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

["Rafael J. Wysocki" <rjw@sisk.pl>]

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