Re: [RFC 1/2] PM / suspend: Add platform_suspend_target_state()

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Wednesday, July 12, 2017 11:08:19 AM Florian Fainelli wrote:
> On 06/29/2017 04:00 PM, Rafael J. Wysocki wrote:
> > On Thursday, June 22, 2017 06:08:36 PM Florian Fainelli wrote:
> >> Add an optional platform_suspend_ops callback: target_state, and a
> >> helper function globally visible to get this called:
> >> platform_suspend_target_state().
> >>
> >> This is useful for platform specific drivers that may need to take a
> >> slightly different suspend/resume path based on the system's
> >> suspend/resume state being entered.
> >>
> >> Although this callback is optional and documented as such, it requires
> >> a platform_suspend_ops::begin callback to be implemented in order to
> >> provide an accurate suspend/resume state within the driver that
> >> implements this platform_suspend_ops.
> >>
> >> Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
> >> ---
> >>  include/linux/suspend.h | 12 ++++++++++++
> >>  kernel/power/suspend.c  | 15 +++++++++++++++
> >>  2 files changed, 27 insertions(+)
> >>
> >> diff --git a/include/linux/suspend.h b/include/linux/suspend.h
> >> index d9718378a8be..d998a04a90a2 100644
> >> --- a/include/linux/suspend.h
> >> +++ b/include/linux/suspend.h
> >> @@ -172,6 +172,15 @@ static inline void dpm_save_failed_step(enum suspend_stat_step step)
> >>   *	Called by the PM core if the suspending of devices fails.
> >>   *	This callback is optional and should only be implemented by platforms
> >>   *	which require special recovery actions in that situation.
> >> + *
> >> + * @target_state: Returns the suspend state the suspend_ops will be entering.
> >> + * 	Called by device drivers that need to know the platform specific suspend
> >> + * 	state the system is about to enter.
> >> + * 	This callback is optional and should only be implemented by platforms
> >> + * 	which require special handling of power management states within
> >> + * 	drivers. It does require @begin to be implemented to provide the suspend
> >> + * 	state. Return value is platform_suspend_ops specific, and may be a 1:1
> >> + * 	mapping to suspend_state_t when relevant.
> >>   */
> >>  struct platform_suspend_ops {
> >>  	int (*valid)(suspend_state_t state);
> >> @@ -184,6 +193,7 @@ struct platform_suspend_ops {
> >>  	bool (*suspend_again)(void);
> >>  	void (*end)(void);
> >>  	void (*recover)(void);
> >> +	int (*target_state)(void);
> > 
> > I would use unsigned int (the sign should not matter).
> > 
> >>  };
> > 
> > That's almost what I was thinking about except that the values returned by
> > ->target_state should be unique, so it would be good to do something to
> > ensure that.
> > 
> > The concern is as follows.
> > 
> > Say you have a driver develped for platform X where ->target_state returns
> > A for "mem" and B for "standby".  Then, the same IP is re-used on platform Y
> > returning B for "mem" and C for "standby" and now the driver cannot
> > distinguish between them.
> > 
> > Moreover, even if they both returned A for "mem" there might be differences
> > in how "mem" was defined by each of them and therefore in what the driver was
> > expected to do to handle "mem" on X and Y.
> 
> That makes sense, would you need the core implementation in
> platform_suspend_target_state() to range check what
> suspend_ops->target_state() returns against a set of reserved value say,
> checking from 0 up to ACPI_S_STATE_COUNT or is there another range you
> would like to see being used?

I had an idea of using an enum type encompassing all of the power states
defined for various platforms and serving both as a registry (to ensure the
uniqueness of the values assigned to the states) and a common ground
between platforms and drivers.

Something like:

enum platform_target_state {
	PLATFORM_STATE_UNKNOWN = -1,
	PLATFORM_STATE_WORKING = 0,
	PLATFORM_STATE_ACPI_S1,
	PLATFORM_STATE_ACPI_S2,
	PLATFORM_STATE_ACPI_S3,
	PLATFORM_STATE_MY_BOARD_1_GATE_CLOCKS,
	PLATFORM_STATE_MY_BOARD_1_GATE_POWER,
	PLATFORM_STATE_ANOTHER_BOARD_DO_CRAZY_STUFF,
	...
};

and define ->target_state to return a value of this type.

Then, if a driver sees one of these and recognizes that value, it should
know exactly what to do.

Thanks,
Rafael