[PATCH v2 0/1] pwm: meson: fix scheduling while atomic issue

[PATCH v2 0/1] pwm: meson: fix scheduling while atomic issue

[Martin Blumenstingl]

Back in January a "BUG: scheduling while atomic" error showed up during
boot on my Meson8b Odroid-C1 (which uses a PWM regulator as CPU supply).
The call trace comes down to:
  __mutex_lock
  clk_prepare_lock
  clk_core_get_rate
  meson_pwm_apply
  ..
  dev_pm_opp_set_rate
  ..

Jerome has also seen the same problem but from pwm-leds (instead of a
pwm-regulator). He posted a patch which replaces the spinlock with a
mutex. That works. I believe we can optimize this by reducing the time
where the lock is held - that also allows to keep the spin-lock.

Analyzing this issue helped me understand the pwm-meson driver better.
My plan is to send some cleanups after this single fix is merged. The
goal of these cleanups is to re-use more of the goodies from the PWM
core in the pwm-meson driver as well as to address issues spotted by
Uwe Kleine-König (these issues violate the PWM API, but none of these
result in breakage of the boards/.dts that we currently have). These
follow-up patches can be found here: [1].

Dependencies: none

Target version: please queue this for -fixes so it makes it's way into
v5.1-rc (so we can get it backported from there, because this issue has
existed since the pwm-meson driver was introduced).


changes since v1 at [2]:
- added comment about usage of the spinlock as suggested by
  Uwe Kleine-König (thank you)
- collected Uwe Kleine-König's Reviewed-by


[0] http://lists.infradead.org/pipermail/linux-amlogic/2019-January/009690.html
[1] https://github.com/xdarklight/linux/commits/meson-pwm-for-5.2-v1
[2] https://patchwork.kernel.org/cover/10867757/


Martin Blumenstingl (1):
  pwm: meson: use the spin-lock only to protect register modifications

 drivers/pwm/pwm-meson.c | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

-- 
2.21.0


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[PATCH v2 1/1] pwm: meson: use the spin-lock only to protect register modifications

[Martin Blumenstingl]

Holding the spin-lock for all of the code in meson_pwm_apply() can
result in a "BUG: scheduling while atomic". This can happen because
clk_get_rate() (which is called from meson_pwm_calc()) may sleep.
Only hold the spin-lock when modifying registers to solve this.

The reason why we need a spin-lock in the driver is because the
REG_MISC_AB register is shared between the two channels provided by one
PWM controller. The only functions where REG_MISC_AB is modified are
meson_pwm_enable() and meson_pwm_disable() so the register reads/writes
in there need to be protected by the spin-lock.

The original code also used the spin-lock to protect the values in
struct meson_pwm_channel. This could be necessary if two consumers can
use the same PWM channel. However, PWM core doesn't allow this so we
don't need to protect the values in struct meson_pwm_channel with a
lock.

Fixes: 211ed630753d2f ("pwm: Add support for Meson PWM Controller")
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
---
 drivers/pwm/pwm-meson.c | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/drivers/pwm/pwm-meson.c b/drivers/pwm/pwm-meson.c
index c1ed641b3e26..f6e738ad7bd9 100644
--- a/drivers/pwm/pwm-meson.c
+++ b/drivers/pwm/pwm-meson.c
@@ -111,6 +111,10 @@ struct meson_pwm {
 	const struct meson_pwm_data *data;
 	void __iomem *base;
 	u8 inverter_mask;
+	/*
+	 * Protects register (write) access to the REG_MISC_AB register
+	 * that is shared between the two PWMs.
+	 */
 	spinlock_t lock;
 };
 
@@ -235,6 +239,7 @@ static void meson_pwm_enable(struct meson_pwm *meson,
 {
 	u32 value, clk_shift, clk_enable, enable;
 	unsigned int offset;
+	unsigned long flags;
 
 	switch (id) {
 	case 0:
@@ -255,6 +260,8 @@ static void meson_pwm_enable(struct meson_pwm *meson,
 		return;
 	}
 
