linux-kernel - Re: [linux-pm] [PATCH v2] Add suspend/resume for HPET

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Date:	Sat, 31 Mar 2007 10:55:42 -0700
From:	David Brownell <david-b@...bell.net>
To:	linux-pm@...ts.linux-foundation.org
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	Jeff Chua <jeff.chua.linux@...il.com>,
	linux-ide@...r.kernel.org,
	Sergei Shtylyov <sshtylyov@...mvista.com>, gregkh@...e.de,
	linux-pm@...ts.osdl.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Adrian Bunk <bunk@...sta.de>, linux-acpi@...r.kernel.org,
	linux-pci@...ey.karlin.mff.cuni.cz,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	"Michael S. Tsirkin" <mst@...lanox.co.il>,
	Ingo Molnar <mingo@...e.hu>, jgarzik@...ox.com,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [linux-pm] [PATCH v2] Add suspend/resume for HPET

On Saturday 31 March 2007 9:53 am, Linus Torvalds wrote:
> 
> On Sat, 31 Mar 2007, Thomas Gleixner wrote:
> > 
> > Right, but clock - sources/events need to be extremly late suspended and
> > early resumed. How can we ensure this ?
> 
> Make them be at the top of the device tree by adding them early. That's 
> the whole point of the device tree after all - we have an ordering that is 
> enforced by its topology, and that we sort by when things were added.

Right, but "when things get added" doesn't correlate well to
"when they should get suspended/resumed".  It's also in basic
conflict with runtime PM models, where devices may be suspended
at essentially any time.  And sysdevs are even stranger.


One way out:  rather than constructing that list as devices get
enumerated, it could be constructed by a (linear-time, non-recursive)
walk of the device tree(s) before they get suspended.

(Or equivalently:  construct lists at enumeration time, but just
adding them *right after their parent* rather than at the end of
the list.)

Would that solve the problem here?  Potentially ... if the tree is
structured to meet Thomas' rules:

> > The required resume order is:
> > 
> > clocksources
> > timekeeping
> > clockevents
> > tick management

> So the only thing that needs to be done is to make sure that we add the 
> timer devices early during bootup - something we have to do *anyway*. If a 
> device is added early in bootup, that automatically means that it will be 
> suspended late, and resumed early - because we maintain that order all the 
> way through..

>             -- "clocksource" -- +-- HPET
>                                 |
>                                 +-- TSC
>                                 |
>                                 +-- i8259
>                                 |
>                                 +-- lapic timer
>                                 |
>                                 .. whatever else

If each of those were a device node, with "clocksource" suspend/resume
methods handling Thomas' "timekeeping" item, and simlarly for "clockevent"
devices ... I could see that all working neatly.

- Dave

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