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SubjectRe: [PATCH] PM: sleep: call devfreq_suspend/resume and cpufreq_suspend/resume in pairs.
Hi Rafael,

(Behalf Of Vincent Wang)

Thanks for your comments, please see my answers below.

On Wed, 22 Apr 2020 at 17:05, Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Wed, Apr 22, 2020 at 7:15 AM Chunyan Zhang <zhang.lyra@gmail.com> wrote:
> >
> > From: Vincent Wang <vincent.wang@unisoc.com>
> >
> > If dpm_prepare() fails in dpm_suspend_start(), dpm_suspend() can't be
> > called.
>
> That's correct.
>
> > And then, devfreq_suspend() and cpufreq_suspend() will not be
> > called in the suspend flow.
>
> Right.
>
> > But in the resiume flow, devfreq_resume() and cpufreq_resume() will
> > be called.
>
> Right, and they are expected to cope with the situation.
>
> > This patch will ensure that devfreq_suspend/devfreq_resume and
> > cpufreq_suspend/cpufreq_resume are called in pairs.
>
> So why is it better to do this than to make devfreq_resume() meet the
> expectations?

Yes,we found an issue with cpufreq schedutil governor on kernel4.14,
and I think the issue should haven't been changed on the latest
version of kernel.

In the function dpm_suspend_start(), dpm_suspend() would not be
exceuted if return error from device_prepare() [1]. So
cpufreq_cpufreq() will not be called, then
cpufreq_remove_update_util_hook() will not be called either, and so
cpufreq_update_util_data will not be NULL.

In the dpm resume flow, sugov_start() will be called, in which
sg_cpu.update_util will be set to 0.

And since cpufreq_update_util_data is not NULL, data->func will not be
set and is still NULL which actually is sg_cpu.update_util.

void cpufreq_add_update_util_hook(int cpu, struct update_util_data *data,
void (*func)(struct update_util_data *data, u64 time,
unsigned int flags))
{
[...]
if (WARN_ON(per_cpu(cpufreq_update_util_data, cpu)))
return;

data->func = func;
[...]
}

When cpufreq_update_util() is called by scheduler, there will be a
NULL pointer issue.


[1] https://elixir.bootlin.com/linux/v5.7-rc1/source/drivers/base/power/main.c#L2052


>
> > Signed-off-by: Vincent Wang <vincent.wang@unisoc.com>
> > Signed-off-by: Samer Xie <samer.xie@unisoc.com>
> > Signed-off-by: Chunyan Zhang <zhang.lyra@gmail.com>
> > ---
> > drivers/base/power/main.c | 6 +++---
> > 1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/base/power/main.c b/drivers/base/power/main.c
> > index fdd508a78ffd..eb3d987d43e0 100644
> > --- a/drivers/base/power/main.c
> > +++ b/drivers/base/power/main.c
> > @@ -1866,9 +1866,6 @@ int dpm_suspend(pm_message_t state)
> > trace_suspend_resume(TPS("dpm_suspend"), state.event, true);
> > might_sleep();
> >
> > - devfreq_suspend();
> > - cpufreq_suspend();
> > -
> > mutex_lock(&dpm_list_mtx);
> > pm_transition = state;
> > async_error = 0;
> > @@ -1988,6 +1985,9 @@ int dpm_prepare(pm_message_t state)
> > trace_suspend_resume(TPS("dpm_prepare"), state.event, true);
> > might_sleep();
> >
> > + devfreq_suspend();
> > + cpufreq_suspend();
> > +
> > /*
> > * Give a chance for the known devices to complete their probes, before
> > * disable probing of devices. This sync point is important at least
> > --
> > 2.20.1
> >

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