linux-kernel - Re: [linux-pm] [PATCH 2/8] PM: suspend_block: Add driver to access suspend blockers from user-space

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Date:	Thu, 27 May 2010 00:34:44 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Cornelia Huck <cornelia.huck@...ibm.com>,
	Len Brown <len.brown@...el.com>,
	Jim Collar <jim.collar@...are.net>, linux-doc@...r.kernel.org,
	"Greg Kroah-Hartman" <gregkh@...e.de>,
	linux-kernel@...r.kernel.org, Avi Kivity <avi@...hat.com>,
	Ryusuke Konishi <konishi.ryusuke@....ntt.co.jp>,
	Magnus Damm <damm@...l.co.jp>,
	linux-pm@...ts.linux-foundation.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [linux-pm] [PATCH 2/8] PM: suspend_block: Add driver to access suspend blockers from user-space

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 HjÃ¸nnevÃ¥g 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
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