pwm: atmel: Rework tracking updates pending in hardware

This improves the driver's behavior in several ways:

 - The lock is held for shorter periods and so a channel that is currently
   waited for doesn't block disabling another channel.

 - It's easier to understand because the procedure is split into more
   semantic units and documentation is improved

 - A channel is only set to pending when such an event is actually
   scheduled in hardware (by writing the CUPD register).

 - Also wait in .get_state() to report the last configured state instead
   of (maybe) the previous one. This fixes the read back duty cycle and so
   prevents a warning being emitted when PWM_DEBUG is on.

Tested on an AriettaG25.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Author: Uwe Kleine-König

drivers/pwm/pwm-atmel.c

@@ -84,9 +84,19 @@ struct atmel_pwm_chip {
 	void __iomem *base;
 	const struct atmel_pwm_data *data;
 
-	unsigned int updated_pwms;
-	/* ISR is cleared when read, ensure only one thread does that */
-	struct mutex isr_lock;
+	/*
+	 * The hardware supports a mechanism to update a channel's duty cycle at
+	 * the end of the currently running period. When such an update is
+	 * pending we delay disabling the PWM until the new configuration is
+	 * active because otherwise pmw_config(duty_cycle=0); pwm_disable();
+	 * might not result in an inactive output.
+	 * This bitmask tracks for which channels an update is pending in
+	 * hardware.
+	 */
+	u32 update_pending;
+
+	/* Protects .update_pending */
+	spinlock_t lock;
 };
 
 static inline struct atmel_pwm_chip *to_atmel_pwm_chip(struct pwm_chip *chip)
@@ -123,6 +133,64 @@ static inline void atmel_pwm_ch_writel(struct atmel_pwm_chip *chip,
 	atmel_pwm_writel(chip, base + offset, val);
 }
 
+static void atmel_pwm_update_pending(struct atmel_pwm_chip *chip)
+{
+	/*
+	 * Each channel that has its bit in ISR set started a new period since
+	 * ISR was cleared and so there is no more update pending.  Note that
+	 * reading ISR clears it, so this needs to handle all channels to not
+	 * loose information.
+	 */
+	u32 isr = atmel_pwm_readl(chip, PWM_ISR);
+
+	chip->update_pending &= ~isr;
+}
+
+static void atmel_pwm_set_pending(struct atmel_pwm_chip *chip, unsigned int ch)
+{
+	spin_lock(&chip->lock);
+
+	/*
+	 * Clear pending flags in hardware because otherwise there might still
+	 * be a stale flag in ISR.
+	 */
+	atmel_pwm_update_pending(chip);
+
+	chip->update_pending |= (1 << ch);
+
+	spin_unlock(&chip->lock);
+}
+
+static int atmel_pwm_test_pending(struct atmel_pwm_chip *chip, unsigned int ch)
+{
+	int ret = 0;
+
+	spin_lock(&chip->lock);
+
+	if (chip->update_pending & (1 << ch)) {
+		atmel_pwm_update_pending(chip);
+
+		if (chip->update_pending & (1 << ch))
+			ret = 1;
+	}
+
+	spin_unlock(&chip->lock);
+
+	return ret;
+}
+
+static int atmel_pwm_wait_nonpending(struct atmel_pwm_chip *chip, unsigned int ch)
+{
+	unsigned long timeout = jiffies + 2 * HZ;
+	int ret;
+
+	while ((ret = atmel_pwm_test_pending(chip, ch)) &&
+	       time_before(jiffies, timeout))
+		usleep_range(10, 100);
+
+	return ret ? -ETIMEDOUT : 0;
+}
+
 static int atmel_pwm_calculate_cprd_and_pres(struct pwm_chip *chip,
 					     unsigned long clkrate,
 					     const struct pwm_state *state,
@@ -185,6 +253,7 @@ static void atmel_pwm_update_cdty(struct pwm_chip *chip, struct pwm_device *pwm,
 
