Date:	Mon, 07 Jul 2014 18:10:20 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-watchdog@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	Wim Van Sebroeck <wim@...ana.be>,
	Catalin Marinas <catalin.marinas@....com>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Will Deacon <will.deacon@....com>,
	Arnd Bergmann <arnd@...db.de>,
	Heiko Stuebner <heiko@...ech.de>,
	Russell King <linux@....linux.org.uk>,
	Jonas Jensen <jonas.jensen@...il.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/7] kernel: Add support for restart notifier call
 chain

On 07/07/2014 02:14 PM, Andrew Morton wrote:
> On Sun,  6 Jul 2014 16:38:14 -0700 Guenter Roeck <linux@...ck-us.net> 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


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