
 
+	guard(pwmchip)(chip);
+
+	if (!chip->operational)
+		return -ENODEV;
+
 	return __pwm_apply(pwm, state);
 }
 EXPORT_SYMBOL_GPL(pwm_apply_atomic);
@@ -334,8 +365,18 @@ static int pwm_capture(struct pwm_device *pwm, struct pwm_capture *result,
 	if (!ops->capture)
 		return -ENOSYS;
 
+	/*
+	 * Holding the pwm_lock is probably not needed. If you use pwm_capture()
+	 * and you're interested to speed it up, please convince yourself it's
+	 * really not needed, test and then suggest a patch on the mailing list.
+	 */
 	guard(mutex)(&pwm_lock);
 
+	guard(pwmchip)(chip);
+
+	if (!chip->operational)
+		return -ENODEV;
+
 	return ops->capture(chip, pwm, result, timeout);
 }
 
@@ -368,6 +409,14 @@ static int pwm_device_request(struct pwm_device *pwm, const char *label)
 	if (test_bit(PWMF_REQUESTED, &pwm->flags))
 		return -EBUSY;
 
+	/*
+	 * This function is called while holding pwm_lock. As .operational only
+	 * changes while holding this lock, checking it here without holding the
+	 * chip lock is fine.
+	 */
+	if (!chip->operational)
+		return -ENODEV;
+
 	if (!try_module_get(chip->owner))
 		return -ENODEV;
 
@@ -396,7 +445,9 @@ static int pwm_device_request(struct pwm_device *pwm, const char *label)
 		 */
 		struct pwm_state state = { 0, };
 
-		err = ops->get_state(chip, pwm, &state);
+		scoped_guard(pwmchip, chip)
+			err = ops->get_state(chip, pwm, &state);
+
 		trace_pwm_get(pwm, &state, err);
 
 		if (!err)
@@ -1020,6 +1071,7 @@ struct pwm_chip *pwmchip_alloc(struct device *parent, unsigned int npwm, size_t
 
 	chip->npwm = npwm;
 	chip->uses_pwmchip_alloc = true;
+	chip->operational = false;
 
 	pwmchip_dev = &chip->dev;
 	device_initialize(pwmchip_dev);
@@ -1125,6 +1177,11 @@ int __pwmchip_add(struct pwm_chip *chip, struct module *owner)
 
 	chip->owner = owner;
 
+	if (chip->atomic)
+		spin_lock_init(&chip->atomic_lock);
+	else
+		mutex_init(&chip->nonatomic_lock);
+
 	guard(mutex)(&pwm_lock);
 
 	ret = idr_alloc(&pwm_chips, chip, 0, 0, GFP_KERNEL);
@@ -1138,13 +1195,19 @@ int __pwmchip_add(struct pwm_chip *chip, struct module *owner)
 	if (IS_ENABLED(CONFIG_OF))
 		of_pwmchip_add(chip);
 
+	scoped_guard(pwmchip, chip)
+		chip->operational = true;
+
 	ret = device_add(&chip->dev);
 	if (ret)
 		goto err_device_add;
 
 	return 0;
 
 err_device_add:
+	scoped_guard(pwmchip, chip)
+		chip->operational = false;
+
 	if (IS_ENABLED(CONFIG_OF))
 		of_pwmchip_remove(chip);
 
@@ -1164,11 +1227,27 @@ void pwmchip_remove(struct pwm_chip *chip)
 {
 	pwmchip_sysfs_unexport(chip);
 
-	if (IS_ENABLED(CONFIG_OF))
-		of_pwmchip_remove(chip);
+	scoped_guard(mutex, &pwm_lock) {
+		unsigned int i;
+
+		scoped_guard(pwmchip, chip)
+			chip->operational = false;
+
+		for (i = 0; i < chip->npwm; ++i) {
+			struct pwm_device *pwm = &chip->pwms[i];
+
+			if (test_and_clear_bit(PWMF_REQUESTED, &pwm->flags)) {
+				dev_warn(&chip->dev, "Freeing requested PWM #%u\n", i);
+				if (pwm->chip->ops->free)
+					pwm->chip->ops->free(pwm->chip, pwm);
+			}
+		}
+
+		if (IS_ENABLED(CONFIG_OF))
+			of_pwmchip_remove(chip);
 
-	scoped_guard(mutex, &pwm_lock)
 		idr_remove(&pwm_chips, chip->id);
+	}
 
 	device_del(&chip->dev);
 }
@@ -1538,12 +1617,17 @@ void pwm_put(struct pwm_device *pwm)
 
 	guard(mutex)(&pwm_lock);
 
-	if (!test_and_clear_bit(PWMF_REQUESTED, &pwm->flags)) {
+	/*
+	 * Trigger a warning if a consumer called pwm_put() twice.
+	 * If the chip isn't operational, PWMF_REQUESTED was already cleared in
+	 * pwmchip_remove(). So don't warn in this case.
+	 */
+	if (chip->operational && !test_and_clear_bit(PWMF_REQUESTED, &pwm->flags)) {
 		pr_warn("PWM device already freed\n");
 		return;
 	}
 
-	if (chip->ops->free)
+	if (chip->operational && chip->ops->free)
 		pwm->chip->ops->free(pwm->chip, pwm);
 
 	pwm->label = NULL;

include/linux/pwm.h

@@ -275,6 +275,9 @@ struct pwm_ops {
  * @of_xlate: request a PWM device given a device tree PWM specifier
  * @atomic: can the driver's ->apply() be called in atomic context
  * @uses_pwmchip_alloc: signals if pwmchip_allow was used to allocate this chip
+ * @operational: signals if the chip can be used (or is already deregistered)
+ * @nonatomic_lock: mutex for nonatomic chips
+ * @atomic_lock: mutex for atomic chips
  * @pwms: array of PWM devices allocated by the framework
  */
 struct pwm_chip {
@@ -290,6 +293,16 @@ struct pwm_chip {
 
 	/* only used internally by the PWM framework */
 	bool uses_pwmchip_alloc;
+	bool operational;
+	union {
+		/*
+		 * depending on the chip being atomic or not either the mutex or
+		 * the spinlock is used. It protects .operational and
+		 * synchronizes the callbacks in .ops
+		 */
+		struct mutex nonatomic_lock;
+		spinlock_t atomic_lock;
+	};
 	struct pwm_device pwms[] __counted_by(npwm);
 };