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@xxxxxxxxxxxxxxx>
---
drivers/pwm/pwm-samsung.c | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)
diff --git a/drivers/pwm/pwm-samsung.c b/drivers/pwm/pwm-samsung.c
index 3e9b583..3e252dc 100644
--- a/drivers/pwm/pwm-samsung.c
+++ b/drivers/pwm/pwm-samsung.c
@@ -335,6 +335,27 @@ 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, trigger a manual update to
+ * prevent unexpected results when disabling the pwm */
+ if (chan->period_ns/chan->tin_ns == chan->duty_ns/chan->tin_ns) {
+ unsigned int tcon_chan = to_tcon_channel(pwm->hwpwm);
+ u32 tcon;
+ unsigned long flags;
+
+ dev_dbg(our_chip->chip.dev, "Forcing manual update");
+
+ spin_lock_irqsave(&samsung_pwm_lock, flags);
+
+ tcon = readl(our_chip->base + REG_TCON);
+ tcon |= TCON_MANUALUPDATE(tcon_chan);
+ writel(tcon, our_chip->base + REG_TCON);
+
+ tcon &= ~TCON_MANUALUPDATE(tcon_chan);
+ writel(tcon, our_chip->base + REG_TCON);
+
+ spin_unlock_irqrestore(&samsung_pwm_lock, flags);
+ }
+
chan->period_ns = period_ns;
chan->tin_ns = tin_ns;
chan->duty_ns = duty_ns;
--
2.1.4
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