pwm: samsung: Fix output race on disabling

When disabling the Samsung PWM the output state remains at the level it
was at the end of a PWM cycle. In other words, calling pwm_disable()
when at 100% duty cycle will keep the output active, while at all other
settings 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 cycle 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 pwm_config()
and pwm_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) fixed, only force an update
of the settings when at 100% duty cycle, which shouldn't have any
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>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Acked-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

1 files changed, 31 insertions, 1 deletions

diff --git a/drivers/pwm/pwm-samsung.c b/drivers/pwm/pwm-samsung.c
index 3e9b5835a4af..ff201e1b9219 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 +355,16 @@ 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 cycle, force a manual
+         * update to prevent the signal staying high if 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;