[PATCH 03/12] PCI/ACPI: Add aux power grant notifier

[Rafael J. Wysocki]

On Tue, Apr 1, 2025 at 10:13 PM Bjorn Helgaas <helgaas at kernel.org> wrote:
>
> On Tue, Apr 01, 2025 at 09:02:16PM +0530, Anshuman Gupta wrote:
> > Adding a notifier to notify all PCIe child devices about the
> > block main power removal. It is needed because theoretically
> > multiple PCIe Endpoint devices on same Root Port
> > can request for AUX power and _DSM method can return with
> > 80000000h signifies that the hierarchy connected via
> > the slot cannot support main power removal when in D3Cold.
>
> I wish the spec used different language here.  "D3cold" *means* "main
> power is removed" (PCIe r6.0, sec 5.3.1.4.2), so it doesn't make sense
> to say that "the slot cannot support main power removal when in
> D3cold".  If a device is in D3cold, its main power has been removed by
> definition.
>
> I suppose the spec is trying to say if the driver has called this _DSM
> with 80000000h, it means the platform must not remove main power from
> the device while the system is in S0?  Is that what you think it
> means?
>
> The 2h return value description says it "indicates that the platform
> will not remove main power from the slot while the system is in S0,"
> so I guess that must be it.
>
> In this series, pci_acpi_aux_power_setup() only supplies a 16-bit
> aux_pwr_limit value, so the driver cannot *request* that the platform
> not remove main power.
>
> So I guess the only scenario where the _DSM returns 80000000h is when
> the platform itself or other devices prevent the removal of main
> power.  And the point of the notifier is to tell devices that their
> main power will never be removed while the system is in S0.  Is that
> right?
>
> > This may disrupt functionality of other child device.
>
> What sort of disruption could happen?  I would think that if the _DSM
> returns 80000000h, it just means the device will not have main power
> removed, so it won't see that power state transition.  The only
> "disruption" would be that we're using more power.
>
> > Let's notify all PCIe devices requested Aux power resource
> > and Let PCIe End Point driver handle it in its callback.
>
> Wrap to fill 75 columns.
>
> s/Adding/Add/
> s/Let's notify/Notify/
> s/and Let/and let/
> s/End Point/Endpoint/ (several places here and below)
>
> > Signed-off-by: Anshuman Gupta <anshuman.gupta at intel.com>
> > ---
> >  drivers/pci/pci-acpi.c   | 34 +++++++++++++++++++++++++++++++---
> >  include/linux/pci-acpi.h | 13 +++++++++++++
> >  2 files changed, 44 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
> > index 04149f037664..d1ca1649e6e8 100644
> > --- a/drivers/pci/pci-acpi.c
> > +++ b/drivers/pci/pci-acpi.c
> > @@ -1421,6 +1421,32 @@ static void pci_acpi_optimize_delay(struct pci_dev *pdev,
> >       ACPI_FREE(obj);
> >  }
> >
> > +static BLOCKING_NOTIFIER_HEAD(pci_acpi_aux_power_notify_list);
> > +
> > +/**
> > + * pci_acpi_register_aux_power_notifier - Register driver notifier
> > + * @nb: notifier block
> > + *
> > + * This function shall be called by PCIe End Point device requested the Aux
> > + * power resource in order to handle the any scenario gracefully when other
> > + * child PCIe devices Aux power request returns with No main power removal.
> > + * PCIe devices which register this notifier shall handle No main power
> > + * removal scenario accordingly.
>
> This would actually be called by the *driver* (not by the device).

Apart from this, there seems to be a design issue here because it
won't notify every driver that has requested the Aux power, just the
ones that have also registered notifiers.

So this appears to be an opt-in from getting notifications on Aux
power request rejection responses to requests from other drivers
requesting Aux power for the same Root Port, but the changelog of the
first patch already claimed that the aggregation of requests was not
supported.  So if only one driver will be allowed to request the Aux
power for the given Root Port, why would the notifiers be necessary
after all?