Re: [PATCH v7 05/12] PCI/PM: Disable device wakeups when halting or powering off system

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

On Wed, Sep 10, 2025 at 7:24 PM Mario Limonciello <superm1@kernel.org> wrote:
>
> On 9/10/25 12:11 PM, Bjorn Helgaas wrote:
> > On Wed, Sep 10, 2025 at 11:52:00AM -0500, Mario Limonciello wrote:
> >> On 9/10/25 10:06 AM, Bjorn Helgaas wrote:
> >>> On Tue, Sep 09, 2025 at 02:16:12PM -0500, Mario Limonciello (AMD) wrote:
> >>>> PCI devices can be configured as wakeup sources from low power states.
> >>>> However, when the system is halting or powering off such wakeups are
> >>>> not expected and may lead to spurious behavior.
> >>>
> >>> I'm a little unclear on the nomenclature for these low power states,
> >>> so I think it would be helpful to connect to the user action, e.g.,
> >>> suspend/hibernate/etc, and the ACPI state, e.g.,
> >>>
> >>>     ... when the system is hibernating (e.g., transitioning to ACPI S4
> >>>     and halting) or powering off (e.g., transitioning to ACPI S5 soft
> >>>     off), such wakeups are not expected ...
> >>
> >> I will try to firm it up in the commit message.  But yes you're getting the
> >> intent, having a wakeup occur at S5 would be unexpected, and would likely
> >> change semantics of what people "think" powering off a machine means.
> >>
> >>> When I suspend or power off my laptop from the GUI user interface, I
> >>> want to know if keyboard or mouse activity will resume or if I need to
> >>> press the power button.
> >>
> >> The way the kernel is set up today you get a single wakeup sysfs file for a
> >> device and that wakeup file means 3 things:
> >> * abort the process of entering a suspend state or hibernate
> >> * wake up the machine from a suspend state
> >> * wake up the machine from hibernate
> >>
> >>>> ACPI r6.5, section 16.1.5 notes:
> >>>>
> >>>>       "Hardware does allow a transition to S0 due to power button press
> >>>>        or a Remote Start."
> >>>
> >>> Important to note here that sec 16.1.5 is specifically for "S5
> >>> Soft Off State".
> >>>
> >>> S4 is a sleeping state and presumably sec 16.1.6 ("Transitioning
> >>> from the Working to the Sleeping State") applies.  That section
> >>> mentions wakeup devices, so it's not obvious to me that PCI device
> >>> wakeup should be disabled for S4.
> >>
> >> It actually /shouldn't/ be disabled for S4 - it should only be
> >> disabled for S5.
> >>
> >> Are you implying a bug in the flow?  I didn't think there was one:
> >>
> >> During entering hibernate the poweroff() call will have system_state
> >> = SYSTEM_SUSPEND so wakeups would be enabled.
> >>
> >> For powering off the system using hibernate flows poweroff() call
> >> would have system_state = SYSTEM_HALT or SYSTEM_POWER_OFF.
> >
> > OK.  I assumed that since you check for two states (SYSTEM_HALT or
> > SYSTEM_POWER_OFF), one must be hibernate (ending up in S4?) and the
> > other a soft power off (ending up in S5?).
> >
> > But it sounds like there are two ways to power off.  I'm just confused
> > about the correspondence between hibernate, soft poweroff, S4, S5,
> > SYSTEM_HALT, and SYSTEM_POWER_OFF.
> >
> > *Do* both SYSTEM_HALT and SYSTEM_POWER_OFF lead to S5 on an ACPI
> > system?  If so, what's the difference between them?
>
> The two functions are kernel_halt() and kernel_power_off().
>
> And looking again, Ahhhh!  kernel_power_off() is the only thing that
> actually leads to machine_power_off().  Halt just stops the CPUs.
>
> I think we should only be using the hibernate flows for SYSTEM_POWER_OFF.

That's correct.