Re: [PATCH v2 1/7] kernel: Add support for restart notifier call chain

[Guenter Roeck]

On 07/07/2014 02:14 PM, Andrew Morton wrote:
> On Sun,  6 Jul 2014 16:38:14 -0700 Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
>
> > Various drivers implement architecture and/or device specific means
> > to restart (reset) the system. Various mechanisms have been implemented
> > to support those schemes. The best known mechanism is arm_pm_restart,
> > which is a function pointer to be set either from platform specific code
> > or from drivers. Another mechanism is to use hardware watchdogs to issue
> > a reset; this mechanism is used if there is no other method available
> > to reset a board or system. Two examples are alim7101_wdt, which currently
> > uses the reboot notifier to trigger a reset, and moxart_wdt, which registers
> > the arm_pm_restart function.
> >
> > The existing mechanisms have a number of drawbacks. Typically only one scheme
> > to restart the system is supported (at least if arm_pm_restart is used).
> > At least in theory there can be mutliple means to restart the system, some of
> > which may be less desirable (for example one mechanism may only reset the CPU,
> > while another may reset the entire system). Using arm_pm_restart can also be
> > racy if the function pointer is set from a driver, as the driver may be in
> > the process of being unloaded when arm_pm_restart is called.
> > Using the reboot notifier is always racy, as it is unknown if and when
> > other functions using the reboot notifier have completed execution
> > by the time the watchdog fires.
> >
> > To solve the problem, introduce a system restart notifier. This notifier
> > is expected to be called from the architecture specific machine_restart()
> > function. Drivers providing system restart functionality (such as the watchdog
> > drivers mentioned above) are expected to register with this notifier.
>
> It all looks sane to my unfamiliar eye.
>
> >   /*
> > + *	Notifier list for kernel code which wants to be called
> > + *	to restart the system.
> > + */
>
> hm, is this all we have to say?
>
> > --- a/kernel/reboot.c
> > +++ b/kernel/reboot.c
> > @@ -104,6 +104,38 @@ int unregister_reboot_notifier(struct notifier_block *nb)
> >   }
> >   EXPORT_SYMBOL(unregister_reboot_notifier);
> >
> > +/**
> > + *	register_restart_notifier - Register function to be called to reset
> > + *				    the system
> > + *	@nb: Info about notifier function to be called
> > + *
> > + *	Registers a function with the list of functions
> > + *	to be called to restart the system.
> > + *
> > + *	Currently always returns zero, as blocking_notifier_chain_register()
> > + *	always returns zero.
> > + */
>
> This would be a good place to describe what those notifier callbacks
> actually do.  Why they exist, what their role is, under what
> circumstances they are called, what values they should return, etc.

Makes sense. Done.

Guenter


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/