Message-ID: <53D9010B.7070700@roeck-us.net>
Date:	Wed, 30 Jul 2014 07:28:27 -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 07:10 AM, Guenter Roeck wrote:
> 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.
>

After re-reading your patches, I guess you refer to the s3c2410 changes.
Yes, that could use a delay. Care to send an updated patch ?

Thanks,
Guenter

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