Re: Power-managing devices that are not of interest at some point in time

[Dmitry Torokhov <dtor@google.com>]

On Thursday, July 17, 2014 10:39:16 AM Alan Stern wrote:
> On Wed, 16 Jul 2014, Dmitry Torokhov wrote:
> > We are not planning on implementing the policy in kernel, that's
> > indeed task for userspace; but unless we bring in the heavy hammer of
> > forcibly unbinding drivers, we do not currently have universal
> > mechanism of quiescing devices.
> 
> We sort of do: the ->freeze() callback.  But it wasn't intended for
> this kind of use; drivers may very well expect that userspace will
> already be frozen when the callback runs.  Besides, ->freeze() is
> supposed to quiesce devices without powering them down, whereas you
> want to do both.

Right.

> 
> What you're asking for is different from anything the PM subsystem has
> done before.

Right.

> Given this fact, I don't see any alternatives to adding a
> new API or repurposing an existing API.  Either one would be somewhat
> painful.
> 
> For example, we could arrange to invoke ->suspend().  However, since
> the circumstances would be unusual (userspace is still running,
> ->prepare() was not called beforehand, ->suspend_irq() won't be called
> afterward), subsystems and drivers may very well react inappropriately.

I do not think anybody expects that drivers would not have to be modified to 
support this functionality; I expect drivers would have to declare themselves 
"queiscable" and therefore would assert that they will act according to 
whatever rules we set up. I only want to make sure that this new state is 
added to existing list of PM states rather than creating completely new 
facility, so that driver authors have a chance to understand PM state 
transitions that involve their driver. 

Thanks,
Dmitry