linux-kernel - [PATCH 1/2] pwm: sunxi: allow the pwm to finish its pulse before disable

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Message-Id: <1472147411-30424-2-git-send-email-oliver@schinagl.nl>
Date:   Thu, 25 Aug 2016 19:50:10 +0200
From:   Olliver Schinagl <oliver@...inagl.nl>
To:     Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Chen-Yu Tsai <wens@...e.org>
Cc:     linux-pwm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, Olliver Schinagl <oliver@...inagl.nl>
Subject: [PATCH 1/2] pwm: sunxi: allow the pwm to finish its pulse before disable

When we inform the PWM block to stop toggeling the output, we may end up
in a state where the output is not what we would expect (e.g. not the
low-pulse) but whatever the output was at when the clock got disabled.

To counter this we have to wait for maximally the time of one whole
period to ensure the pwm hardware was able to finish. Since we already
told the PWM hardware to disable it self, it will not continue toggling
but merly finish its current pulse.

If a whole period is considered to much, it may be contemplated to use a
half period + a little bit to ensure we get passed the transition.

Signed-off-by: Olliver Schinagl <oliver@...inagl.nl>
---
 drivers/pwm/pwm-sun4i.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/pwm/pwm-sun4i.c b/drivers/pwm/pwm-sun4i.c
index 03a99a5..5e97c8a 100644
--- a/drivers/pwm/pwm-sun4i.c
+++ b/drivers/pwm/pwm-sun4i.c
@@ -8,6 +8,7 @@
 
 #include <linux/bitops.h>
 #include <linux/clk.h>
+#include <linux/delay.h>
 #include <linux/err.h>
 #include <linux/io.h>
 #include <linux/module.h>
@@ -245,6 +246,16 @@ static void sun4i_pwm_disable(struct pwm_chip *chip, struct pwm_device *pwm)
 	spin_lock(&sun4i_pwm->ctrl_lock);
 	val = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
 	val &= ~BIT_CH(PWM_EN, pwm->hwpwm);
+	sun4i_pwm_writel(sun4i_pwm, val, PWM_CTRL_REG);
+	spin_unlock(&sun4i_pwm->ctrl_lock);
+
+	/* Allow for the PWM hardware to finish its last toggle. The pulse
+	 * may have just started and thus we should wait a full period.
+	 */
+	ndelay(pwm_get_period(pwm));
+
+	spin_lock(&sun4i_pwm->ctrl_lock);
+	val = sun4i_pwm_readl(sun4i_pwm, PWM_CTRL_REG);
 	val &= ~BIT_CH(PWM_CLK_GATING, pwm->hwpwm);
 	sun4i_pwm_writel(sun4i_pwm, val, PWM_CTRL_REG);
 	spin_unlock(&sun4i_pwm->ctrl_lock);
-- 
2.8.1