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

From: Guenter Roeck
Date: Wed Jul 30 2014 - 10:10:57 EST

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

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

registers three notifiers.

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 [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@xxxxxxxxx>



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