linux-kernel - Re: [PATCH 1/8] PM: Opportunistic suspend support.

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Date:	Thu, 27 May 2010 17:34:57 +1000
From:	Neil Brown <neilb@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Arve Hjønnevåg <arve@...roid.com>,
	Florian Mickler <florian@...kler.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Alan Stern <stern@...land.harvard.edu>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
	Randy Dunlap <rdunlap@...otime.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Tejun Heo <tj@...nel.org>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Nigel Cunningham <nigel@...onice.net>,
	Ming Lei <tom.leiming@...il.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Maxim Levitsky <maximlevitsky@...il.com>,
	linux-doc@...r.kernel.org, Matthew Garrett <mjg59@...f.ucam.org>,
	Greg KH <gregkh@...e.de>, tytso@....edu,
	James Bottomley <James.Bottomley@...e.de>
Subject: Re: [PATCH 1/8] PM: Opportunistic suspend support.

On Wed, 26 May 2010 12:21:02 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, 2010-05-26 at 03:17 -0700, Arve Hjønnevåg wrote:
> > > With a single suspend manager process that manages the suspend state you
> > > can achieve the same goal.
> > >
> > 
> > Yes we don't need the /dev interface, but it is useful. Without it any
> > program that needs to block suspend has to make a blocking ipc call
> > into the suspend manager process. Android already does this for java
> > code, but system processes written in C block suspend directly with
> > the kernel since they cannot use the java APIs. 
> 
> So provide a C interface to it as well?
> 
> Surely you can have the java thing have a unix socket or something a C
> app can talk to. That shouldn't be hard at all.
> 
> Or make the suspend manager a C proglet and provide a JNI interface, or
> whatever.

I fail to understand the modern fascination with complex IPC mechanisms.

If you have a userspace process that initiates suspends, and you want other
user-space processes to be able to block that suspend, then I would suggest
the use of a lock-file.  /var/run/suspend/blocked maybe.

To block suspend, you open the file and get a read lock.
To initiate a suspend you take a write-lock (blocking if necessary), then
ask the kernel to suspend.
To restrict access to particular users you use permissions - either group
based or ACLs (or both).
This is all easy to do from C or python or perl or presumably even java..
(I use this mechanism on my Freerunner and even have shell scripts that
happily prevent suspend).

NeilBrown
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