[PATCH] pwm: pca9685: fix pwm/gpio inter-operation

[Sven Van Asbroeck]

This driver allows pwms to be requested as gpios via gpiolib.
Obviously, it should not be allowed to request a gpio when its
corresponding pwm is already requested (and vice versa).
So it requires some exclusion code.

Given that the pwm and gpio cores are not synchronized with
respect to each other, this exclusion code will also require
proper synchronization.

Such a mechanism was in place, but was inadvertently removed
by Uwe's clean-up patch.

Upon revisiting the synchronization mechanism, we found that
theoretically, it could allow two threads to successfully
request conflicting pwms / gpios.

Replace with a bitmap which tracks pwm in-use, plus a mutex.
As long as pwm and gpio's respective request/free functions
modify the in-use bitmap while holding the mutex, proper
synchronization will be guaranteed.

Reported-by: YueHaibing <yuehaibing@xxxxxxxxxx>
Fixes: e926b12c611c ("pwm: Clear chip_data in pwm_put()")
Cc: Mika Westerberg <mika.westerberg@xxxxxxxxxxxxxxx>
Cc: Uwe Kleine-KÃnig <u.kleine-koenig@xxxxxxxxxxxxxx>
Link: https://lkml.org/lkml/2019/5/31/963
Signed-off-by: Sven Van Asbroeck <TheSven73@xxxxxxxxx>
---

This approach will also prevent the request of the "all" pwm channel, if any
other pwm channel is already in use. Is this correct behaviour?

 drivers/pwm/pwm-pca9685.c | 64 +++++++++++++++++++++------------------
 1 file changed, 35 insertions(+), 29 deletions(-)

diff --git a/drivers/pwm/pwm-pca9685.c b/drivers/pwm/pwm-pca9685.c
index 567f5e2771c4..f9927cd106d0 100644
--- a/drivers/pwm/pwm-pca9685.c
+++ b/drivers/pwm/pwm-pca9685.c
@@ -31,6 +31,7 @@
 #include <linux/slab.h>
 #include <linux/delay.h>
 #include <linux/pm_runtime.h>
+#include <linux/bitmap.h>
 
 /*
  * Because the PCA9685 has only one prescaler per chip, changing the period of
@@ -85,6 +86,7 @@ struct pca9685 {
 #if IS_ENABLED(CONFIG_GPIOLIB)
 	struct mutex lock;
 	struct gpio_chip gpio;
+	DECLARE_BITMAP(pwms_inuse, PCA9685_MAXCHAN);
 #endif
 };
 
@@ -97,48 +99,45 @@ static inline struct pca9685 *to_pca(struct pwm_chip *chip)
 static int pca9685_pwm_gpio_request(struct gpio_chip *gpio, unsigned int offset)
 {
 	struct pca9685 *pca = gpiochip_get_data(gpio);
-	struct pwm_device *pwm;
 
 	mutex_lock(&pca->lock);
 
-	pwm = &pca->chip.pwms[offset];
-
-	if (pwm->flags & (PWMF_REQUESTED | PWMF_EXPORTED)) {
+	if (test_and_set_bit(offset, pca->pwms_inuse)) {
 		mutex_unlock(&pca->lock);
 		return -EBUSY;
 	}
 
-	pwm_set_chip_data(pwm, (void *)1);
-
 	mutex_unlock(&pca->lock);
 	pm_runtime_get_sync(pca->chip.dev);
 	return 0;
 }
 
-static bool pca9685_pwm_is_gpio(struct pca9685 *pca, struct pwm_device *pwm)
+static bool
+pca9685_pwm_test_set_inuse(struct pca9685 *pca, struct pwm_device *pwm)
 {
-	bool is_gpio = false;
+	bool is_inuse;
 
 	mutex_lock(&pca->lock);
+	/*
+	 * Check if any of the PWMs are requested and in that case
+	 * prevent using the "all LEDs" channel.
+	 */
+	if (pwm->hwpwm >= PCA9685_MAXCHAN &&
+			!bitmap_empty(pca->pwms_inuse, PCA9685_MAXCHAN))
+		is_inuse = true;
+	else
+		is_inuse = test_and_set_bit(pwm->hwpwm, pca->pwms_inuse);
+	mutex_unlock(&pca->lock);
 
