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

[Leif Middelschulte]

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

A race condition might occur, ultimately leading to switching off the
PWM that is supposed to be turned on.
The condition is more likely, if `CONFIG_PWM_DEBUG` is set and the PWM
has been enabled before Linux is booted.

After writing some value to the register linked to the duty cycle
(`MX3_PWMSAR`), the related debug function
(`core.c:pwm_apply_state_debug`) reads back (`.get_state`)
a wrong value (`0`) as the configured duty cycle. This value is stored
as part of a temporary state variable that it subsequently reapplies
to the PWM for testing purposes. Which, effectively, turns off the PWM.

This patch postpones the return of the `.apply` function until either
the written value can be read back as expected or the operation times
out.

Signed-off-by: Leif Middelschulte <Leif.Middelschulte at klsmartin.com>
---
 drivers/pwm/pwm-imx27.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/drivers/pwm/pwm-imx27.c b/drivers/pwm/pwm-imx27.c
index 29a3089c534c..9b473fe10cb9 100644
--- a/drivers/pwm/pwm-imx27.c
+++ b/drivers/pwm/pwm-imx27.c
@@ -15,6 +15,7 @@
 #include <linux/delay.h>
 #include <linux/err.h>
 #include <linux/io.h>
+#include <linux/iopoll.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/of.h>
@@ -223,7 +224,7 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	unsigned long long c;
 	unsigned long long clkrate;
 	int ret;
-	u32 cr;
+	u32 cr, val;
 
 	pwm_get_state(pwm, &cstate);
 
@@ -290,6 +291,18 @@ static int pwm_imx27_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	if (!state->enabled)
 		pwm_imx27_clk_disable_unprepare(imx);
 
+	/*
+	 * According to imx pwm RM the value can be:
+	 * - written at any time
+	 * - only be read, if the pwm is enabled
+	 * Yet it returns a wrong value (i.e. within `pwm_imx27_get_state`) if it is subsequently read (while enabled).
+	 * Apparently it takes the value some cycles to propagate.
+	 * Wait a bit to make sure the right value can be read by other functions, before returning.
+	 */
+	ret = readl_relaxed_poll_timeout(imx->mmio_base + MX3_PWMSAR, val, val == duty_cycles, 20000, 300000);
+	if (ret)
+		return ret;
+
 	return 0;
 }
 
-- 
2.39.1