linux-kernel - Re: [1/2] pwm: sunxi: allow the pwm to finish its pulse before disable

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Date:   Sat, 24 Sep 2016 00:02:13 +1000
From:   Jonathan Liu <net147@...il.com>
To:     oliver@...inagl.nl
Cc:     Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Chen-Yu Tsai <wens@...e.org>, linux-pwm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [1/2] pwm: sunxi: allow the pwm to finish its pulse before disable

On 26 August 2016 at 03:50,  <oliver@...inagl.nl> wrote:
> When we inform the PWM block to stop toggeling the output, we may end up
> in a state where the output is not what we would expect (e.g. not the
> low-pulse) but whatever the output was at when the clock got disabled.
>
> To counter this we have to wait for maximally the time of one whole
> period to ensure the pwm hardware was able to finish. Since we already
> told the PWM hardware to disable it self, it will not continue toggling
> but merly finish its current pulse.
>
> If a whole period is considered to much, it may be contemplated to use a
> half period + a little bit to ensure we get passed the transition.
>
> Signed-off-by: Olliver Schinagl <oliver@...inagl.nl>
> ---
>  drivers/pwm/pwm-sun4i.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
>
> diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
> index 03a99a5..5e97c8a 100644
> --- a/drivers/pwm/pwm-sun4i.c
> +++ b/drivers/pwm/pwm-sun4i.c
> @@ -8,6 +8,7 @@
>
>  #include <linux/bitops.h>
>  #include <linux/clk.h>
> +#include <linux/delay.h>
>  #include <linux/err.h>
>  #include <linux/io.h>
>  #include <linux/module.h>
> @@ -245,6 +246,16 @@ static void sun4i_pwm_disable(struct pwm_chip *chip, struct pwm_device *pwm)
>         spin_lock(&sun4i_pwm->ctrl_lock);
>         val = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
>         val &= ~BIT_CH(PWM_EN, pwm->hwpwm);
> +       sun4i_pwm_writel(sun4i_pwm, val, PWM_CTRL_REG);
> +       spin_unlock(&sun4i_pwm->ctrl_lock);
> +
> +       /* Allow for the PWM hardware to finish its last toggle. The pulse
> +        * may have just started and thus we should wait a full period.
> +        */
> +       ndelay(pwm_get_period(pwm));
> +
> +       spin_lock(&sun4i_pwm->ctrl_lock);
> +       val = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
>         val &= ~BIT_CH(PWM_CLK_GATING, pwm->hwpwm);
>         sun4i_pwm_writel(sun4i_pwm, val, PWM_CTRL_REG);
>         spin_unlock(&sun4i_pwm->ctrl_lock);

I had some issues where setting the brightness for pwm-backlight to 0
was not turning off the backlight sometimes.
This patch fixes the issue for me. Thanks.

Tested-by: Jonathan Liu <net147@...il.com>