Re: [PATCH v2 1/5] PCI: host-common: Add helper to determine host bridge D3cold eligibility

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Feb 23, 2026 at 07:46:08PM +0100, Rafael J. Wysocki wrote:
> On Mon, Feb 23, 2026 at 7:10 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
> > On Tue, Feb 17, 2026 at 04:49:06PM +0530, Krishna Chaitanya Chundru wrote:
> > > Add a common helper, pci_host_common_can_enter_d3cold(), to determine
> > > whether a PCI host bridge can safely transition to D3cold.
> > >
> > > This helper is intended to be used by PCI host controller drivers to
> > > decide whether they may safely put the host bridge into D3cold based on
> > > the power state and wakeup capabilities of downstream endpoints.
> > >
> > > The helper walks all devices on the bridge's primary bus and only allows
> > > the host bridge to enter D3cold if all PCIe endpoints are already in
> > > PCI_D3hot. This ensures that we do not power off the host bridge while
> > > any active endpoint still requires the link to remain powered.
> > >
> > > For devices that may wake the system, the helper additionally requires
> > > that the device supports PME wake from D3cold (via WAKE#). Devices that
> > > do not have wakeup enabled are not restricted by this check and do not
> > > block the host bridge from entering D3cold.
> > >
> > > Devices without a bound driver and with PCI not enabled via sysfs are
> > > treated as inactive and therefore do not prevent the host bridge from
> > > entering D3cold. This allows controllers to power down more aggressively
> > > when there are no actively managed endpoints.
> >
> > This series is currently structured so it's only applicable to native
> > host bridge drivers, i.e., things using DT.  Is there anything that
> > prevents using D3cold for host bridges in ACPI systems?
> 
> Do you mean in principle or in practice?

Just in principle.  I was hoping for some way to tie this decision
making back to a spec.  Everything downstream being in D3 already, and
any wakeup devices supporting PME from D3cold sounds plausible to me,
but I don't know about any prescriptive spec language, and I wouldn't
want to get locked into a scheme that couldn't be supported on ACPI.

> x86 platforms don't support PCI host bridge PM AFAICS and the ACPI
> driver doesn't support PM either.  I'd rather not add it ad hoc
> because of this patch series.

Agreed.

> > > Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
> > > ---
> > >  drivers/pci/controller/pci-host-common.c | 45 ++++++++++++++++++++++++++++++++
> > >  drivers/pci/controller/pci-host-common.h |  2 ++
> > >  2 files changed, 47 insertions(+)
> > >
> > > diff --git a/drivers/pci/controller/pci-host-common.c b/drivers/pci/controller/pci-host-common.c
> > > index d6258c1cffe5ec480fd2a7e50b3af39ef6ac4c8c..b0a4a3c995e80e0245657f0273a349334071013c 100644
> > > --- a/drivers/pci/controller/pci-host-common.c
> > > +++ b/drivers/pci/controller/pci-host-common.c
> > > @@ -106,5 +106,50 @@ void pci_host_common_remove(struct platform_device *pdev)
> > >  }
> > >  EXPORT_SYMBOL_GPL(pci_host_common_remove);
> > >
> > > +static int pci_host_common_check_d3cold(struct pci_dev *pdev, void *userdata)
> > > +{
> > > +     bool *d3cold_allow = userdata;
> > > +
> > > +     if (pci_pcie_type(pdev) != PCI_EXP_TYPE_ENDPOINT)
> > > +             return 0;
> > > +
> > > +     if (!pdev->dev.driver && !pci_is_enabled(pdev))
> > > +             return 0;
> > > +
> > > +     if (pdev->current_state != PCI_D3hot)
> > > +             goto exit;
> > > +
> > > +     if (device_may_wakeup(&pdev->dev) && !pci_pme_capable(pdev, PCI_D3cold))
> > > +             goto exit;
> > > +
> > > +     return 0;
> > > +exit:
> > > +     *d3cold_allow = false;
> > > +     return -EBUSY;
> > > +}
> > > +
> > > +/**
> > > + * pci_host_common_can_enter_d3cold - Determine whether a host bridge may enter D3cold
> > > + * @bridge: PCI host bridge to check
> > > + *
> > > + * Walk downstream PCIe endpoint devices and determine whether the host bridge
> > > + * is permitted to transition to D3cold.
> > > + *
> > > + * The host bridge may enter D3cold only if all active PCIe endpoints are in
> > > + * %PCI_D3hot and any wakeup-enabled endpoint is capable of generating PME from
> > > + * D3cold. Inactive endpoints are ignored.
> > > + *
> > > + * Return: %true if the host bridge may enter D3cold, otherwise %false.
> > > + */
> > > +bool pci_host_common_can_enter_d3cold(struct pci_host_bridge *bridge)
> > > +{
> > > +     bool d3cold_allow = true;
> > > +
> > > +     pci_walk_bus(bridge->bus, pci_host_common_check_d3cold, &d3cold_allow);
> > > +
> > > +     return d3cold_allow;
> > > +}
> > > +EXPORT_SYMBOL_GPL(pci_host_common_can_enter_d3cold);
> > > +
> > >  MODULE_DESCRIPTION("Common library for PCI host controller drivers");
> > >  MODULE_LICENSE("GPL v2");
> > > diff --git a/drivers/pci/controller/pci-host-common.h b/drivers/pci/controller/pci-host-common.h
> > > index b5075d4bd7eb31fbf1dc946ef1a6afd5afb5b3c6..18a731bca058828340bca84776d0e91da1edbbf7 100644
> > > --- a/drivers/pci/controller/pci-host-common.h
> > > +++ b/drivers/pci/controller/pci-host-common.h
> > > @@ -20,4 +20,6 @@ void pci_host_common_remove(struct platform_device *pdev);
> > >
> > >  struct pci_config_window *pci_host_common_ecam_create(struct device *dev,
> > >       struct pci_host_bridge *bridge, const struct pci_ecam_ops *ops);
> > > +
> > > +bool pci_host_common_can_enter_d3cold(struct pci_host_bridge *bridge);
> > >  #endif
> > >
> > > --
> > > 2.34.1
> > >