Message-ID: <53D8FCEA.9020506@roeck-us.net>
Date:	Wed, 30 Jul 2014 07:10:50 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Heiko Stübner <heiko@...ech.de>
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>,
	Russell King <linux@....linux.org.uk>,
	Jonas Jensen <jonas.jensen@...il.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
	David Woodhouse <dwmw2@...radead.org>,
	Tomasz Figa <t.figa@...sung.com>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/7] kernel: Add support for restart handler call chain

On 07/30/2014 05:16 AM, Heiko Stübner wrote:
> Hi Guenter,
>
> Am Dienstag, 29. Juli 2014, 18:50:47 schrieb Guenter Roeck:
>> On 07/18/2014 12:34 AM, Guenter Roeck 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. Several other restart drivers for
>>> arm, all directly calling arm_pm_restart, are in the process of being
>>> integrated into the kernel. All those drivers would benefit from the new
>>> API.
>>>
>>> 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 multiple 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.
>>>
>>> Introduce a system restart handler call chain to solve the described
>>> problems. This call chain is expected to be executed 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 call chain. By using the
>>> priority field in the notifier block, callers can control restart handler
>>> execution sequence and thus ensure that the restart handler with the
>>> optimal restart capabilities for a given system is called first.
>>>
>>> Since the first revision of this patchset, a number of separate patch
>>> submissions have been made which either depend on it or could make use of
>>> it.
>>>
>>> http://www.spinics.net/linux/lists/arm-kernel/msg344796.html
>>>
>>> 	registers three notifiers.
>>>
>>> https://lkml.org/lkml/2014/7/8/962
>>>
>>> 	would benefit from it.
>>>
>>> Patch 1 of this series implements the restart handler function. Patches 2
>>> and 3 implement calling the restart handler chain from arm and arm64
>>> restart code.
>>>
>>> Patch 4 modifies the restart-poweroff driver to no longer call
>>> arm_pm_restart directly but machine_restart. This is done to avoid
>>> calling arm_pm_restart from more than one place. The change makes the
>>> driver architecture independent, so it would be possible to drop the arm
>>> dependency from its Kconfig entry.
>>>
>>> Patch 5 and 6 convert existing restart handlers in the watchdog subsystem
>>> to use the restart handler. Patch 7 unexports arm_pm_restart to ensure
>>> that no one gets the idea to implement a restart handler as module.
>>>
>>> ---
>>> v5: Rebased series to v3.16-rc5
>>>
>>>       Function renames:
>>>       register_restart_notifier -> register_restart_handler
>>>       unregister_restart_notifier -> unregister_restart_handler
>>>       kernel_restart_notify -> do_kernel_restart
>>>
>>> v4: Document restart notifier priorities
>>>
>>>       Select 128 as default priority for newly introduced notifiers
>>>       Fix checkpatch warning (line too long) in moxart patch
>>>
>>> v3: Drop RFC.
>>>
>>>       Add kernel_restart_notify wrapper function to execute notifier
>>>       Improve documentation.
>>>       Move restart_notifier_list into kernel/reboot.c and make it static.
>>>
>>> v2: Add patch 4.
>>>
>>>       Only call blocking notifier call chain if arm_pm_restart was not set.
>>>
>>> --
>>
>> To get more test coverage, this series plus a few add-on patches which
>> depend on it are now available in the restart-staging branch of my
>> repository at kernel.org [1]. The branch is currently based on 3.16-rc7.
>> Extensive build test results are available at [2]; look for the column
>> marked 'restart-staging' on the far right of the tables.
>>
>> I would encourage everyone interested in this series to send me Reviewed-by:
>> or at least Acked-by: tags. Note that I removed all of the earlier tags
>> since I feel that the changes made subsequently warrant updated tags. An
>> Acked-by: from affected maintainers would also be very helpful.
>
> Thanks for adapting my Samsung restart-patches to the API changes.
>
> The one thing I found is, in
> 	"clk: samsung: register restart handlers for s3c2412 and s3c2443"
> you seem to have forgotten the priority in the clk-s3c2443.c part while in
> clk-s3c2412.c it is present.
>
Yes, you are correct. I fixed that after I got the message from Fengguang's
robot. Which means that my auto-builder misses to build that file. Too bad
that arm:allmodconfig fails with other errors and is unusable. Do you know
if there is a defconfig which builds clk-s3c2443.c ?

> And I'm not sure if there shouldn't be some sort of delay, to give the
> watchdog some time to work, as Tomasz suggested in my initial submission?
>
In your case the reset handler is in the clock code, itn's it ? Question
would be if the write to SWRST results in an immediate reset or if it needs
a delay. From the context, it looks to me as if reaction would be immediate,
but obviously I don't have the specification so that is just a wild guess.

>
> I'll update the Rockchip specific restart-patches after the merge window, when
> the core clock support has landed.
>
> And I still like the whole concept very much, so
> Acked-by: Heiko Stuebner <heiko@...ech.de>

Thanks!

Guenter

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