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

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Message-ID: <20141029153641.GB20565@roeck-us.net>
Date:	Wed, 29 Oct 2014 08:36:41 -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 02:35:26PM +0100, Johan Hovold wrote:
> On Wed, Oct 29, 2014 at 06:20:40AM -0700, Guenter Roeck wrote:
> > On 10/29/2014 05:34 AM, Johan Hovold wrote:
> > > On Tue, Oct 28, 2014 at 03:16:10PM +0000, Russell King - ARM Linux wrote:
> > >> On Tue, Oct 28, 2014 at 02:12:57PM +0100, Johan Hovold wrote:
> > >>> That's not what I was trying to refer to. But the patch set explicitly
> > >>> allows for multiple, prioritised power-off handlers, which can power
> > >>> off a board in different ways and with various degrees of success.
> > >>> Specifically, it allows for fallback handlers in case one or more
> > >>> power-off handlers fail.
> > >>>
> > >>> So if we allow for that, what is to prevent the final power-off handler
> > >>> from failing? And should this not be logged by arch code in the same way
> > >>> as failure to restart is?
> > >>
> > >> 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).
> >
> > Good that I just replaced notifier chain with an open coded implementation.
> 
> Good to hear.
> 
> > Sure, that is possible, but I would prefer to do that as a follow-up commit,
> > and it should be discussed in the context of the power-off handler patch set.
> 
> Fine with me.
> 
> > > 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?
> > 
> > That delay would depend on the individual power-off handler, so I think
> > the current implementation works just fine (where power-off handlers
> > implement the delay).
> 
> Some don't, and could possibly unknowingly have been relying on the fact
> that they could return to user space and be powered off at some later
> time. With systemd that would have caused a panic.
> 
Agreed, but there are two cases to consider: What should be the delay
before the next power-off handler is called, and what should the system
do if all power-off handlers fail (or if there are none). The current
behavior isn't exactly well defined. Ok, with systemd that results in
a crash, but I am not really sure if one can or should blame systemd
for that. The discussion about systemd and its philosophy should not
cloud the fact that power-off behavior isn't exactly well defined.

> Also consider generic power-off handlers such as gpio-poweroff. It
> currently hard-codes a three-second delay but the actual delay would
> really be board specific.
> 
A configurable delay would address that. The actually required delay
could be provided in platform data or as devicetree property.

Thanks,
Guenter
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