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

["Rafael J. Wysocki" <rafael@kernel.org>]

On Wed, Aug 3, 2016 at 2:28 PM, Lukas Wunner <lukas@wunner.de> wrote:
> 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.)

Right, I obviously forgot about PCI_UNKNOWN.

> 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;

That should work (even though it is a little clumsy IMO).

Thanks,
Rafael