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

Sjoerd Simons
2015-03-05T08:14:08Z

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;
--
2.1.4