linux-kernel - Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

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Message-Id: <201006020000.52992.rjw@sisk.pl>
Date:	Wed, 2 Jun 2010 00:00:52 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Arve Hjønnevåg <arve@...roid.com>
Cc:	Neil Brown <neilb@...e.de>, Thomas Gleixner <tglx@...utronix.de>,
	Alan Stern <stern@...land.harvard.edu>,
	Felipe Balbi <felipe.balbi@...ia.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul@...p1.linux-foundation.org" <Paul@...p1.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Florian Mickler <florian@...kler.org>,
	Linux OMAP Mailing List <linux-omap@...r.kernel.org>,
	Linux PM <linux-pm@...ts.linux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

On Tuesday 01 June 2010, Arve Hjønnevåg wrote:
> On Mon, May 31, 2010 at 3:05 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > On Monday 31 May 2010, Neil Brown wrote:
> >> On Thu, 27 May 2010 23:40:29 +0200 (CEST)
> >> Thomas Gleixner <tglx@...utronix.de> wrote:
> >>
> >> > On Thu, 27 May 2010, Rafael J. Wysocki wrote:
> >> >
> >> > > On Thursday 27 May 2010, Thomas Gleixner wrote:
> >> > > > On Thu, 27 May 2010, Alan Stern wrote:
> >> > > >
> >> > > > > On Thu, 27 May 2010, Felipe Balbi wrote:
> >> > > > >
> >> > > > > > On Thu, May 27, 2010 at 05:06:23PM +0200, ext Alan Stern wrote:
> >> > > > > > >If people don't mind, here is a greatly simplified summary of the
> >> > > > > > >comments and objections I have seen so far on this thread:
> >> > > > > > >
> >> > > > > > >   The in-kernel suspend blocker implementation is okay, even
> >> > > > > > >   beneficial.
> >> > > > > >
> >> > > > > > I disagree here. I believe expressing that as QoS is much better. Let
> >> > > > > > the kernel decide which power state is better as long as I can say I
> >> > > > > > need 100us IRQ latency or 100ms wakeup latency.
> >> > > > >
> >> > > > > Does this mean you believe "echo mem >/sys/power/state" is bad and
> >> > > > > should be removed?  Or "echo disk >/sys/power/state"?  They pay no
> >> > > >
> >> > > > mem should be replaced by an idle suspend to ram mechanism
> >> > >
> >> > > Well, what about when I want the machine to suspend _regardless_ of whether
> >> > > or not it's idle at the moment?  That actually happens quite often to me. :-)
> >> >
> >> > Fair enough. Let's agree on a non ambigous terminology then:
> >> >
> >> >      forced:
> >> >
> >> >          suspend which you enforce via user interaction, which
> >> >                  also implies that you risk losing wakeups depending on
> >> >                  the hardware properties
> >>
> >> Reasonable definition I think.  However the current implementation doesn't
> >> exactly match it.
> >> With the current implementation you risk losing wakeups *independent* of the
> >> hardware properties.
> >
> > Define "losing", please.
> >
> > Currently, we simply don't regard hardware signals occuring _during_ the
> > suspend operation itself as wakeups (unless they are wakeup interrupts to be
> > precise, because these _are_ taken into account by our current code).
> >
> > The reason is that the meaning of given event may be _different_ at run time
> > and after the system has been suspended.  For example, consider a power button
> > on a PC box.  If it's pressed at run time, it usually means "power off the
> > system" to the kernel.  After the system has been suspended, however, it means
> > "wake up".  So, you have to switch from one interpretation of the event to the
> > other and that's not an atomic operaition (to put it lightly).
> >
> >> Even with ideal hardware events can be lost - by which I mean that they will
> >> not be seen until some other event effects a wake-up.
> >> e.g. the interrupt which signals the event happens immediately before the
> >> suspend is requested (or maybe at the same time as), but the process which
> >> needs to handle the event doesn't get a chance to see it before the suspend
> >> procedure freezes that process, and even if it did it would have no way to
> >> abort the suspend.
> >>
> >> So I submit that the current implementation doesn't match your description of
> >> "forced", is therefore buggy, and that if it were fixed, that would be
> >> sufficient to meet the immediate needs of android.
> >
> > I don't really think it may be fixed with respect to every possible kind of
> > hardware.  On platforms where I/O interrupts are wakeup events it should
> > work right now.  On other platforms it may be impossible to overcome hardware
> > limitations.
> >
> 
> There is no reason you cannot make the rtc alarms work reliably on x86
> hardware.

They usually work, but not in 100% of cases.

Generally, "x86 hardware" is a broad category, where modern ones have much more
interesting capabilities than the older.

> Even if you may loose key events while suspending I think it
> is still valuable to have reliable alarms.

I don't think anyone will argue with that. :-)

Rafael
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