Re: [linux-pm] [PATCH 1/8] PM: Add suspend block api.

[Tony Lindgren <tony@atomide.com>]

* Alan Stern <stern@rowland.harvard.edu> [100506 11:42]:
> On Thu, 6 May 2010, Tony Lindgren wrote:
> 
> > Well if your hardware runs off-while-idle or even just
> > retention-while-idle, then the basic shell works just fine waking up
> > every few seconds or so.
> > 
> > Then you could keep init/shell/suspend policy deamon running until
> > it's time to suspend the whole device. To cut down runaway timers,
> > you could already freeze the desktop/GUI/whatever earlier.
> 
> This comes down mostly to efficiency.  Although the suspend blocker
> patch does the actual suspending in a workqueue thread, AFAIK there's
> no reason it couldn't use a user thread instead.
> 
> The important difference lies in what happens when a suspend fails
> because a driver is busy.  Without suspend blockers, the kernel has to
> go through the whole procedure of freezing userspace and kernel threads
> and then suspending a bunch of drivers before hitting the one that's
> busy.  Then all that work has to be undone.  By contrast, with suspend
> blockers the suspend attempt can fail right away with minimal overhead.

But does that really matter if you're only few tens of times times per
day or so? I don't understand why you would want to try to suspend except
after some timeout of no user or network activity.
 
> There's also a question of reliability.  With suspends controlled by 
> userspace there is a possibility of races, which could lead the system 
> to suspend when it shouldn't.  With control in the kernel, these races 
> can be eliminated.

I agree the suspend needs to happen without races. But I think the
logic for when to suspend should be done in the userspace as it
can be device or user specific.

Regards,

Tony