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Subject[PATCH AUTOSEL 4.19 031/141] pwm: meson: Use the spin-lock only to protect register modifications
Date
From: Martin Blumenstingl <martin.blumenstingl@googlemail.com>

[ Upstream commit f173747fffdf037c791405ab4f1ec0eb392fc48e ]

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>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 c1ed641b3e266..f6e738ad7bd92 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,
--
2.20.1
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