| Message ID | 1421962893-23078-1-git-send-email-sjoerd.simons@collabora.co.uk (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
+To Kukjin Jingoo, Kukjin, could one of you review this patch to ensure it's the right thing to do on samsung hardware? On Thu, 2015-01-22 at 22:41 +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;
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;
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(-)