diff mbox series

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

Message ID 20190324220217.15813-2-martin.blumenstingl@googlemail.com
State Changes Requested
Headers show
Series pwm: meson: fix scheduling while atomic issue | expand

Commit Message

Martin Blumenstingl March 24, 2019, 10:02 p.m. UTC 
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>
---
 drivers/pwm/pwm-meson.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

Comments

Uwe Kleine-König March 25, 2019, 8:48 a.m. UTC | #1 
Hello,

On Sun, Mar 24, 2019 at 11:02:17PM +0100, 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>

Looks right.

Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

Bonus points for adding a comment to struct meson_pwm::lock. Something
like:

	/*
	 * Protects register access to the MISC_AB register that is
	 * shared between the two PWMs.
	 */

Best regards
Uwe
diff mbox series

Patch

diff --git a/drivers/pwm/pwm-meson.c b/drivers/pwm/pwm-meson.c
index c1ed641b3e26..2b03938039b6 100644
--- a/drivers/pwm/pwm-meson.c
+++ b/drivers/pwm/pwm-meson.c
@@ -235,6 +235,7 @@  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 +256,8 @@  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 +270,14 @@  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 +292,13 @@  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 +306,16 @@  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 +336,7 @@  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 +348,7 @@  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,