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From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Alan Stern <stern@rowland.harvard.edu>
Subject: Re: [linux-pm] [PATCH 2/8] PM: suspend_block: Add driver to access suspend blockers from user-space
Date: Thu, 27 May 2010 00:34:44 +0200
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Cc: Peter Zijlstra <peterz@infradead.org>,
       Cornelia Huck <cornelia.huck@de.ibm.com>,
       Len Brown <len.brown@intel.com>, Jim Collar <jim.collar@eqware.net>,
       linux-doc@vger.kernel.org, "Greg Kroah-Hartman" <gregkh@suse.de>,
       linux-kernel@vger.kernel.org, Avi Kivity <avi@redhat.com>,
       Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>,
       Magnus Damm <damm@igel.co.jp>, linux-pm@lists.linux-foundation.org,
       Andrew Morton <akpm@linux-foundation.org>
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On Thursday 27 May 2010, Alan Stern wrote:
> On Wed, 26 May 2010, Rafael J. Wysocki wrote:
> 
> > On Wednesday 26 May 2010, Peter Zijlstra wrote:
> > > On Fri, 2010-05-21 at 15:46 -0700, Arve Hjønnevåg wrote:
> > > > +To create a suspend blocker from user space, open the suspend_blocker
> > > > special
> > > > +device file:
> > > > +
> > > > +    fd = open("/dev/suspend_blocker", O_RDWR | O_CLOEXEC);
> > > > +
> > > > +then optionally call:
> > > > +
> > > > +    ioctl(fd, SUSPEND_BLOCKER_IOCTL_SET_NAME(strlen(name)), name);
> > > > +
> > > > +To activate the suspend blocker call:
> > > > +
> > > > +    ioctl(fd, SUSPEND_BLOCKER_IOCTL_BLOCK);
> > > > +
> > > > +To deactivate it call:
> > > > +
> > > > +    ioctl(fd, SUSPEND_BLOCKER_IOCTL_UNBLOCK);
> > > > +
> > > > +To destroy the suspend blocker, close the device:
> > > > +
> > > > +    close(fd);
> > > 
> > > Urgh, please let the open() be BLOCK, the close() be UNBLOCK, and keep
> > > the SET_NAME thing if you really care.
> > 
> > SET_NAME wouldn't serve any purpose in that case.
> > 
> > This whole thing is related to the statistics part, which Arve says is
> > essential to him.  He wants to collect statistics for each suspend blocker
> > activated and deactivated so that he can tell who's causing problems by
> > blocking suspend too often.  The name also is a part of this.
> > 
> > In fact, without the statistics part the whole thing might be implemented
> > as a single reference counter such that suspend would happen when it went down
> > to zero.
> 
> There's also Arve's other main point: These suspend blockers tend to
> get used quite a lot.  Opening and closing a file each time has much
> higher overhead than a simple ioctl.
> 
> All these topics have been covered earlier in this discussion.  Peter 
> should go back and read the emails in the linux-pm archive.

Yeah.

Although with a reference counter the special device file won't be necessary.

Rafael