-	if (pwm->hwpwm >= PCA9685_MAXCHAN) {
-		unsigned int i;
-
-		/*
-		 * Check if any of the GPIOs are requested and in that case
-		 * prevent using the "all LEDs" channel.
-		 */
-		for (i = 0; i < pca->gpio.ngpio; i++)
-			if (gpiochip_is_requested(&pca->gpio, i)) {
-				is_gpio = true;
-				break;
-			}
-	} else if (pwm_get_chip_data(pwm)) {
-		is_gpio = true;
-	}
+	return is_inuse;
+}
 
+static void pca9685_pwm_clear_inuse(struct pca9685 *pca, struct pwm_device *pwm)
+{
+	mutex_lock(&pca->lock);
+	if (pwm->hwpwm < PCA9685_MAXCHAN)
+		clear_bit(pwm->hwpwm, pca->pwms_inuse);
 	mutex_unlock(&pca->lock);
-	return is_gpio;
 }
 
 static int pca9685_pwm_gpio_get(struct gpio_chip *gpio, unsigned int offset)
@@ -170,12 +169,11 @@ static void pca9685_pwm_gpio_set(struct gpio_chip *gpio, unsigned int offset,
 static void pca9685_pwm_gpio_free(struct gpio_chip *gpio, unsigned int offset)
 {
 	struct pca9685 *pca = gpiochip_get_data(gpio);
-	struct pwm_device *pwm;
 
+	mutex_lock(&pca->lock);
 	pca9685_pwm_gpio_set(gpio, offset, 0);
 	pm_runtime_put(pca->chip.dev);
-	mutex_lock(&pca->lock);
-	pwm = &pca->chip.pwms[offset];
+	clear_bit(offset, pca->pwms_inuse);
 	mutex_unlock(&pca->lock);
 }
 
@@ -228,12 +226,17 @@ static int pca9685_pwm_gpio_probe(struct pca9685 *pca)
 	return devm_gpiochip_add_data(dev, &pca->gpio, pca);
 }
 #else
-static inline bool pca9685_pwm_is_gpio(struct pca9685 *pca,
+static inline bool pca9685_pwm_test_set_inuse(struct pca9685 *pca,
 				       struct pwm_device *pwm)
 {
 	return false;
 }
 
+static inline void
+pca9685_pwm_clear_inuse(struct pca9685 *pca, struct pwm_device *pwm)
+{
+}
+
 static inline int pca9685_pwm_gpio_probe(struct pca9685 *pca)
 {
 	return 0;
@@ -417,7 +420,7 @@ static int pca9685_pwm_request(struct pwm_chip *chip, struct pwm_device *pwm)
 {
 	struct pca9685 *pca = to_pca(chip);
 
-	if (pca9685_pwm_is_gpio(pca, pwm))
+	if (pca9685_pwm_test_set_inuse(pca, pwm))
 		return -EBUSY;
 	pm_runtime_get_sync(chip->dev);
 
@@ -426,8 +429,11 @@ static int pca9685_pwm_request(struct pwm_chip *chip, struct pwm_device *pwm)
 
 static void pca9685_pwm_free(struct pwm_chip *chip, struct pwm_device *pwm)
 {
+	struct pca9685 *pca = to_pca(chip);
+
 	pca9685_pwm_disable(chip, pwm);
 	pm_runtime_put(chip->dev);
+	pca9685_pwm_clear_inuse(pca, pwm);
 }
 
 static const struct pwm_ops pca9685_pwm_ops = {
-- 
2.17.1