linux-kernel - Re: [linux-sunxi] [RFC PATCH 4/4] pwm: sun4i: Delay after writing the period

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Date:   Tue, 21 Apr 2020 22:43:37 -0500
From:   Samuel Holland <samuel@...lland.org>
To:     dev@...calroeleven.nl, Thierry Reding <thierry.reding@...il.com>,
        Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
        Maxime Ripard <mripard@...nel.org>,
        Chen-Yu Tsai <wens@...e.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        linux-pwm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Cc:     linux-sunxi@...glegroups.com
Subject: Re: [linux-sunxi] [RFC PATCH 4/4] pwm: sun4i: Delay after writing the
 period

Hello Pascal,

On 3/17/20 10:59 AM, Pascal Roeleven wrote:
> When disabling, ensure the period write is complete before continuing.
> This fixes an issue on some devices when the write isn't complete before
> the panel is turned off but the clock gate is still on.
> 
> Signed-off-by: Pascal Roeleven <dev@...calroeleven.nl>
> ---
>  drivers/pwm/pwm-sun4i.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
> index a11d00f96..75250fd4c 100644
> --- a/drivers/pwm/pwm-sun4i.c
> +++ b/drivers/pwm/pwm-sun4i.c
> @@ -299,6 +299,10 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  	sun4i_pwm_writel(sun4i_pwm, val, PWM_CH_PRD(pwm->hwpwm));
>  	next_period = jiffies + usecs_to_jiffies(cstate.period / 1000 + 1);
>  
> +	/* When disabling, make sure the period register is written first */
> +	if (!state->enabled && cstate.enabled)
> +		sun4i_pwm_wait(next_period);
> +

It is not visible from the context of this patch, but this call to
sun4i_pwm_wait() ends up calling msleep() inside a spinlock, which isn't
allowed. The spinlock should probably be converted to a mutex, considering that
sun4i_pwm_apply() already sleeps and takes mutexes.

Regards,
Samuel

>  	if (state->polarity != PWM_POLARITY_NORMAL)
>  		ctrl &= ~BIT_CH(PWM_ACT_STATE, pwm->hwpwm);
>  	else
> @@ -320,6 +324,7 @@ static int sun4i_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
>  		return 0;
>  
>  	/* We need a full period to elapse before disabling the channel. */
> +	next_period = jiffies + usecs_to_jiffies(cstate.period / 1000 + 1);
>  	sun4i_pwm_wait(next_period);
>  
>  	spin_lock(&sun4i_pwm->ctrl_lock);
>