pwm: meson: Use the spin-lock only to protect register modifications

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: 211ed630 ("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>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

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 ||
@@ -324,7 +335,7 @@ 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;
@@ -336,9 +347,7 @@ 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,