pwm: imx27: Cache duty cycle register value

The hardware register containing the duty cycle value cannot be accessed
when the PWM is disabled. This causes the ->get_state() callback to read
back a duty cycle value of 0, which can confuse consumer drivers.
Tested-by: Michal Vokáč <michal.vokac@ysoft.com>
Tested-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-imx27.c

@@ -85,6 +85,13 @@ struct pwm_imx27_chip {
 	struct clk	*clk_per;
 	void __iomem	*mmio_base;
 	struct pwm_chip	chip;
+
+	/*
+	 * The driver cannot read the current duty cycle from the hardware if
+	 * the hardware is disabled. Cache the last programmed duty cycle
+	 * value to return in that case.
+	 */
+	unsigned int duty_cycle;
 };
 
 #define to_pwm_imx27_chip(chip)	container_of(chip, struct pwm_imx27_chip, chip)
@@ -155,14 +162,17 @@ static void pwm_imx27_get_state(struct pwm_chip *chip,
 	tmp = NSEC_PER_SEC * (u64)(period + 2);
 	state->period = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);
 
-	/* PWMSAR can be read only if PWM is enabled */
-	if (state->enabled) {
+	/*
+	 * PWMSAR can be read only if PWM is enabled. If the PWM is disabled,
+	 * use the cached value.
+	 */
+	if (state->enabled)
 		val = readl(imx->mmio_base + MX3_PWMSAR);
-		tmp = NSEC_PER_SEC * (u64)(val);
-		state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);
-	} else {
-		state->duty_cycle = 0;
-	}
+	else
+		val = imx->duty_cycle;
+
+	tmp = NSEC_PER_SEC * (u64)(val);
+	state->duty_cycle = DIV_ROUND_CLOSEST_ULL(tmp, pwm_clk);
 
 	if (!state->enabled)
 		pwm_imx27_clk_disable_unprepare(chip);
@@ -261,6 +271,13 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 		writel(duty_cycles, imx->mmio_base + MX3_PWMSAR);
 		writel(period_cycles, imx->mmio_base + MX3_PWMPR);
 
+		/*
+		 * Store the duty cycle for future reference in cases where
+		 * the MX3_PWMSAR register can't be read (i.e. when the PWM
+		 * is disabled).
+		 */
+		imx->duty_cycle = duty_cycles;
+
 		cr = MX3_PWMCR_PRESCALER_SET(prescale) |
 		     MX3_PWMCR_STOPEN | MX3_PWMCR_DOZEN | MX3_PWMCR_WAITEN |
 		     FIELD_PREP(MX3_PWMCR_CLKSRC, MX3_PWMCR_CLKSRC_IPG_HIGH) |