Fix bugs in kona pwm driver and pwm core

Update the driver so that settings are applied in accordance with the most recent version of the hardware spec. The revised sequence clears the trigger bit, waits 400ns, writes settings, sets the trigger bit, and waits another 400ns. This corrects an issue where occasionally a requested change was not properly reflected in the PWM output.

Reviewed-by Arun Ramamurthy <[email protected]>

Reviewed-by Scott Branden <[email protected]>

Tested-by Scott Branden <[email protected]>

Reviewed-by Tim Kryger <[email protected]>

Signed-off-by Jonathan Richardson <[email protected]>

---
 drivers/pwm/pwm-bcm-kona.c |   47 +++++++++++++++++++++++++++++++++++---------
 1 file changed, 38 insertions(+), 9 deletions(-)

drivers/pwm/pwm-bcm-kona.c +38 -9

index 7af8fea..9b1d397 100644
@@ -76,19 +76,36 @@ static inline struct kona_pwmc *to_kona_pwmc(struct pwm_chip *_chip)
 	return container_of(_chip, struct kona_pwmc, chip);
 }
 
-static void kona_pwmc_apply_settings(struct kona_pwmc *kp, unsigned int chan)
+/*
+ * Clear trigger bit but set smooth bit to maintain old output.
+ */
+static void kona_pwmc_prepare_for_settings(struct kona_pwmc *kp,
+	unsigned int chan)
 {
 	unsigned int value = readl(kp->base + PWM_CONTROL_OFFSET);
 
-	/* Clear trigger bit but set smooth bit to maintain old output */
 	value |= 1 << PWM_CONTROL_SMOOTH_SHIFT(chan);
 	value &= ~(1 << PWM_CONTROL_TRIGGER_SHIFT(chan));
 	writel(value, kp->base + PWM_CONTROL_OFFSET);
 
+	/*
+	 * There must be a min 400ns delay between clearing trigger and setting
+	 * it. Failing to do this may result in no PWM signal.
+	 */
+	ndelay(400);
+}
+
+static void kona_pwmc_apply_settings(struct kona_pwmc *kp, unsigned int chan)
+{
+	unsigned int value = readl(kp->base + PWM_CONTROL_OFFSET);
+
 	/* Set trigger bit and clear smooth bit to apply new settings */
 	value &= ~(1 << PWM_CONTROL_SMOOTH_SHIFT(chan));
 	value |= 1 << PWM_CONTROL_TRIGGER_SHIFT(chan);
 	writel(value, kp->base + PWM_CONTROL_OFFSET);
+
+	/* Trigger bit must be held high for at least 400 ns. */
+	ndelay(400);
 }
 
 static int kona_pwmc_config(struct pwm_chip *chip, struct pwm_device *pwm,

@@ -133,8 +150,14 @@ static int kona_pwmc_config(struct pwm_chip *chip, struct pwm_device *pwm,
 			return -EINVAL;
 	}
 
-	/* If the PWM channel is enabled, write the settings to the HW */
+	/*
+	 * Don't apply settings if disabled. The period and duty cycle are
+	 * always calculated above to ensure the new values are
+	 * validated immediately instead of on enable.
+	 */
 	if (test_bit(PWMF_ENABLED, &pwm->flags)) {
+		kona_pwmc_prepare_for_settings(kp, chan);
+
 		value = readl(kp->base + PRESCALE_OFFSET);
 		value &= ~PRESCALE_MASK(chan);
 		value |= prescale << PRESCALE_SHIFT(chan);

@@ -164,6 +187,8 @@ static int kona_pwmc_set_polarity(struct pwm_chip *chip, struct pwm_device *pwm,
 		return ret;
 	}
 
+	kona_pwmc_prepare_for_settings(kp, chan);
+
 	value = readl(kp->base + PWM_CONTROL_OFFSET);
 
 	if (polarity == PWM_POLARITY_NORMAL)

@@ -175,9 +200,6 @@ static int kona_pwmc_set_polarity(struct pwm_chip *chip, struct pwm_device *pwm,
 
 	kona_pwmc_apply_settings(kp, chan);
 
