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

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Date:	Mon, 7 Jul 2014 14:14:54 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Guenter Roeck <linux@...ck-us.net>
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 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.


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