Re: [PATCH v2] iio: adc: stm32-adc: fix runtime autosuspend delay when slow polling

[Jonathan Cameron <jic23@kernel.org>]

On Wed, 29 Apr 2020 14:29:08 +0200
Fabrice Gasnier <fabrice.gasnier@st.com> wrote:

> 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: 9bdbb1139ca1 ("iio: adc: stm32-adc: add power management support")
> 
> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>

Whilst this seems 'sensible' to me, I really don't have a good enough grasp
of runtime pm to be sure. 

I see something similar looking in the greybus driver, but not sure on the
reason it is there.

Hence, ideally looking for an ack from Rafael on this one!

Thanks,

Jonathan

> ---
> Changes in v2:
> - Use runtime_idle callback in stm32-adc-core driver, instead of refreshing
>   last_busy from the child (for the parent) at many place. Initial patch v1
>   looked like "somewhat adhoc solution" as commented by Jonathan.
> ---
>  drivers/iio/adc/stm32-adc-core.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iio/adc/stm32-adc-core.c b/drivers/iio/adc/stm32-adc-core.c
> index 2df88d2..ebe5dbc 100644
> --- a/drivers/iio/adc/stm32-adc-core.c
> +++ b/drivers/iio/adc/stm32-adc-core.c
> @@ -803,6 +803,13 @@ 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 = {
> @@ -810,7 +817,7 @@ 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 = {