linux-kernel - Re: [PATCH v2 00/20] rtc: omap: fixes and power-off feature

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Message-ID: <20141029152502.GA20565@roeck-us.net>
Date:	Wed, 29 Oct 2014 08:25:02 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Johan Hovold <johan@...nel.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Felipe Balbi <balbi@...com>,
	Alessandro Zummo <a.zummo@...ertech.it>,
	Tony Lindgren <tony@...mide.com>,
	Benoît Cousson <bcousson@...libre.com>,
	Lokesh Vutla <lokeshvutla@...com>, nsekhar@...com,
	t-kristo@...com, j-keerthy@...com, linux-omap@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, devicetree@...r.kernel.org,
	rtc-linux@...glegroups.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 00/20] rtc: omap: fixes and power-off feature

On Wed, Oct 29, 2014 at 02:22:44PM +0100, Johan Hovold wrote:
> On Wed, Oct 29, 2014 at 01:10:20PM +0000, Russell King - ARM Linux wrote:
> > On Wed, Oct 29, 2014 at 01:34:18PM +0100, Johan Hovold wrote:
> > > On Tue, Oct 28, 2014 at 03:16:10PM +0000, Russell King - ARM Linux wrote:
> > > > And how is that different from having a set of power-off handlers, and
> > > > reporting when each individual one fails?  Don't you want to know if
> > > > your primary high priority reboot handler fails, just as much as you
> > > > want to know if your final last-resort power-off handler fails?
> > > 
> > > Good point. Failed power-off should probably be logged by the power-off
> > > call chain implementation (which seems to makes notifier chains a bad
> > > fit).
> > > 
> > > And what about any power-off latencies? Should this always be dealt with
> > > in the power-off handler?
> > > 
> > > Again, if it's predictable and high, as in the OMAP RTC case, it should
> > > go in the handler. But what if it's just normal bus latencies
> > > (peripheral busses, i2c, or whatever people may come up with)?
> > > 
> > > Should there always be a short delay before calling the next handler?
> > 
> > If the handler has determined that it has failed, then why delay before
> > trying the next handler?  At the point it has decided it has failed,
> > surely that's after it has waited sufficient time to determine that
> > failure?
> 
> The current handlers we have are not expecting any other handler to be
> run after they return. My question was whether all these handlers should
> get a short mdelay added to them (e.g. to compensate for bus latencies)

Some of them do add a delay.

> or if this could be done in the power-off handler (e.g. before printing
> the error message).
> 
That might make sense, but it would have to be configurable, since the delay
is platform specific and power-off handler does not know how long to wait
(the longest delay I have seen is 10 seconds).

> > > > Or different from having no power-off handlers.
> > > 
> > > That is actually quite different, as in that case we call machine_halt
> > > instead (via kernel_halt).
> > 
> > Today, ARM does exactly what x86 does.  If there's no power off handler
> > registered, machine_power_off() shuts down other CPUs and returns.
> 
> No, if there are no power-off handlers registered, kernel/reboot.c will
> never call machine_power_off:
> 
> 	/* Instead of trying to make the power_off code look like
> 	 * halt when pm_power_off is not set do it the easy way.
> 	 */
> 	if ((cmd == LINUX_REBOOT_CMD_POWER_OFF) && !pm_power_off)
> 		cmd = LINUX_REBOOT_CMD_HALT;
> 
> So in that case on arm, a system-halted message is printed, and we never
> return to user-space.
> 
Some architectures do that, or go into an endless loop. Others do return
from machine_power_off. Having a well defined behavior would be nice
(such as dumping an error mesasge and calling machine_halt if
machine_power_off returns). Only question would be where to put it.
kernel_power_off() might be a good place; only problem is that there
are direct callers of machine_power_off().

Guenter
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