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SubjectRe: [PATCH 2/4] PM / devfreq: Handle monitor suspend/resume in the devfreq core
Hi Chanwoo,

On Thu, Feb 14, 2019 at 11:10:00PM +0900, Chanwoo Choi wrote:
> Hi Matthias,
>
> 2019년 2월 14일 (목) 오후 7:16, Matthias Kaehlcke <mka@chromium.org>님이 작성:
> >
> > devfreq expects governors to call devfreq_monitor_suspend/resume()
> > in response to DEVFREQ_GOV_SUSPEND/RESUME events. Since the devfreq
> > core itself generates these events and invokes the governor's event
> > handler the suspend/resume of the load monitoring can be done in the
> > common code.
> >
> > Call devfreq_monitor_suspend/resume() when the governor reports a
> > successful handling of DEVFREQ_GOV_SUSPEND/RESUME and remove these
> > calls from the simpleondemand and tegra governors. Make
> > devfreq_monitor_suspend/resume() static since these functions are now
> > only used by the devfreq core.
>
> The devfreq core generates the all events including
> DEVFREQ_GOV_START/STOP/INTERVAL.
> It is possible to make 'devfreq_monitor_*()' function as the static
> instead of exported function.
> And call them in devfreq.c as this patch as following:
>
> --- a/drivers/devfreq/devfreq.c
> +++ b/drivers/devfreq/devfreq.c
> @@ -728,6 +728,8 @@ struct devfreq *devfreq_add_device(struct device *dev,
> goto err_init;
> }
>
> + devfreq_monitor_start(devfreq);
> +
> list_add(&devfreq->node, &devfreq_list);
>
> mutex_unlock(&devfreq_list_lock);
> @@ -760,6 +762,7 @@ int devfreq_remove_device(struct devfreq *devfreq)
> if (devfreq->governor)
> devfreq->governor->event_handler(devfreq,
> DEVFREQ_GOV_STOP, NULL);
> + devfreq_monitor_stop(devfreq);
> device_unregister(&devfreq->dev);
>
> return 0;
> @@ -1259,6 +1262,9 @@ static ssize_t polling_interval_store(struct device *dev,
> df->governor->event_handler(df, DEVFREQ_GOV_INTERVAL, &value);
> ret = count;
>
> + if (!ret)
> + devfreq_interval_update(devfreq, (unsigned int *)data);
> +
> return ret;
> }
> static DEVICE_ATTR_RW(polling_interval);
> diff --git a/drivers/devfreq/tegra-devfreq.c b/drivers/devfreq/tegra-devfreq.c
> index 79efa1e..515fb85 100644
> --- a/drivers/devfreq/tegra-devfreq.c
> +++ b/drivers/devfreq/tegra-devfreq.c
> @@ -580,13 +580,11 @@ static int tegra_governor_event_handler(struct
> devfreq *devfreq,
>
> switch (event) {
> case DEVFREQ_GOV_START:
> - devfreq_monitor_start(devfreq);
> tegra_actmon_enable_interrupts(tegra);
> break;
>
> case DEVFREQ_GOV_STOP:
> tegra_actmon_disable_interrupts(tegra);
> - devfreq_monitor_stop(devfreq);
> break;

indeed, that's similar to "[4/4] PM / devfreq: Handle monitor
start/stop in the devfreq core" of this series.

> Instead,
>
> If the governor should execute some codes before and after of
> DEVFREQ_GOV_SUSPEND, DEVFREQ_GOV_RESUME,
> it is impossible to change the order between devfreq_monitor_*() function
> and the specific governor in the event_handler callback function of
> each governor.
>
> For example, if some govenor requires the following sequencue,
> after this patch, it is not possible.
>
> case DEVFREQ_GOV_SUSPEND:
> /* execute some code before devfreq_monitor_suspend() */
> devfreq_monitor_suspend()
> /* execute some code after devfreq_monitor_suspend() */

I agree that the patch introduces this limitation, however I'm not
convinced it is a problem in practice.

For suspend we'd essentially end up with:

governor_suspend
governor->suspend

monitor_suspend
update_status

stop polling

What would a governor want to do after polling is stopped? Is there
any real world or halfway reasonable hypothetical example?


And for resume:

governor_resume
governor->resume

monitor_resume
start polling

Same question here, what would the governor realistically want to do
after polling has started that it couldn't have done before?

Cheers

Matthias

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