-	/* Wait for waveform to settle before gating off the clock */
-	ndelay(400);
-
 	clk_disable_unprepare(kp->clk);
 
 	return 0;

@@ -207,13 +229,20 @@ static void kona_pwmc_disable(struct pwm_chip *chip, struct pwm_device *pwm)
 {
 	struct kona_pwmc *kp = to_kona_pwmc(chip);
 	unsigned int chan = pwm->hwpwm;
+	unsigned int value;
+
+	kona_pwmc_prepare_for_settings(kp, chan);
 
 	/* Simulate a disable by configuring for zero duty */
 	writel(0, kp->base + DUTY_CYCLE_HIGH_OFFSET(chan));
-	kona_pwmc_apply_settings(kp, chan);
+	writel(0, kp->base + PERIOD_COUNT_OFFSET(chan));
 
-	/* Wait for waveform to settle before gating off the clock */
-	ndelay(400);
+	/* Set prescale to 0 for this channel */
+	value = readl(kp->base + PRESCALE_OFFSET);
+	value &= ~PRESCALE_MASK(chan);
+	writel(value, kp->base + PRESCALE_OFFSET);
+
+	kona_pwmc_apply_settings(kp, chan);
 
 	clk_disable_unprepare(kp->clk);
 }

Update the driver so that settings are applied in accordance with the most recent version of the hardware spec. The revised sequence clears the trigger bit, waits 400ns, writes settings, sets the trigger bit, and waits another 400ns. This corrects an issue where occasionally a requested change was not properly reflected in the PWM output.

Reviewed-by Arun Ramamurthy <[email protected]>

Reviewed-by Scott Branden <[email protected]>

Tested-by Scott Branden <[email protected]>

Reviewed-by Tim Kryger <[email protected]>

Signed-off-by Jonathan Richardson <[email protected]>

---
 drivers/pwm/pwm-bcm-kona.c |   47 +++++++++++++++++++++++++++++++++++---------
 1 file changed, 38 insertions(+), 9 deletions(-)

The pwm_enable function didn't clear the enabled bit if a call to a clients enable function returned an error. The result was that the state of the pwm core was wrong. Clearing the bit when enable returns an error ensures the state is properly set.

Tested-by Jonathan Richardson <[email protected]>

Reviewed-by Dmitry Torokhov <[email protected]>

Signed-off-by Jonathan Richardson <[email protected]>

---
 drivers/pwm/core.c  |   19 ++++++++++++++++---
 include/linux/pwm.h |    2 ++
 2 files changed, 18 insertions(+), 3 deletions(-)

drivers/pwm/core.c +8 -3

index 76b0386..c255267 100644
@@ -263,6 +263,7 @@ 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);
 	}

@@ -474,10 +475,22 @@ EXPORT_SYMBOL_GPL(pwm_set_polarity);
  */
 int pwm_enable(struct pwm_device *pwm)
 {
-	if (pwm && !test_and_set_bit(PWMF_ENABLED, &pwm->flags))
-		return pwm->chip->ops->enable(pwm->chip, pwm);
+	int err = 0;
 
-	return pwm ? 0 : -EINVAL;
+	if (!pwm)
+		return -EINVAL;
+
+	mutex_lock(&pwm->lock);
+
+	if (!test_and_set_bit(PWMF_ENABLED, &pwm->flags)) {

Quoted 45 lines · Jonathan Richardson

The pwm_enable function didn't clear the enabled bit if a call to a clients enable function returned an error. The result was that the state of the pwm core was wrong. Clearing the bit when enable returns an error ensures the state is properly set.

