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

[Fabrice Gasnier <fabrice.gasnier@st.com>]

On 2/2/20 4:33 PM, Jonathan Cameron wrote:
> On Fri, 24 Jan 2020 09:26:09 +0100
> 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, add an explicit call to pm_runtime_mark_last_busy() for the parent
>> driver (stm32-adc-core), synchronously with the child driver (stm32-adc),
>> to avoid this.
>>
>> Fixes: 9bdbb1139ca1 ("iio: adc: stm32-adc: add power management support")
>>
>> Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
> 
> Hi Fabrice,
> 
> Whilst this will clearly work, it seems like a somewhat adhoc solution.
> Power management specialists (cc'd):  Is this what we should be doing, or
> have Fabrice and I both missed something that we should be doing here?

Hi all, PM specialists,

As per my understanding, pm_runtime_mark_last_busy() doesn't cascade to
the parent device:

- in pm_runtime.h:
static inline void pm_runtime_mark_last_busy(struct device *dev)
{
	WRITE_ONCE(dev->power.last_busy, ktime_get_mono_fast_ns());
}

STM32 ADC driver uses a model with an autosupsend delay for
- a parent driver to handle common resources, registers etc.
- child drivers for each ADC.

So the question is on how to fix the behavior I described:
1: Child  activity with "short" autosuspend_delay
2: Parent activity with "longer" autosuspend_delay
     _     _     _     _     _       _     _     _
1: _| |___| |___| |___| |___| |_..._| |___| |___| |_...

    v v   v v   v v   v v   v v ... v v   v v   v v
    | |                                     |   |
    | +- pm_runtime_mark_last_busy()        |   |
    | +- pm_runtime_put_autosuspend()       v   |
    |                                       |   |
    +--- pm_runtime_get_sync()              |   v
    |                                       |   |
    +---> expires after autosuspend_delay   |   |
    |                                       |   |
    v                                       v   v
     _______________________________________     ___...
2: _|                           ...         |___|

Glitches on parent dev near autosuspend_delay ^

- does the child driver needs to "kick" parent driver with
pm_runtime_mark_last_busy(), as proposed in current patch ?

- or is it something that should be done by PM runtime core routines ?
e.g. make pm_runtime_mark_last_busy() recursive or something else ?

Please advise
Best regards,
Fabrice

> 
> Thanks,
> 
> Jonathan
> 
>> ---
>>  drivers/iio/adc/stm32-adc.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/drivers/iio/adc/stm32-adc.c b/drivers/iio/adc/stm32-adc.c
>> index 3b291d7..670157e 100644
>> --- a/drivers/iio/adc/stm32-adc.c
>> +++ b/drivers/iio/adc/stm32-adc.c
>> @@ -1157,6 +1157,7 @@ static int stm32_adc_single_conv(struct iio_dev *indio_dev,
>>  
>>  	stm32_adc_conv_irq_disable(adc);
>>  
>> +	pm_runtime_mark_last_busy(dev->parent);
>>  	pm_runtime_mark_last_busy(dev);
>>  	pm_runtime_put_autosuspend(dev);
>>  
>> @@ -1278,6 +1279,7 @@ static int stm32_adc_update_scan_mode(struct iio_dev *indio_dev,
>>  	adc->num_conv = bitmap_weight(scan_mask, indio_dev->masklength);
>>  
>>  	ret = stm32_adc_conf_scan_seq(indio_dev, scan_mask);
>> +	pm_runtime_mark_last_busy(dev->parent);
>>  	pm_runtime_mark_last_busy(dev);
>>  	pm_runtime_put_autosuspend(dev);
>>  
>> @@ -1329,6 +1331,7 @@ static int stm32_adc_debugfs_reg_access(struct iio_dev *indio_dev,
>>  	else
>>  		*readval = stm32_adc_readl(adc, reg);
>>  
>> +	pm_runtime_mark_last_busy(dev->parent);
>>  	pm_runtime_mark_last_busy(dev);
>>  	pm_runtime_put_autosuspend(dev);
>>  
>> @@ -1451,6 +1454,7 @@ static int __stm32_adc_buffer_postenable(struct iio_dev *indio_dev)
>>  err_clr_trig:
>>  	stm32_adc_set_trig(indio_dev, NULL);
>>  err_pm_put:
>> +	pm_runtime_mark_last_busy(dev->parent);
>>  	pm_runtime_mark_last_busy(dev);
>>  	pm_runtime_put_autosuspend(dev);
>>  
>> @@ -1487,6 +1491,7 @@ static void __stm32_adc_buffer_predisable(struct iio_dev *indio_dev)
>>  	if (stm32_adc_set_trig(indio_dev, NULL))
>>  		dev_err(&indio_dev->dev, "Can't clear trigger\n");
>>  
>> +	pm_runtime_mark_last_busy(dev->parent);
>>  	pm_runtime_mark_last_busy(dev);
>>  	pm_runtime_put_autosuspend(dev);
>>  }
>> @@ -1874,6 +1879,7 @@ static int stm32_adc_probe(struct platform_device *pdev)
>>  		goto err_hw_stop;
>>  	}
>>  
>> +	pm_runtime_mark_last_busy(dev->parent);
>>  	pm_runtime_mark_last_busy(dev);
>>  	pm_runtime_put_autosuspend(dev);
>>  
>