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

Alan Stern

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