Re: Run-time PM idea (was: Re: [linux-pm] [RFC][PATCH 0/2] PM: Rearrange core suspend code)

[Matthew Garrett <mjg59@srcf.ucam.org>]

On Mon, Jun 08, 2009 at 03:22:35PM +0200, Ingo Molnar wrote:
> 
> * Matthew Garrett <mjg59@srcf.ucam.org> wrote:
> > The difficulty is in determining when it's viable to autosuspend a 
> > given device. There's a limit to how much we can determine purely 
> > from kernel state (for instance, we could suspend ahci when 
> > there's no pending disk access, but we'd lose hotplug 
> > notifications) so there's going to have to be some level of 
> > userspace policy determination. Having the infrastructure in the 
> > kernel is an important part of this, but there'll be some distance 
> > to go after that.
> 
> What will the 'user space policy' bit do what the kernel cannot?

How does the kernel know whether the user cares about SATA hotplug or 
not?

> If you mean the user has to configure something manually - that wont 
> really happen in practice. We are happy if they know where to put 
> those USB sticks in ;-)

It'll be up to the distributions to provide sane defaults and let them 
be reconfigured as necessary, depending on the information we have from 
the user and maybe platform-specific knowledge. But this is a difficult 
problem - we need to be smart about all the potential sources of 
information in order to pick an appropriate policy, and the kernel's not 
the right layer to do some of this information collection.

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