Re: [PATCH] acpi: bus: handle power manageable but no _PSC/_PRx case

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Friday, September 07, 2012, Aaron Lu wrote:
> On Thu, Sep 06, 2012 at 11:53:34PM +0200, Rafael J. Wysocki wrote:
> > On Thursday, September 06, 2012, Rafael J. Wysocki wrote:
> > > On Monday, August 27, 2012, Aaron Lu wrote:
> > > > Currently, when we are trying to get the power state of an acpi device,
> > > > we will do the following:
> > > > If device is not power manageable, init its power state as its parent or
> > > > if it does not have a parent, init as D0;
> > > > If device is power manageable, evaluate _PSC and then refine with
> > > > acpi_power_get_inferred_state.
> > > > 
> > > > But there exist some devices with _PSx defined, but no _PSC or _PRx.
> > > > It is power manageable, but the above method to get power state does
> > > > not cover this case and its power state will be UNKNOWN(255).
> > > > 
> > > > So change the check of power manageable to whether _PSC and _PRx
> > > > defined.
> > > > 
> > > > Signed-off-by: Aaron Lu <aaron.lu@intel.com>
> > > 
> > > Applied to the linux-next branch of the linux-pm.git tree.
> > > 
> > > I think it should go to -stable too and therefore it would be good to have
> > > it in v3.6, so I'd like to push it to Linus for -rc5, if Len has no objections.
> > 
> > On a second thought, perhaps the initial power state of those devices
> > _should_ be "unknown", hmm?
> > 
> > After all, we don't know what power state the device is in.
> 
> Agree here but I think this is a safe assumption that on initial system
> boot, all devices should be at D0. We have already assumed this fact if
> the device is not power manageable and has no parent.

That is mandated by the spec, though.

> And this patch just changed the condition to not judging if power
> manageable but if it has _PSC or _PRx, since we will use that to know
> the device's power state, so I think using _PSC or _PRx as the
> condition check is more precise.

It is not clear if we can assume anything about the initial power states
of devices having _PSx defined if they cannot be determined through
_PSC or power resources.

> > Is there any practical user-visible problem this causes to happen?
> 
> Yes, on a test system, when I try to put a device into D3 cold and ACPI
> will complain that I can't due to its parent is in a even lower power
> state UNKNOWN(255), this parent device is power manageable but has no
> _PSC and _PRx defined.

Perhaps we can force _PS0 for such devices to start with, so that we know
for sure that the initial state is D0?

Rafael