Re: [PATCH v5 01/48] kernel: Add support for power-off handler call chain

Guenter Roeck
2014-11-06T22:27:14Z

On Thu, Nov 06, 2014 at 11:30:59PM +0100, Rafael J. Wysocki wrote:
> On Thursday, November 06, 2014 08:42:45 AM Guenter Roeck wrote:
> > Various drivers implement architecture and/or device specific means to
> > power off the system. For the most part, those drivers set the global
> > variable pm_power_off to point to a function within the driver.
> >
> > This mechanism has a number of drawbacks. Typically only one scheme
> > to remove power is supported (at least if pm_power_off is used).
> > At least in theory there can be multiple means remove power, some of
> > which may be less desirable. For example, some mechanisms may only
> > power off the CPU or the CPU card, while another may power off the
> > entire system. Others may really just execute a restart sequence
> > or drop into the ROM monitor. Using pm_power_off can also be racy
> > if the function pointer is set from a driver built as module, as the
> > driver may be in the process of being unloaded when pm_power_off is
> > called. If there are multiple power-off handlers in the system, removing
> > a module with such a handler may inadvertently reset the pointer to
> > pm_power_off to NULL, leaving the system with no means to remove power.
> >
> > Introduce a system power-off handler call chain to solve the described
> > problems. This call chain is expected to be executed from the architecture
> > specific machine_power_off() function. Drivers and architeceture code
> > providing system power-off functionality are expected to register with
> > this call chain. When registering a power-off handler, callers can
> > provide a priority to control power-off handler execution sequence
> > and thus ensure that the power-off handler with the optimal capabilities
> > to remove power for a given system is called first.
> >
> > Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
> > Cc: Alexander Graf <agraf@suse.de>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> > Cc: Heiko Stuebner <heiko@sntech.de>
> > Cc: Lee Jones <lee.jones@linaro.org>
> > Cc: Len Brown <len.brown@intel.com>
> > Cc: Pavel Machek <pavel@ucw.cz>
> > Cc: Philippe Rétornaz <philippe.retornaz@gmail.com>
> > Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
> > Cc: Romain Perier <romain.perier@gmail.com>
> > Acked-by: Pavel Machek <pavel@ucw.cz>
> > Acked-by: Heiko Stuebner <heiko@sntech.de>
> > Signed-off-by: Guenter Roeck <linux@roeck-us.net>
> > ---
> > v5:
> > - Rebase to v3.18-rc3
> > v4:
> > - Do not use notifiers but internal functions and data structures to manage
> > the list of power-off handlers. Drop unused parameters from callbacks, and
> > make the power-off function type void.
> > Code to manage and walk the list of callbacks is derived from notifier.c.
> > v3:
> > - Rename new file to power_off_handler.c
> > - Replace poweroff in all newly introduced variables and in text
> > with power_off or power-off as appropriate
> > - Replace POWEROFF_PRIORITY_xxx with POWER_OFF_PRIORITY_xxx
> > - Execute power-off handlers without any locks held
> > v2:
> > - poweroff -> power_off
> > - Add defines for default priorities
> > - Use raw notifiers protected by spinlocks instead of atomic notifiers
> > - Add register_poweroff_handler_simple
> > - Add devm_register_power_off_handler
> >
> > include/linux/pm.h | 28 ++++
> > kernel/power/Makefile | 1 +
> > kernel/power/power_off_handler.c | 293 +++++++++++++++++++++++++++++++++++++++
> > 3 files changed, 322 insertions(+)
> > create mode 100644 kernel/power/power_off_handler.c
> >
> > diff --git a/include/linux/pm.h b/include/linux/pm.h
> > index 383fd68..a4d6bf8 100644
> > --- a/include/linux/pm.h
> > +++ b/include/linux/pm.h
> > @@ -35,6 +35,34 @@ extern void (*pm_power_off)(void);
> > extern void (*pm_power_off_prepare)(void);
> >
> > struct device; /* we have a circular dep with device.h */
> > +
> > +/*
> > + * Data structures and callbacks to manage power-off handlers
> > + */
> > +
> > +struct power_off_handler_block {
> > + void (*handler)(struct power_off_handler_block *);
> > + struct power_off_handler_block __rcu *next;
> > + int priority;
> > +};
> > +
> > +int register_power_off_handler(struct power_off_handler_block *);
> > +int devm_register_power_off_handler(struct device *,
> > + struct power_off_handler_block *);
> > +int register_power_off_handler_simple(void (*function)(void), int priority);
> > +int unregister_power_off_handler(struct power_off_handler_block *);
> > +void do_kernel_power_off(void);
> > +bool have_kernel_power_off(void);
> > +
> > +/*
> > + * Pre-defined power-off handler priorities
> > + */
> > +#define POWER_OFF_PRIORITY_FALLBACK 0
> > +#define POWER_OFF_PRIORITY_LOW 64
> > +#define POWER_OFF_PRIORITY_DEFAULT 128
> > +#define POWER_OFF_PRIORITY_HIGH 192
> > +#define POWER_OFF_PRIORITY_HIGHEST 255
>
> I'm not sure why we need these gaps in the priority space.
>
> I guess it might be possible to use
>
> enum power_off_priority {
> POWER_OFF_PRIORITY_FALLBACK = 0,
> POWER_OFF_PRIORITY_LOW,
> POWER_OFF_PRIORITY_DEFAULT,
> POWER_OFF_PRIORITY_HIGH,
> POWER_OFF_PRIORITY_HIGHEST,
> POWER_OFF_PRIORITY_LIMIT,
> };

I retained the large number space on purpose, specifically to permit in-between
priorities. In other words, I want people to be able to say "priority for this
handler is higher than low but lower than default". After all, the defines were
intended as hints, not as a "Thou shall use those and only those priorities".

Having said that, the important part is to get the series accepted, so I won't
argue if that is what it takes to get an Ack. Let me know.

Thanks,
Guenter

>
> and then make register_ complain if priority is POWER_OFF_PRIORITY_LIMIT
> or greater.
>
> But I'm OK with the rest, so if no one else sees a problem here,
> I'm not going to make a fuss about it.
>
> Rafael
>