pwm: lpss: Avoid reconfiguring while UPDATE bit is still enabled

PWM Configuration register has SW_UPDATE bit that is set when a new
configuration is written to the register. The bit is automatically
cleared at the start of the next output cycle by the IP block.

If one writes a new configuration to the register while it still has
the bit enabled, PWM may freeze. That is, while one can still write
to the register, it won't have an effect. Thus, we try to sleep long
enough that the bit gets cleared and make sure the bit is not
enabled while we update the configuration.

Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Tested-by: Richard Griffiths <richard.a.griffiths@intel.com>
Signed-off-by: Ilkka Koskinen <ilkka.koskinen@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-lpss.c

@@ -15,6 +15,7 @@
 
 #include <linux/delay.h>
 #include <linux/io.h>
+#include <linux/iopoll.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/pm_runtime.h>
@@ -80,17 +81,37 @@ static inline void pwm_lpss_write(const struct pwm_device *pwm, u32 value)
 	writel(value, lpwm->regs + pwm->hwpwm * PWM_SIZE + PWM);
 }
 
-static void pwm_lpss_update(struct pwm_device *pwm)
+static int pwm_lpss_update(struct pwm_device *pwm)
 {
-	/*
-	 * Set a limit for busyloop since not all implementations correctly
-	 * clear PWM_SW_UPDATE bit (at least it's not visible on OS side).
-	 */
-	unsigned int count = 10;
+	struct pwm_lpss_chip *lpwm = to_lpwm(pwm->chip);
+	const void __iomem *addr = lpwm->regs + pwm->hwpwm * PWM_SIZE + PWM;
+	const unsigned int ms = 500 * USEC_PER_MSEC;
+	u32 val;
+	int err;
 
 	pwm_lpss_write(pwm, pwm_lpss_read(pwm) | PWM_SW_UPDATE);
-	while (pwm_lpss_read(pwm) & PWM_SW_UPDATE && --count)
-		usleep_range(10, 20);
+
+	/*
+	 * PWM Configuration register has SW_UPDATE bit that is set when a new
+	 * configuration is written to the register. The bit is automatically
+	 * cleared at the start of the next output cycle by the IP block.
+	 *
+	 * If one writes a new configuration to the register while it still has
+	 * the bit enabled, PWM may freeze. That is, while one can still write
+	 * to the register, it won't have an effect. Thus, we try to sleep long
+	 * enough that the bit gets cleared and make sure the bit is not
+	 * enabled while we update the configuration.
+	 */
+	err = readl_poll_timeout(addr, val, !(val & PWM_SW_UPDATE), 40, ms);
+	if (err)
+		dev_err(pwm->chip->dev, "PWM_SW_UPDATE was not cleared\n");
+
+	return err;
+}
+
+static inline int pwm_lpss_is_updating(struct pwm_device *pwm)
+{
+	return (pwm_lpss_read(pwm) & PWM_SW_UPDATE) ? -EBUSY : 0;
 }
 
 static void pwm_lpss_prepare(struct pwm_lpss_chip *lpwm, struct pwm_device *pwm,
@@ -129,16 +150,29 @@ static int pwm_lpss_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 			  struct pwm_state *state)
 {
 	struct pwm_lpss_chip *lpwm = to_lpwm(chip);
+	int ret;
 
 	if (state->enabled) {
 		if (!pwm_is_enabled(pwm)) {
 			pm_runtime_get_sync(chip->dev);
+			ret = pwm_lpss_is_updating(pwm);
+			if (ret) {
+				pm_runtime_put(chip->dev);
+				return ret;
+			}
 			pwm_lpss_prepare(lpwm, pwm, state->duty_cycle, state->period);
-			pwm_lpss_update(pwm);
+			ret = pwm_lpss_update(pwm);
+			if (ret) {
+				pm_runtime_put(chip->dev);
+				return ret;
+			}
 			pwm_lpss_write(pwm, pwm_lpss_read(pwm) | PWM_ENABLE);
 		} else {
+			ret = pwm_lpss_is_updating(pwm);
+			if (ret)
+				return ret;
 			pwm_lpss_prepare(lpwm, pwm, state->duty_cycle, state->period);
-			pwm_lpss_update(pwm);
+			return pwm_lpss_update(pwm);
 		}
 	} else if (pwm_is_enabled(pwm)) {
 		pwm_lpss_write(pwm, pwm_lpss_read(pwm) & ~PWM_ENABLE);