Re: [PATCH v2 Resend] pwm: samsung: Fix output race on disabling

Sjoerd Simons
2015-03-18T07:50:50Z

Hey Jingoo, Kukjijn, Lukasz,

Pinging on this one again, could you please review this patch so it can
be merged through the PWM tree?

On Thu, 2015-03-05 at 09:14 +0100, Sjoerd Simons wrote:
> When disabling the samsung PWM the output state remains at the level it
> was in the end of a pwm cycle. In other words, calling pwm_disable when
> at 100% duty will keep the output active, while at all other setting the
> output will go/stay inactive. On top of that the samsung PWM settings are
> double-buffered, which means the new settings only get applied at the
> start of a new PWM cycle.
>
> This results in a race if the PWM is at 100% duty and a driver calls:
> pwm_config (pwm, 0, period);
> pwm_disable (pwm);
>
> In this case the PWMs output will unexpectedly stay active, unless a new
> PWM cycle happened to start between the register writes in _config and
> _disable. As far as i can tell this is a regression introduced by 3bdf878,
> before that a call to pwm_config would call pwm_samsung_enable which,
> while heavy-handed, made sure the expected settings were live.
>
> To resolve this, while not re-introducing the issues 3bdf878 (flickering
> as the PWM got reset while in a PWM cycle). Only force an update of the
> settings when at 100% duty, which shouldn't have a noticeable effect on
> the output but is enough to ensure the behaviour is as expected on
> disable.
>
> Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
> ---
> Changes since v1:
> Fix small issues pointed out by Tomasz Figa
> - Correct various coding style issues
> - Read the current value of the tcmp register for comparison rather then
> using a non-trivial comparison to decide whether the current state was
> 100% duty
> - Move the code to force manual update out into its own function
> - Clarify the comment indicating why a manual update is sometimes required
>
> drivers/pwm/pwm-samsung.c | 31 ++++++++++++++++++++++++++++++-
> 1 file changed, 30 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/pwm/pwm-samsung.c b/drivers/pwm/pwm-samsung.c
> index 3e9b583..649f6c4 100644
> --- a/drivers/pwm/pwm-samsung.c
> +++ b/drivers/pwm/pwm-samsung.c
> @@ -269,12 +269,31 @@ static void pwm_samsung_disable(struct pwm_chip *chip, struct pwm_device *pwm)
> spin_unlock_irqrestore(&samsung_pwm_lock, flags);
> }
>
> +static void pwm_samsung_manual_update(struct samsung_pwm_chip *chip,
> + struct pwm_device *pwm)
> +{
> + unsigned int tcon_chan = to_tcon_channel(pwm->hwpwm);
> + u32 tcon;
> + unsigned long flags;
> +
> + spin_lock_irqsave(&samsung_pwm_lock, flags);
> +
> + tcon = readl(chip->base + REG_TCON);
> + tcon |= TCON_MANUALUPDATE(tcon_chan);
> + writel(tcon, chip->base + REG_TCON);
> +
> + tcon &= ~TCON_MANUALUPDATE(tcon_chan);
> + writel(tcon, chip->base + REG_TCON);
> +
> + spin_unlock_irqrestore(&samsung_pwm_lock, flags);
> +}
> +
> static int pwm_samsung_config(struct pwm_chip *chip, struct pwm_device *pwm,
> int duty_ns, int period_ns)
> {
> struct samsung_pwm_chip *our_chip = to_samsung_pwm_chip(chip);
> struct samsung_pwm_channel *chan = pwm_get_chip_data(pwm);
> - u32 tin_ns = chan->tin_ns, tcnt, tcmp;
> + u32 tin_ns = chan->tin_ns, tcnt, tcmp, oldtcmp;
>
> /*
> * We currently avoid using 64bit arithmetic by using the
> @@ -288,6 +307,7 @@ static int pwm_samsung_config(struct pwm_chip *chip, struct pwm_device *pwm,
> return 0;
>
> tcnt = readl(our_chip->base + REG_TCNTB(pwm->hwpwm));
> + oldtcmp = readl(our_chip->base + REG_TCMPB(pwm->hwpwm));
>
> /* We need tick count for calculation, not last tick. */
> ++tcnt;
> @@ -335,6 +355,15 @@ static int pwm_samsung_config(struct pwm_chip *chip, struct pwm_device *pwm,
> writel(tcnt, our_chip->base + REG_TCNTB(pwm->hwpwm));
> writel(tcmp, our_chip->base + REG_TCMPB(pwm->hwpwm));
>
> + /* In case the PWM is currently at 100% duty, force a manual update
> + * to prevent the signal staying high in the pwm is disabled shortly
> + * afer this update (before it autoreloaded the new values) .
> + */
> + if (oldtcmp == (u32) -1) {
> + dev_dbg(our_chip->chip.dev, "Forcing manual update");
> + pwm_samsung_manual_update(our_chip, pwm);
> + }
> +
> chan->period_ns = period_ns;
> chan->tin_ns = tin_ns;
> chan->duty_ns = duty_ns;