 	atmel_pwm_ch_writel(atmel_pwm, pwm->hwpwm,
 			    atmel_pwm->data->regs.duty_upd, cdty);
+	atmel_pwm_set_pending(atmel_pwm, pwm->hwpwm);
 }
 
 static void atmel_pwm_set_cprd_cdty(struct pwm_chip *chip,
@@ -205,20 +274,8 @@ static void atmel_pwm_disable(struct pwm_chip *chip, struct pwm_device *pwm,
 	struct atmel_pwm_chip *atmel_pwm = to_atmel_pwm_chip(chip);
 	unsigned long timeout = jiffies + 2 * HZ;
 
-	/*
-	 * Wait for at least a complete period to have passed before disabling a
-	 * channel to be sure that CDTY has been updated
-	 */
-	mutex_lock(&atmel_pwm->isr_lock);
-	atmel_pwm->updated_pwms |= atmel_pwm_readl(atmel_pwm, PWM_ISR);
-
-	while (!(atmel_pwm->updated_pwms & (1 << pwm->hwpwm)) &&
-	       time_before(jiffies, timeout)) {
-		usleep_range(10, 100);
-		atmel_pwm->updated_pwms |= atmel_pwm_readl(atmel_pwm, PWM_ISR);
-	}
+	atmel_pwm_wait_nonpending(atmel_pwm, pwm->hwpwm);
 
-	mutex_unlock(&atmel_pwm->isr_lock);
 	atmel_pwm_writel(atmel_pwm, PWM_DIS, 1 << pwm->hwpwm);
 
 	/*
@@ -292,10 +349,6 @@ static int atmel_pwm_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 			val |= PWM_CMR_CPOL;
 		atmel_pwm_ch_writel(atmel_pwm, pwm->hwpwm, PWM_CMR, val);
 		atmel_pwm_set_cprd_cdty(chip, pwm, cprd, cdty);
-		mutex_lock(&atmel_pwm->isr_lock);
-		atmel_pwm->updated_pwms |= atmel_pwm_readl(atmel_pwm, PWM_ISR);
-		atmel_pwm->updated_pwms &= ~(1 << pwm->hwpwm);
-		mutex_unlock(&atmel_pwm->isr_lock);
 		atmel_pwm_writel(atmel_pwm, PWM_ENA, 1 << pwm->hwpwm);
 	} else if (cstate.enabled) {
 		atmel_pwm_disable(chip, pwm, true);
@@ -326,6 +379,9 @@ static void atmel_pwm_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
 		tmp <<= pres;
 		state->period = DIV64_U64_ROUND_UP(tmp, rate);
 
+		/* Wait for an updated duty_cycle queued in hardware */
+		atmel_pwm_wait_nonpending(atmel_pwm, pwm->hwpwm);
+
 		cdty = atmel_pwm_ch_readl(atmel_pwm, pwm->hwpwm,
 					  atmel_pwm->data->regs.duty);
 		tmp = (u64)(cprd - cdty) * NSEC_PER_SEC;
@@ -416,9 +472,10 @@ static int atmel_pwm_probe(struct platform_device *pdev)
 	if (!atmel_pwm)
 		return -ENOMEM;
 
-	mutex_init(&atmel_pwm->isr_lock);
 	atmel_pwm->data = of_device_get_match_data(&pdev->dev);
-	atmel_pwm->updated_pwms = 0;
+
+	atmel_pwm->update_pending = 0;
+	spin_lock_init(&atmel_pwm->lock);
 
 	atmel_pwm->base = devm_platform_ioremap_resource(pdev, 0);
 	if (IS_ERR(atmel_pwm->base))
@@ -460,7 +517,6 @@ static int atmel_pwm_remove(struct platform_device *pdev)
 	pwmchip_remove(&atmel_pwm->chip);
 
 	clk_unprepare(atmel_pwm->clk);
-	mutex_destroy(&atmel_pwm->isr_lock);
 
 	return 0;
 }