Re: [PATCH] [RFC] pwm: sun4i: Move pwm_calculate out of spin_lock

Re: [PATCH] [RFC] pwm: sun4i: Move pwm_calculate out of spin_lock

[Uwe Kleine-König]

Hello Clément,

On Fri, Jan 10, 2020 at 12:31:06AM +0100, Clément Péron wrote:
> pwm_calculate calls clk_get_rate while holding a spin_lock.
> 
> This create an issue as clk_get_rate() may sleep.

Slightly orthogonal to this issue, it might be a good idea to add a
might_sleep() to clk_get_rate(). (Added clk maintainers to Cc: for this
suggestion.)

> Move pwm_calculate out of this spin_lock.
> 
> Fixes: c32c5c50d4fe ("pwm: sun4i: Switch to atomic PWM")
> Reported-by: Alex Mobigo <alex.mobigo@gmail.com>
> Suggested-by: Vasily Khoruzhick <anarsoul@gmail.com>
> Signed-off-by: Clément Péron <peron.clem@gmail.com>
> ---
> 
> Hi,
> 
> this issue has been reported on linux-sunxi Google groups.
> 
> I don't have a board with PWM to confirm it.
> 
> Please wait a tested-by.
> 
> Thanks,
> Clément
> 
>  drivers/pwm/pwm-sun4i.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
> index 1afd41ebd3fd..6b230029dc49 100644
> --- a/drivers/pwm/pwm-sun4i.c
> +++ b/drivers/pwm/pwm-sun4i.c
> @@ -248,19 +248,18 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  		}
>  	}
>  
> -	spin_lock(&sun4i_pwm->ctrl_lock);
> -	ctrl = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
> -
>  	ret = sun4i_pwm_calculate(sun4i_pwm, state, &duty, &period, &prescaler,
>  				  &bypass);
>  	if (ret) {
>  		dev_err(chip->dev, "period exceeds the maximum value\n");
> -		spin_unlock(&sun4i_pwm->ctrl_lock);
>  		if (!cstate.enabled)
>  			clk_disable_unprepare(sun4i_pwm->clk);
>  		return ret;
>  	}
>  
> +	spin_lock(&sun4i_pwm->ctrl_lock);
> +	ctrl = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
> +
>  	if (sun4i_pwm->data->has_direct_mod_clk_output) {
>  		if (bypass) {
>  			ctrl |= BIT_CH(PWM_BYPASS, pwm->hwpwm);

As sun4i_pwm_calculate does nothing that depends on (or modifies)
hardware state (apart from clk_get_rate(sun4i_pwm->clk) which can be
assumed to be constant) the change looks good.

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

Best regards
Uwe

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