linux-kernel - Re: [PATCH] power: reset: Add MAX77620 support

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Message-ID: <20170119230036.sjfqvqxgxutis6xs@earth>
Date:   Fri, 20 Jan 2017 00:00:36 +0100
From:   Sebastian Reichel <sre@...nel.org>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Laxman Dewangan <ldewangan@...dia.com>,
        Martin Michlmayr <tbm@...ius.com>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Chaitanya Bandi <bandik@...dia.com>,
        Guenter Roeck <linux@...ck-us.net>
Subject: Re: [PATCH] power: reset: Add MAX77620 support

Hi Thierry,

> > > [...]
> >
> > Please use register_restart_handler() instead. It has support for
> > priorities, is not arm specific and properly supports unregistering
> > (some handler with lower priority will take over). You can check
> > gpio-restart as an example for the API.
> > 
> > If you have some other arm_pm_restart handler (those are tried first)
> > I suggest to convert that to the restart handler framework. Actually
> > it may be a good idea to convert all of them and drop arm_pm_restart,
> > since there are only 4 left:
> > 
> > $ git grep "arm_pm_restart ="
> > drivers/firmware/psci.c:        arm_pm_restart = psci_sys_reset;
> > arch/arm/mach-prima2/rstc.c:    arm_pm_restart = sirfsoc_restart;
> > arch/arm/xen/enlighten.c:       arm_pm_restart = xen_restart;
> > arch/arm/kernel/setup.c:                arm_pm_restart = mdesc->restart;
> > 
> > The first 3 should be easy to update and the last one could
> > be solved by registering a wrapper function as restart handler,
> > which calls mdesc->restart().
> 
> I remember this not being the first time that this confuses me. And from
> looking around a little it seems like I'm not the only one.
> 
> Do you know if there's ever been any attempts to formalize all of this
> by adding some sort of framework for this? That way some of the
> confusion may be removed and architecture-specific bits could be
> eliminated more easily.

We have a framework for restart handlers, which has been written
by Guenter Roeck [0] in 2014. You just have to call register_restart_handler()
during probe and unregister_restart_handler() during removal of
your reboot driver. All reboot drivers in drivers/power/reset use
that framework.

The restart handlers are invoked by calling do_kernel_restart()
based on their configured priority. That function is called by
the architecture's machine_restart() function. It's currently
used by mips, ppc, arm & arm64 as far as I can see. ARM's
implementation looks like this:

if (arm_pm_restart)
	arm_pm_restart()
else
	do_kernel_restart() /* reboot handler framework */

No such thing exists for poweroff. Guenter also wrote something for
that [1], but Linus intervened [2]. Anyways, pm_power_off is at
least architecture independent.

[0] https://lwn.net/Articles/605298/
[1] https://lwn.net/Articles/617345/
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-June/351369.html

-- Sebastian

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