pwm: Ensure a struct pwm has the same lifetime as its pwm_chip

It's required to not free the memory underlying a requested PWM
while a consumer still has a reference to it. While currently a pwm_chip
doesn't live long enough in all cases, linking the struct pwm to the
pwm_chip results in the right lifetime as soon as the pwmchip is living
long enough. This happens with the following commits.

Note this is a breaking change for all pwm drivers that don't use
pwmchip_alloc().

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org> # for struct_size() and __counted_by()
Link: https://lore.kernel.org/r/7e9e958841f049026c0023b309cc9deecf0ab61d.1710670958.git.u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

drivers/pwm/core.c

@@ -987,7 +987,7 @@ static void pwmchip_sysfs_unexport(struct pwm_chip *chip)
 
 static void *pwmchip_priv(struct pwm_chip *chip)
 {
-	return (void *)chip + ALIGN(sizeof(*chip), PWMCHIP_ALIGN);
+	return (void *)chip + ALIGN(struct_size(chip, pwms, chip->npwm), PWMCHIP_ALIGN);
 }
 
 /* This is the counterpart to pwmchip_alloc() */
@@ -1001,8 +1001,10 @@ struct pwm_chip *pwmchip_alloc(struct device *parent, unsigned int npwm, size_t
 {
 	struct pwm_chip *chip;
 	size_t alloc_size;
+	unsigned int i;
 
-	alloc_size = size_add(ALIGN(sizeof(*chip), PWMCHIP_ALIGN), sizeof_priv);
+	alloc_size = size_add(ALIGN(struct_size(chip, pwms, npwm), PWMCHIP_ALIGN),
+			      sizeof_priv);
 
 	chip = kzalloc(alloc_size, GFP_KERNEL);
 	if (!chip)
@@ -1014,6 +1016,12 @@ struct pwm_chip *pwmchip_alloc(struct device *parent, unsigned int npwm, size_t
 
 	pwmchip_set_drvdata(chip, pwmchip_priv(chip));
 
+	for (i = 0; i < chip->npwm; i++) {
+		struct pwm_device *pwm = &chip->pwms[i];
+		pwm->chip = chip;
+		pwm->hwpwm = i;
+	}
+
 	return chip;
 }
 EXPORT_SYMBOL_GPL(pwmchip_alloc);
@@ -1085,7 +1093,6 @@ static bool pwm_ops_check(const struct pwm_chip *chip)
  */
 int __pwmchip_add(struct pwm_chip *chip, struct module *owner)
 {
-	unsigned int i;
 	int ret;
 
 	if (!chip || !pwmchip_parent(chip) || !chip->ops || !chip->npwm)
@@ -1105,28 +1112,16 @@ int __pwmchip_add(struct pwm_chip *chip, struct module *owner)
 
 	chip->owner = owner;
 
-	chip->pwms = kcalloc(chip->npwm, sizeof(*chip->pwms), GFP_KERNEL);
-	if (!chip->pwms)
-		return -ENOMEM;
-
 	mutex_lock(&pwm_lock);
 
 	ret = idr_alloc(&pwm_chips, chip, 0, 0, GFP_KERNEL);
 	if (ret < 0) {
 		mutex_unlock(&pwm_lock);
-		kfree(chip->pwms);
 		return ret;
 	}
 
 	chip->id = ret;
 
-	for (i = 0; i < chip->npwm; i++) {
-		struct pwm_device *pwm = &chip->pwms[i];
-
-		pwm->chip = chip;
-		pwm->hwpwm = i;
-	}
-
 	mutex_unlock(&pwm_lock);
 
 	if (IS_ENABLED(CONFIG_OF))
@@ -1156,8 +1151,6 @@ void pwmchip_remove(struct pwm_chip *chip)
 	idr_remove(&pwm_chips, chip->id);
 
 	mutex_unlock(&pwm_lock);
-
-	kfree(chip->pwms);
 }
 EXPORT_SYMBOL_GPL(pwmchip_remove);
 

include/linux/pwm.h

@@ -290,7 +290,7 @@ struct pwm_chip {
 	/* only used internally by the PWM framework */
 	bool uses_pwmchip_alloc;
 	void *driver_data;
-	struct pwm_device *pwms;
+	struct pwm_device pwms[] __counted_by(npwm);
 };
 
 static inline struct device *pwmchip_parent(const struct pwm_chip *chip)