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

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Date:   Wed, 3 Apr 2019 13:21:24 +0200
From:   Neil Armstrong <narmstrong@...libre.com>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        thierry.reding@...il.com, linux-pwm@...r.kernel.org,
        linux-amlogic@...ts.infradead.org
Cc:     jbrunet@...libre.com, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, u.kleine-koenig@...gutronix.de
Subject: Re: [PATCH v2 1/1] pwm: meson: use the spin-lock only to protect
 register modifications

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@...glemail.com>
> Reviewed-by: Uwe Kleine-König <u.kleine-koenig@...gutronix.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@...libre.com>