Tested-by Jonathan Richardson <[email protected]>

Reviewed-by Dmitry Torokhov <[email protected]>

Signed-off-by Jonathan Richardson <[email protected]>

---
 drivers/pwm/core.c  |   19 ++++++++++++++++---
 include/linux/pwm.h |    2 ++
 2 files changed, 18 insertions(+), 3 deletions(-)

drivers/pwm/core.c +13 -3

index 76b0386..c255267 100644
@@ -263,6 +263,7 @@ 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);
 	}

@@ -474,10 +475,22 @@ EXPORT_SYMBOL_GPL(pwm_set_polarity);
  */
 int pwm_enable(struct pwm_device *pwm)
 {
-	if (pwm && !test_and_set_bit(PWMF_ENABLED, &pwm->flags))
-		return pwm->chip->ops->enable(pwm->chip, pwm);
+	int err = 0;
 
-	return pwm ? 0 : -EINVAL;
+	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);
+	}
+

drivers/pwm/core.c EXPORT_SYMBOL_GPL(pwm_set_polarity); · line 495

Quoted 5 lines

drivers/pwm/core.c +3 -0

+	mutex_unlock(&pwm->lock);
+
+	return err;
 }
 EXPORT_SYMBOL_GPL(pwm_enable);

I meant to add the mutex check in disable also, but what about when PWMF_ENABLED is checked in pwm_set_polarity() and pwm_dbg_show()?

Quoted 46 lines · Jonathan Richardson

Quoted 45 lines · Jonathan Richardson

The pwm_enable function didn't clear the enabled bit if a call to a clients enable function returned an error. The result was that the state of the pwm core was wrong. Clearing the bit when enable returns an error ensures the state is properly set.

Tested-by Jonathan Richardson <[email protected]>

Reviewed-by Dmitry Torokhov <[email protected]>

Signed-off-by Jonathan Richardson <[email protected]>

---
 drivers/pwm/core.c  |   19 ++++++++++++++++---
 include/linux/pwm.h |    2 ++
 2 files changed, 18 insertions(+), 3 deletions(-)

drivers/pwm/core.c +13 -3

index 76b0386..c255267 100644
@@ -263,6 +263,7 @@ 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);
 	}

@@ -474,10 +475,22 @@ EXPORT_SYMBOL_GPL(pwm_set_polarity);
  */
 int pwm_enable(struct pwm_device *pwm)
 {
-	if (pwm && !test_and_set_bit(PWMF_ENABLED, &pwm->flags))
-		return pwm->chip->ops->enable(pwm->chip, pwm);
+	int err = 0;
 
-	return pwm ? 0 : -EINVAL;
+	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);
+	}
+

Quoted 8 lines

drivers/pwm/core.c int pwm_set_polarity(struct pwm_device *pwm, enum pwm_polarity polarity)

Quoted 5 lines

drivers/pwm/core.c +3 -0

+	mutex_unlock(&pwm->lock);
+
+	return err;
 }
 EXPORT_SYMBOL_GPL(pwm_enable);

I meant to add the mutex check in disable also, but what about when PWMF_ENABLED is checked in pwm_set_polarity() and pwm_dbg_show()?

I think for debugfs we're fine since there's no potential to race there. It'll simply show the state of the PWM at the point where it was queried even though that may change immediately after. I suppose we could keep the lock across the body of the loop just to make sure debugfs will show a consistent view of the PWM.

For pwm_disable() I don't think we need the lock, since the test_and_ clear_bit() is atomic and ->disable() cannot fail.

As for pwm_set_polarity(), I think it would need to be something like the below:

---- >8 ----

drivers/pwm/core.c +7 -3

index 3f9df3ea3350..8488c7a19bf6 100644
@@ -473,16 +473,22 @@ int pwm_set_polarity(struct pwm_device *pwm, enum pwm_polarity polarity)
        if (!pwm->chip->ops->set_polarity)
                return -ENOSYS;
 
-       if (pwm_is_enabled(pwm))
-               return -EBUSY;
+       mutex_lock(&pwm->lock);
+
+       if (pwm_is_enabled(pwm)) {
+               err = -EBUSY;
+               goto unlock;
+       }
 
        err = pwm->chip->ops->set_polarity(pwm->chip, pwm, polarity);
        if (err)
-               return err;
+               goto unlock;