Re: [PATCH v2 10/13] PCI: Avoid going from D3cold to D3hot for system sleep

[Lukas Wunner <lukas@wunner.de>]

On Mon, Jul 18, 2016 at 03:39:15PM +0200, Rafael J. Wysocki wrote:
> On Saturday, June 18, 2016 12:14:07 AM Lukas Wunner wrote:
> > On Fri, Jun 17, 2016 at 04:09:24PM -0500, Bjorn Helgaas wrote:
> > > On Fri, May 13, 2016 at 01:15:31PM +0200, Lukas Wunner wrote:
> > > > There are devices wich are not power-managed by the platform, yet can be
> > > > runtime suspended to D3cold with some other mechanism.  When putting the
> > > > system to sleep, we currently handle such devices improperly by trying
> > > > to transition them from D3cold to D3hot (the default power state defined
> > > > at the beginning of pci_target_state()).  Avoid that.
> > > > 
> > > > An example for devices affected by this are Thunderbolt controllers
> > > > built into Macs which can be put into D3cold with nonstandard ACPI
> > > > methods.
> > > > 
> > > > Signed-off-by: Lukas Wunner <lukas@wunner.de>
> > > 
> > > This needs an ack from Rafael.
> > > 
> > > > ---
> > > >  drivers/pci/pci.c | 2 ++
> > > >  1 file changed, 2 insertions(+)
> > > > 
> > > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > > index 791dfe7..6af9911 100644
> > > > --- a/drivers/pci/pci.c
> > > > +++ b/drivers/pci/pci.c
> > > > @@ -1943,6 +1943,8 @@ static pci_power_t pci_target_state(struct pci_dev *dev)
> > > >  			      && !(dev->pme_support & (1 << target_state)))
> > > >  				target_state--;
> > > >  		}
> > > > +	} else if (dev->current_state == PCI_D3cold) {
> > > > +		target_state = PCI_D3cold;
> > > >  	}
> > > 
> > > This only covers the case of !device_may_wakeup().  So I guess
> > > device_may_wakeup() is false for these Thunderbolt controllers.
> > > Is there a reason you don't want to do this check for devices that
> > > may wakeup?
> > 
> > Fear of breaking things. It would mean that a device would be left in
> > D3cold even though it may not be able to signal wakeup from that power
> > state.
> 
> Then it should not be put into D3_cold at run time too if it is wakeup-
> capable.
> 
> > That's a change of behaviour the consequences of which I cannot
> > estimate. Intuitively, I would expect breakage from such a change.
> 
> That would have been the case if the device had been capable of signaling
> wakeup from D3_cold at run time, but not from system sleep.  However, that
> can only happen when platform_pci_power_manageable() is true AFAICS.
> 
> So I'd change the switch () under the platform_pci_power_manageable() check
> to return "state" in the default case and then do
> 
> 	return dev->current_state < target_state ? target_state : dev->current_state;
> 
> at the end of the function.

That suggestion doesn't seem to be correct because there's another
value besides PCI_D3cold which is also greater than PCI_D3hot,
namely PCI_UNKNOWN. (If the device is in that state, e.g. after
pci_device_remove() has been called, and the system goes to sleep,
we'd leave the device as is and not put it into D3hot as we do now.)

I will update this patch with Bjorn's suggestion to also leave the
device in D3cold if it is wakeup-capable. The idea is to just change
the default state in the first line of the function like this:

-	pci_power_t target_state = PCI_D3hot;
+	pci_power_t target_state =
+		dev->current_state == PCI_D3cold ? PCI_D3cold : PCI_D3hot;

Thanks,

Lukas