+	spin_lock_irqsave(&meson->lock, flags);
+
 	value = readl(meson->base + REG_MISC_AB);
 	value &= ~(MISC_CLK_DIV_MASK << clk_shift);
 	value |= channel->pre_div << clk_shift;
@@ -267,11 +274,14 @@ static void meson_pwm_enable(struct meson_pwm *meson,
 	value = readl(meson->base + REG_MISC_AB);
 	value |= enable;
 	writel(value, meson->base + REG_MISC_AB);
+
+	spin_unlock_irqrestore(&meson->lock, flags);
 }
 
 static void meson_pwm_disable(struct meson_pwm *meson, unsigned int id)
 {
 	u32 value, enable;
+	unsigned long flags;
 
 	switch (id) {
 	case 0:
@@ -286,9 +296,13 @@ static void meson_pwm_disable(struct meson_pwm *meson, unsigned int id)
 		return;
 	}
 
+	spin_lock_irqsave(&meson->lock, flags);
+
 	value = readl(meson->base + REG_MISC_AB);
 	value &= ~enable;
 	writel(value, meson->base + REG_MISC_AB);
+
+	spin_unlock_irqrestore(&meson->lock, flags);
 }
 
 static int meson_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
@@ -296,19 +310,16 @@ static int meson_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 {
 	struct meson_pwm_channel *channel = pwm_get_chip_data(pwm);
 	struct meson_pwm *meson = to_meson_pwm(chip);
-	unsigned long flags;
 	int err = 0;
 
 	if (!state)
 		return -EINVAL;
 
-	spin_lock_irqsave(&meson->lock, flags);
-
 	if (!state->enabled) {
 		meson_pwm_disable(meson, pwm->hwpwm);
 		channel->state.enabled = false;
 
-		goto unlock;
+		return 0;
 	}
 
 	if (state->period != channel->state.period ||
@@ -329,7 +340,7 @@ static int meson_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 		err = meson_pwm_calc(meson, channel, pwm->hwpwm,
 				     state->duty_cycle, state->period);
 		if (err < 0)
-			goto unlock;
+			return err;
 
 		channel->state.polarity = state->polarity;
 		channel->state.period = state->period;
@@ -341,9 +352,7 @@ static int meson_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 		channel->state.enabled = true;
 	}
 
-unlock:
-	spin_unlock_irqrestore(&meson->lock, flags);
-	return err;
+	return 0;
 }
 
 static void meson_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
-- 
2.21.0


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Re: [PATCH v2 1/1] pwm: meson: use the spin-lock only to protect register modifications

[Neil Armstrong]

