Re: [RFC][PATCH -mm 3/3] PM: Disable _request_firmware before hibernation/suspend

[Pavel Machek <pavel@ucw.cz>]

Hi!

> > > With the USB subsystem I have followed the approach taken by the PM
> > > core, which is that tasks are frozen.  But one can -- and Linus has on
> > > at least one occasion -- make a good case that tasks should be left
> > > running while only I/O is frozen.  This would require the subsystem to
> > > distinguish between a selective device suspend and a system-wide
> > > suspend-to-RAM, so that selective resume could be enabled on demand in
> > > one case but not the other.
> > > 
> > > It's quite doable in principle -- it's just not the technique I used.
> > 
> > I guess we need to do that. Random user should not be able to prevent
> > machine from sleeping.
> 
> Just to be clear about this, let's agree that we're talking about 
> suspend-to-RAM here, not hibernation.

Yes.

> It boils down to whether we want to freeze user tasks.  As I recall,
> Linus said that he didn't have any big objection to freezing user
> threads; he was much more concerned about freezing kernel threads.  
> Thanks to Raphael's new notifier chains this will no longer be an
> issue, since kernel threads will be able to stop themselves when they
> receive a suspend notification.
...
> The alternative is to have drivers take over the burden.  I don't like
> this at all.  The most obvious disadvantage is that the necessary
> checks would have to be duplicated many many times and spread out over
> lots of drivers.

I like freezer better :-).

> It's also harder to handle these things at the driver level.  Suppose a
> driver gets an I/O request while a suspend is underway.  What should it
> do?  Return an error?  Block until the suspend is over?  Both
> approaches have their difficulties:
> 
> Returning an error would mean that suspend is no longer transparent.  
> Even an error like -EAGAIN.

No, -EAGAIN is not nice.

> Waiting until the suspend is over is likely to be impractical.  At a 
> minimum it would involve adding code to drop a lock or mutex, enter the 
> freezer (or its equivalent), and then restart the I/O operation.  And 
> then, what if the driver was invoked with O_NONBLOCK?

Blocking would be possible option. I agree it is tricky to
implement... it may also be useful for a harddrive:

"I'm riding a horse at 40kph now, so you'll kill the harddrive if you
access it; just freeze everyone until we are at the other end of
meadow".

...hmm, but this seems to be blockdevice specific, and I can't think
of a network or char driver where similar behaviour would be useful.

> I think it is much better overall to stop I/O requests from being
> generated at the source, either by freezing userspace or preventing it
> from making system calls.  It's hard to imagine that anybody would
> miss the small amount of CPU time they'd be giving up by not allowing
> user threads to run during the time that a suspend is underway!

Agreed.
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html