Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

Matthew Garrett
2010-05-27T17:23:43Z

On Thu, May 27, 2010 at 07:15:31PM +0200, Thomas Gleixner wrote:
> On Thu, 27 May 2010, Matthew Garrett wrote:
> > You still need the in-kernel suspend blockers if you want to guarantee
> > that you can't lose wakeup events. But yes, if you're not concerned
> > handling badly behaved applications then I believe that you can lose
> > opportunistic suspend and just use the scheduler.
>
> No, we do not. We need correctly implemented drivers and a safe
> switchover from normal event delivery to wakeup based.

What is a "Correctly implemented driver" in this case? One that receives
a wakeup event and then prevents suspend being entered until userspace
has acknowledged that event? Because that's what an in-kernel suspend
blocker is.

> > My question was about explicit suspend states, not implicitly handling
> > an identical state based on scheduler constraints. Suspend-as-a-C-state
> > isn't usable on x86 - you have to explicitly trigger it based on some
>
> And why not ? Just because suspend is not implemented as an ACPI
> C-state ?
>
> Nonsense, if we want to push the system into suspend from the idle
> state we can do that. It's just not implemented and we've never tried
> to do it as it requires a non trivial amount of work, but I have done
> it on an ARM two years ago as a prove of concept and it works like a
> charm.

ACPI provides no guarantees about what level of hardware functionality
remains during S3. You don't have any useful ability to determine which
events will generate wakeups. And from a purely practical point of view,
since the latency is in the range of seconds, you'll never have a low
enough wakeup rate to hit it.

> > policy. And if you want to be able to do that without risking the loss
> > of wakeup events then you need in-kernel suspend blockers.
>
> Crap. Stop beating on those lost wakeup events. If we lose them then
> the drivers are broken and do not handle the switch over correctly. Or
> the suspend mechanism is broken as it does not evaluate the system
> state correctly. Blockers are just papering over that w/o tackling the
> real problem.

Ger;kljaserf;kljf;kljer;klj. Suspend blockers are the mechanism for the
driver to indicate whether the wakeup event has been handled. That's
what they're there for. The in-kernel ones don't paper over anything.

--
Matthew Garrett | mjg59@srcf.ucam.org