On 01/04/2019 19:57, Martin Blumenstingl wrote:
> Holding the spin-lock for all of the code in meson_pwm_apply() can
> result in a "BUG: scheduling while atomic". This can happen because
> clk_get_rate() (which is called from meson_pwm_calc()) may sleep.
> Only hold the spin-lock when modifying registers to solve this.
> 
> The reason why we need a spin-lock in the driver is because the
> REG_MISC_AB register is shared between the two channels provided by one
> PWM controller. The only functions where REG_MISC_AB is modified are
> meson_pwm_enable() and meson_pwm_disable() so the register reads/writes
> in there need to be protected by the spin-lock.
> 
> The original code also used the spin-lock to protect the values in
> struct meson_pwm_channel. This could be necessary if two consumers can
> use the same PWM channel. However, PWM core doesn't allow this so we
> don't need to protect the values in struct meson_pwm_channel with a
> lock.
> 
> Fixes: 211ed630753d2f ("pwm: Add support for Meson PWM Controller")
> Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
> Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
> ---
>  drivers/pwm/pwm-meson.c | 25 +++++++++++++++++--------
>  1 file changed, 17 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-meson.c b/drivers/pwm/pwm-meson.c
> index c1ed641b3e26..f6e738ad7bd9 100644
> --- a/drivers/pwm/pwm-meson.c
> +++ b/drivers/pwm/pwm-meson.c
> @@ -111,6 +111,10 @@ struct meson_pwm {
>  	const struct meson_pwm_data *data;
>  	void __iomem *base;
>  	u8 inverter_mask;
> +	/*
> +	 * Protects register (write) access to the REG_MISC_AB register
> +	 * that is shared between the two PWMs.
> +	 */
>  	spinlock_t lock;
>  };
>  
> @@ -235,6 +239,7 @@ static void meson_pwm_enable(struct meson_pwm *meson,
>  {
>  	u32 value, clk_shift, clk_enable, enable;
>  	unsigned int offset;
> +	unsigned long flags;
>  
>  	switch (id) {
>  	case 0:
> @@ -255,6 +260,8 @@ static void meson_pwm_enable(struct meson_pwm *meson,
>  		return;
>  	}
>  
> +	spin_lock_irqsave(&meson->lock, flags);
> +
>  	value = readl(meson->base + REG_MISC_AB);
>  	value &= ~(MISC_CLK_DIV_MASK << clk_shift);
>  	value |= channel->pre_div << clk_shift;
> @@ -267,11 +274,14 @@ static void meson_pwm_enable(struct meson_pwm *meson,
>  	value = readl(meson->base + REG_MISC_AB);
>  	value |= enable;
>  	writel(value, meson->base + REG_MISC_AB);
> +
> +	spin_unlock_irqrestore(&meson->lock, flags);
>  }
>  
>  static void meson_pwm_disable(struct meson_pwm *meson, unsigned int id)
>  {
>  	u32 value, enable;
> +	unsigned long flags;
>  
>  	switch (id) {
>  	case 0:
> @@ -286,9 +296,13 @@ static void meson_pwm_disable(struct meson_pwm *meson, unsigned int id)
>  		return;
>  	}
>  
> +	spin_lock_irqsave(&meson->lock, flags);
> +
>  	value = readl(meson->base + REG_MISC_AB);
>  	value &= ~enable;
>  	writel(value, meson->base + REG_MISC_AB);
> +
> +	spin_unlock_irqrestore(&meson->lock, flags);
>  }
>  
>  static int meson_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
> @@ -296,19 +310,16 @@ static int meson_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  {
>  	struct meson_pwm_channel *channel = pwm_get_chip_data(pwm);
>  	struct meson_pwm *meson = to_meson_pwm(chip);
> -	unsigned long flags;
>  	int err = 0;
>  
>  	if (!state)
>  		return -EINVAL;
>  
> -	spin_lock_irqsave(&meson->lock, flags);
> -
>  	if (!state->enabled) {
>  		meson_pwm_disable(meson, pwm->hwpwm);
>  		channel->state.enabled = false;
>  
> -		goto unlock;
> +		return 0;
>  	}
>  
>  	if (state->period != channel->state.period ||
> @@ -329,7 +340,7 @@ static int meson_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  		err = meson_pwm_calc(meson, channel, pwm->hwpwm,
>  				     state->duty_cycle, state->period);
>  		if (err < 0)
> -			goto unlock;
> +			return err;
>  
>  		channel->state.polarity = state->polarity;
>  		channel->state.period = state->period;
> @@ -341,9 +352,7 @@ static int meson_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  		channel->state.enabled = true;
>  	}
>  
> -unlock:
> -	spin_unlock_irqrestore(&meson->lock, flags);
> -	return err;
> +	return 0;
>  }
>  
>  static void meson_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
> 

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>

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Re: [PATCH v2 1/1] pwm: meson: use the spin-lock only to protect register modifications

[Thierry Reding]

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On Mon, Apr 01, 2019 at 07:57:48PM +0200, Martin Blumenstingl wrote:
> Holding the spin-lock for all of the code in meson_pwm_apply() can
> result in a "BUG: scheduling while atomic". This can happen because
> clk_get_rate() (which is called from meson_pwm_calc()) may sleep.
> Only hold the spin-lock when modifying registers to solve this.
> 
> The reason why we need a spin-lock in the driver is because the
> REG_MISC_AB register is shared between the two channels provided by one
> PWM controller. The only functions where REG_MISC_AB is modified are
> meson_pwm_enable() and meson_pwm_disable() so the register reads/writes
> in there need to be protected by the spin-lock.
> 
> The original code also used the spin-lock to protect the values in
> struct meson_pwm_channel. This could be necessary if two consumers can
> use the same PWM channel. However, PWM core doesn't allow this so we
> don't need to protect the values in struct meson_pwm_channel with a
> lock.
> 
> Fixes: 211ed630753d2f ("pwm: Add support for Meson PWM Controller")
> Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
> Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
> ---
>  drivers/pwm/pwm-meson.c | 25 +++++++++++++++++--------
>  1 file changed, 17 insertions(+), 8 deletions(-)

Applied, thanks.

Thierry

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