[PATCH] PM: s2idle: Introduce syscore callbacks in s2idle flow

[Claude.Yen]

On Tue, 2020-09-01 at 13:57 +0200, Rafael J. Wysocki wrote:
> On Tue, Sep 1, 2020 at 9:05 AM Claude Yen <claude.yen at mediatek.com> wrote:
> >
> > This series based on 5.9-rc1
> > This patch makes s2idle call existing syscore callbacks. Currently,
> > when s2idle is selected as system suspend method, callbacks hooked
> > by register_syscore_ops() will not be triggered. This may induce
> > unexpected results.
> 
> They are not executed by design.
> 
> > For example, sched_clock_suspend() was added to s2idle flow in
> > commit 3f2552f7e9c5 ("timers/sched_clock: Prevent generic sched_clock
> > wrap caused by tick_freeze()") to fix clock wrap problem. However,
> > sched_clock_suspend() is originally registered in syscore callback.
> 
> I'm not sure why this matters here.

If functions in syscore callbacks are needed in s2idle, explicit
migration is needed like commit 3f2552f7e9c5 ("timers/sched_clock:
Prevent generic sched_clock wrap caused by tick_freeze()").
Thus, I am wondering if such effort could be saved.

> > With this patch, if another syscore callback is needed in s2idle,
> > additional migration effort could be saved.
> 
> s2idle cannot execute syscore callbacks, because it doesn' take
> non-boot CPUs offline and it won't do that.
> 
> Thanks!

Yes, the current design of syscore callback needs non-boot CPUs offline.
Considering the following case: in s2idle flow, there is a status that
only one CPU is alive and other CPUs have enter deepest idle state.
This situation is similar to getting non-boot CPUs offline, though all
CPUs are online from kernel's perspective.

Reply from Stephen mentioned that if an operation is needed in both
S2R and s2idle, CPU_PM notifier can be utilized. 
In my opinion, CPU_PM notifier is particularly for CPU entering idle
state. In contrast, syscore callback is for system going low power
state. There exists semantic difference between these two callbacks.

Could the current design of syscore callback be re-designed as
system-wide suspend callback?

Proposed suspend flow in this patch:

    Freeze tasks
        |
        V
    Device suspend callbacks
        |
        |-------------s2idle----------
        |                            |
        V                            |
    Disable nonboot CPUs    Is this CPU last core to enter idle?
        |                            |
        V                            |-------------
    syscore callbacks                |            |
        |                           No           Yes
        V                            |            |
    platform suspend                 V            V
                                 enter idle     syscore callback
                                                  |
                                                  V
                                                enter idle

Regards,
Claude