Re: [PATCH v5 0/7] kernel: Add support for restart handler call chain

[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,
Guenter

---
[1] https://git.kernel.org/cgit/linux/kernel/git/groeck/linux-staging.git/
[2] http://server.roeck-us.net:8010/builders

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