iio: adc: stm32-adc: fix runtime autosuspend delay when slow polling

When the ADC is runtime suspended and starting a conversion, the stm32-adc
driver calls pm_runtime_get_sync() that gets cascaded to the parent
(e.g. runtime resume of stm32-adc-core driver). This also kicks the
autosuspend delay (e.g. 2s) of the parent.
Once the ADC is active, calling pm_runtime_get_sync() again (upon a new
capture) won't kick the autosuspend delay for the parent (stm32-adc-core
driver) as already active.

Currently, this makes the stm32-adc-core driver go in suspend state
every 2s when doing slow polling. As an example, doing a capture, e.g.
cat in_voltageY_raw at a 0.2s rate, the auto suspend delay for the parent
isn't refreshed. Once it expires, the parent immediately falls into
runtime suspended state, in between two captures, as soon as the child
driver falls into runtime suspend state:
- e.g. after 2s, + child calls pm_runtime_put_autosuspend() + 100ms
  autosuspend delay of the child.
- stm32-adc-core switches off regulators, clocks and so on.
- They get switched on back again 100ms later in this example (at 2.2s).

So, use runtime_idle() callback in stm32-adc-core driver to call
pm_runtime_mark_last_busy() for the parent driver (stm32-adc-core),
to avoid this.

Fixes: 9bdbb113 ("iio: adc: stm32-adc: add power management support")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/1593615328-5180-1-git-send-email-fabrice.gasnier@st.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/stm32-adc-core.c

@@ -769,6 +769,13 @@ static int stm32_adc_core_runtime_resume(struct device *dev)
 {
 	return stm32_adc_core_hw_start(dev);
 }
+
+static int stm32_adc_core_runtime_idle(struct device *dev)
+{
+	pm_runtime_mark_last_busy(dev);
+
+	return 0;
+}
 #endif
 
 static const struct dev_pm_ops stm32_adc_core_pm_ops = {
@@ -776,7 +783,7 @@ static const struct dev_pm_ops stm32_adc_core_pm_ops = {
 				pm_runtime_force_resume)
 	SET_RUNTIME_PM_OPS(stm32_adc_core_runtime_suspend,
 			   stm32_adc_core_runtime_resume,
-			   NULL)
+			   stm32_adc_core_runtime_idle)
 };
 
 static const struct stm32_adc_priv_cfg stm32f4_adc_priv_cfg = {