[PATCH v8 06/17] pwm: lpss: Use pwm_lpss_restore() when restoring state on resume

[Hans de Goede <hdegoede@redhat.com>]

Before this commit a suspend + resume of the LPSS PWM controller
would result in the controller being reset to its defaults of
output-freq = clock/256, duty-cycle=100%, until someone changes
to the output-freq and/or duty-cycle are made.

This problem has been masked so far because the main consumer
(the i915 driver) was always making duty-cycle changes on resume.
With the conversion of the i915 driver to the atomic PWM API the
driver now only disables/enables the PWM on suspend/resume leaving
the output-freq and duty as is, triggering this problem.

The LPSS PWM controller has a mechanism where the ctrl register value
and the actual base-unit and on-time-div values used are latched. When
software sets the SW_UPDATE bit then at the end of the current PWM cycle,
the new values from the ctrl-register will be latched into the actual
registers, and the SW_UPDATE bit will be cleared.

The problem is that before this commit our suspend/resume handling
consisted of simply saving the PWM ctrl register on suspend and
restoring it on resume, without setting the PWM_SW_UPDATE bit.
When the controller has lost its state over a suspend/resume and thus
has been reset to the defaults, just restoring the register is not
enough. We must also set the SW_UPDATE bit to tell the controller to
latch the restored values into the actual registers.

Fixing this problem is not as simple as just or-ing in the value which
is being restored with SW_UPDATE. If the PWM was enabled before we must
write the new settings + PWM_SW_UPDATE before setting PWM_ENABLE.
We must also wait for PWM_SW_UPDATE to become 0 again and depending on the
model we must do this either before or after the setting of PWM_ENABLE.

All the necessary logic for doing this is already present inside
pwm_lpss_apply(), so instead of duplicating this inside the resume
handler, this commit adds a new pwm_lpss_restore() helper which mirrors
pwm_lpss_apply() minus the runtime-pm reference handling (which we should
not change on resume).

This fixes the output-freq and duty-cycle being reset to their defaults
on resume.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
---
Changes in v8:
- Drop optimization to skip restore if current ctrl reg is the same as our saved
  ctrl reg value (because this causes issues on some devices)
- Simplify pwm_lpss_restore_state() to not rely on the current state
- Modify commit message to mention the new pwm_lpss_restore_state() helper

Changes in v6:
- Add a pwm_lpss_restore_state() helper for re-applying the PWM state on resume

Changes in v5:
- The changes to pwm_lpss_apply() are much cleaner now thanks to the new
  pwm_lpss_prepare_enable() helper.

Changes in v3:
- This replaces the "pwm: lpss: Set SW_UPDATE bit when enabling the PWM"
  patch from previous versions of this patch-set, which really was a hack
  working around the resume issue which this patch fixes properly.
---
 drivers/pwm/pwm-lpss.c | 40 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 37 insertions(+), 3 deletions(-)

diff --git a/drivers/pwm/pwm-lpss.c b/drivers/pwm/pwm-lpss.c
index 8a136ba2a583..9a7400c6fb6e 100644
--- a/drivers/pwm/pwm-lpss.c
+++ b/drivers/pwm/pwm-lpss.c
@@ -166,6 +166,25 @@ static int pwm_lpss_apply(struct pwm_chip *chip, struct pwm_device *pwm,
 	return ret;
 }
 
+/*
+ * This is a mirror of pwm_lpss_apply() without relying on the current state
+ * (no pwm_is_enabled() calls) and without pm_runtime reference handling,
+ * for restoring the PWM state on resume.
+ */
+static int pwm_lpss_restore_state(struct pwm_lpss_chip *lpwm,
+				  struct pwm_device *pwm,
+				  const struct pwm_state *state)
+{
+	int ret = 0;
+
+	if (state->enabled)
+		ret = pwm_lpss_prepare_enable(lpwm, pwm, state, true);
+	else
+		pwm_lpss_write(pwm, pwm_lpss_read(pwm) & ~PWM_ENABLE);
+
+	return ret;
+}
+
 static void pwm_lpss_get_state(struct pwm_chip *chip, struct pwm_device *pwm,
 			       struct pwm_state *state)
 {
@@ -278,10 +297,25 @@ EXPORT_SYMBOL_GPL(pwm_lpss_suspend);
 int pwm_lpss_resume(struct device *dev)
 {
 	struct pwm_lpss_chip *lpwm = dev_get_drvdata(dev);
-	int i;
+	struct pwm_device *pwm;
+	int i, ret;
 
-	for (i = 0; i < lpwm->info->npwm; i++)
-		writel(lpwm->saved_ctrl[i], lpwm->regs + i * PWM_SIZE + PWM);
+	for (i = 0; i < lpwm->info->npwm; i++) {
+		pwm = &lpwm->chip.pwms[i];
+
+		/*
+		 * We cannot just blindly restore the old value here. Since we
+		 * are changing the settings we must set SW_UPDATE and if the
+		 * PWM was enabled before we must write the new settings +
+		 * PWM_SW_UPDATE before setting PWM_ENABLE. We must also wait
+		 * for PWM_SW_UPDATE to become 0 again and depending on the
+		 * model we must do this either before or after the setting of
+		 * PWM_ENABLE.
+		 */
+		ret = pwm_lpss_restore_state(lpwm, pwm, &pwm->state);
+		if (ret)
+			dev_err(dev, "Error restoring state on resume\n");
+	}
 
 	return 0;
 }
-- 
2.28.0