[PATCH v2] pwm: imx27: fix race condition .apply,.get_state

[Leif Middelschulte]

From: Leif Middelschulte <Leif.Middelschulte at klsmartin.com>

With CONFIG_PWM_DEBUG=y after writing a value to the PWMSAR
register in .apply(), the register is read in .get_state().
Unless a period completed in the meantime, this read yields the
previously used duty cycle configuration. As the PWM_DEBUG code
applies the read out configuration for testing purposes this
effectively undoes the intended effect by rewriting the previous
hardware state.

Note that this change merely implements a sensible heuristic.
The i.MX has a 4 slot FIFO to configure the duty cycle. This FIFO
cannot be read back in its entirety. The "write x then read back
x from hw" semantics are therefore not easily applicable.
With this change, the .get_state() function tries to wait for some
stabilization in the FIFO (empty state). In this state it keeps
applying the last value written to the sample register.

Signed-off-by: Leif Middelschulte <Leif.Middelschulte at klsmartin.com>
---
 drivers/pwm/pwm-imx27.c | 50 ++++++++++++++++++++++++++++++++++++++---
 1 file changed, 47 insertions(+), 3 deletions(-)

diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c
index 29a3089c534c..32389ca2da3e 100644
--- a/drivers/pwm/pwm-imx27.c
+++ b/drivers/pwm/pwm-imx27.c
@@ -75,6 +75,7 @@
 						   (x)) + 1)
 
 #define MX3_PWM_SWR_LOOP		5
+#define MX3_PWM_FIFOAV_EMPTY_LOOP	4
 
 /* PWMPR register value of 0xffff has the same effect as 0xfffe */
 #define MX3_PWMPR_MAX			0xfffe
@@ -118,8 +119,28 @@ static void pwm_imx27_clk_disable_unprepare(struct pwm_imx27_chip *imx)
 	clk_disable_unprepare(imx->clk_ipg);
 }
 
+static int pwm_imx27_wait_fifo_empty(struct pwm_chip *chip,
+				     struct pwm_device *pwm)
+{
+	struct pwm_imx27_chip *imx = to_pwm_imx27_chip(chip);
+	struct device *dev = chip->dev;
+	unsigned int period_ms = DIV_ROUND_UP_ULL(pwm_get_period(pwm), NSEC_PER_MSEC);
+	int tries = MX3_PWM_FIFOAV_EMPTY_LOOP;
+	int fifoav;
+	u32 sr;
+
+	while (tries--) {
+		sr = readl(imx->mmio_base + MX3_PWMSR);
+		fifoav = FIELD_GET(MX3_PWMSR_FIFOAV, sr);
+		if (fifoav == MX3_PWMSR_FIFOAV_EMPTY)
+			return;
+		msleep(period_ms);
+	}
+	dev_warn(dev, "FIFO has been refilled concurrently\n");
+}
+
 static int pwm_imx27_get_state(struct pwm_chip *chip,
-			       struct pwm_device *pwm, struct pwm_state *state)
+				struct pwm_device *pwm, struct pwm_state *state)
 {
 	struct pwm_imx27_chip *imx = to_pwm_imx27_chip(chip);
 	u32 period, prescaler, pwm_clk, val;
@@ -161,10 +182,33 @@ static int pwm_imx27_get_state(struct pwm_chip *chip,
 	 * PWMSAR can be read only if PWM is enabled. If the PWM is disabled,
 	 * use the cached value.
 	 */
-	if (state->enabled)
+	if (state->enabled) {
+		/*
+		 * From the i.MX PWM reference manual:
+		 * "A read on the sample register yields the current FIFO value that
+		 *  is being used, or will be used, by the PWM for generation on the
+		 *  output signal. Therefore, a write and a subsequent read on the
+		 *  sample register may result in different values being obtained."
+		 * Furthermore:
+		 * "When a new value is written, the duty cycle changes after the
+		 *  current period is over."
+		 * Note "changes" vs. "changes to the given value"!
+		 * Finally:
+		 * "The PWM will run at the last set duty-cycle setting if all the
+		 *  values of the FIFO has been utilized, until the FIFO is reloaded
+		 *  or the PWM is disabled."
+		 * Try to be at least a bit more deterministic about which value is
+		 * read by waiting until the FIFO is empty. In this state the last/most
+		 * recently pushed sample (duty cycle) value is continuously applied.
+		 * Beware that this approach is still racy, as a new value could have
+		 * been supplied and a period expired between the call of the wait
+		 * function and the subsequent readl.
+		 */
+		pwm_imx27_wait_fifo_empty(chip, pwm);
 		val = readl(imx->mmio_base + MX3_PWMSAR);
-	else
+	} else {
 		val = imx->duty_cycle;
+	}
 
 	tmp = NSEC_PER_SEC * (u64)(val) * prescaler;
 	state->duty_cycle = DIV_ROUND_UP_ULL(tmp, pwm_clk);
-- 
2.39.1