Re: [PATCH 0/8] Suspend block api (version 6)

[Rafael J. Wysocki]

On Thursday 06 May 2010, Kevin Hilman wrote:
> Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> writes:
> 
> > On Wed, 5 May 2010, Matthew Garrett wrote:
> >
> >> On Wed, May 05, 2010 at 03:20:40PM -0400, Alan Stern wrote:
> >> 
> >> > One the face of it, a runtime-PM solution would dictate that the
> >> > codec's driver ought to turn off the codec whenever the driver thinks
> >> > it isn't being used.  Ergo, if the driver didn't know when a call was
> >> > in progress, it would use runtime PM to turn off the codec during a
> >> > call.
> >> 
> >> Well, part of the problem is that right now most of our beliefs about 
> >> imposed constraints tend to be based on what userspace is doing - "Don't 
> >> power down the audio codec when userspace has it open", for instance. 
> >> But that goes away with opportunistic suspend. In most cases you don't 
> >> want the audio codec to stay awake just because userspace was using it 
> >> to make bouncing cow noises, especially if you've just frozen userspace. 
> >> So the problem becomes more complicated than it would otherwise be.
> >
> > It sounds like the problem can be stated simply enough: At the moment, 
> > nobody knows when the codec should be powered down!  Userspace might 
> > have some idea, but even if its ideas are right it has no way of 
> > communicating them to the kernel.
> >
> > The power/control sysfs attribute was intended for just that purpose,
> > although it was aimed at runtime PM rather than system PM.  
> > Nevertheless, it or something like it could be used.  Of course, there 
> > would still remain the issue of userspace telling the kernel not to 
> > power down the codec while making bouncing cow noises -- but at this 
> > point it's not really a kernel problem any more.
> 
> I guess what we're talking about here is a set of per-device
> constraints that could be used by both [opportunistic|system] suspend
> and runtime PM.  For lack of a better term, per-device PM QoS (as
> compared to the current system-wide PM QoS.)
> 
> For example, if userspace (or some other device) has communicated that
> it has a constraint on the audio HW, then both the suspend path and the
> runtime PM path could check those constraints before making a decision
> on how to act.  Hopefully the phone app would set a constraint and the
> cow-noise app would not.  :)
> 
> On OMAP, we keep track of per-device constraints (currently latency
> and throughput) in order to make proper run-time PM decicions in the
> kernel, but we are realizing that we need a way for userspace to
> communicate these constraints as well, so that userspace can make
> power vs. performance policy decisions instead of the kernel.
> 
> Probably generalizing these into the LDM is the direction to go so
> userspace can set constraints on a per-device (or per-class?) basis:
> 
> /sys/devices/.../power/constraint/throughput
> /sys/devices/.../power/constraint/wakeup_latency
> /sys/devices/.../power/constraint/... ?

That sounds reasonable although it may be a challenge to find a set of
universal constraints common to all devices.

Rafael
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