On Thursday, July 17, 2014 10:39:16 AM Alan Stern wrote:

On Wed, 16 Jul 2014, Dmitry Torokhov wrote:

quoted

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