Re: [PATCH V2 1/2] PCI: Add ACPI StorageD3Enable _DSD support

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

On Wed, Jun 24, 2020 at 11:15 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> On Fri, Jun 12, 2020 at 01:48:19PM -0700, David E. Box wrote:
> > StorageD3Enable is a boolean property that indicates that the platform
> > wants to use D3 for PCIe storage drives during suspend-to-idle. It is a
> > BIOS work around that is currently in use on shipping systems like some
> > Intel Comet Lake platforms. It is meant to change default driver policy for
> > suspend that may cause higher power consumption.
> >
> > Add the DSD property for recognition by fwnode calls and provide an
> > exported symbol for device drivers to use to read the property as needed.
> >
> > Link: https://docs.microsoft.com/en-us/windows-hardware/design/component-guidelines/power-management-for-storage-hardware-devices-intro
> > Signed-off-by: David E. Box <david.e.box@linux.intel.com>
> > ---
> >  drivers/acpi/property.c |  3 +++
> >  drivers/pci/pci-acpi.c  | 59 +++++++++++++++++++++++++++++++++++++++++
> >  include/linux/pci.h     |  2 ++
> >  3 files changed, 64 insertions(+)
> >
> > diff --git a/drivers/acpi/property.c b/drivers/acpi/property.c
> > index e601c4511a8b..c2e2ae774a19 100644
> > --- a/drivers/acpi/property.c
> > +++ b/drivers/acpi/property.c
> > @@ -45,6 +45,9 @@ static const guid_t prp_guids[] = {
> >       /* Thunderbolt GUID for WAKE_SUPPORTED: 6c501103-c189-4296-ba72-9bf5a26ebe5d */
> >       GUID_INIT(0x6c501103, 0xc189, 0x4296,
> >                 0xba, 0x72, 0x9b, 0xf5, 0xa2, 0x6e, 0xbe, 0x5d),
> > +     /* Storage device needs D3 GUID: 5025030f-842f-4ab4-a561-99a5189762d0 */
> > +     GUID_INIT(0x5025030f, 0x842f, 0x4ab4,
> > +               0xa5, 0x61, 0x99, 0xa5, 0x18, 0x97, 0x62, 0xd0),
> >  };
> >
> >  /* ACPI _DSD data subnodes GUID: dbb8e3e6-5886-4ba6-8795-1319f52a966b */
> > diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
> > index d21969fba6ab..732df524e09c 100644
> > --- a/drivers/pci/pci-acpi.c
> > +++ b/drivers/pci/pci-acpi.c
> > @@ -972,6 +972,65 @@ static bool acpi_pci_bridge_d3(struct pci_dev *dev)
> >       return val == 1;
> >  }
> >
> > +/**
> > + * pci_acpi_storage_d3 - whether root port requests D3 for idle suspend
> > + * @pdev: PCI device to check
> > + *
> > + * Returns true if the ACPI companion device contains the "StorageD3Enable"
> > + * _DSD property and the value is 1. This indicates that the root port is
> > + * used by a storage device and the platform is requesting D3 for the
> > + * device during suspend to idle in order to support platform pm.
> > + */
> > +bool pci_acpi_storage_d3(struct pci_dev *dev)
> > +{
> > +     const struct fwnode_handle *fwnode;
> > +     struct acpi_device *adev;
> > +     struct pci_dev *root;
> > +     acpi_handle handle;
> > +     acpi_status status;
> > +     bool ret = false;
> > +     u8 val;
> > +
> > +     /*
> > +      * Look for _DSD property specifying that the storage device on
> > +      * the port must use D3 to support deep platform power savings during
> > +      * suspend-to-idle
> > +      */
> > +     root = pci_find_pcie_root_port(dev);
>
> I think this would need to be updated to apply to v5.8-rc1 after
> 6ae72bfa656e ("PCI: Unify pcie_find_root_port() and
> pci_find_pcie_root_port()").
>
> https://git.kernel.org/linus/6ae72bfa656e
>
> > +     if (!root)
> > +             return false;
> > +
> > +     adev = ACPI_COMPANION(&root->dev);
> > +     if (!adev) {
> > +             /*
> > +              * It is possible that the ACPI companion is not yet bound
> > +              * for the root port so look it up manually here.
> > +              */
> > +             if (!adev && !pci_dev_is_added(root))
> > +                     adev = acpi_pci_find_companion(&root->dev);
>
> I see that you copied this "ACPI companion not yet bound" thing from
> acpi_pci_bridge_d3().  But it's ugly.
>
> Isn't there a way we can bind the ACPI companion during normal PCI
> enumeration so we don't need this exception case?
>
> I really do not like the idea of putting this code in the PCI core
> because AFAICT the PCI core can do nothing with this information.
>
> If we could make sure during enumeration that the root port always has
> an ACPI companion, this code could go to the nvme driver itself.  And
> we could also clean up the ugliness in acpi_pci_bridge_d3().
>
> Rafael, is that possible?  I don't really know how the companion
> device gets set.

That's a bit convoluted.

device_add() calls device_platform_notify(), before calling bus_add_device().

device_platform_notify() calls acpi_platform_notify() which invokes
acpi_device_notify() that looks for the companion via
type->find_companion() which for PCI points to
acpi_pci_find_companion().  If found, the companion is attached to the
dev structure as a physical_node, via acpi_bind_one().

So by the time bus_probe_device() runs, the companion should be there
already - if it is there at all.

The parent ACPI companion should be present when the child is probing
too, as per the above.

> Maybe this is could be done somewhere around pci_device_add()?

It is done in there.

It is not necessary to call acpi_pci_find_companion() from
pci_acpi_storage_d3() as long as that function is required to be
called by the target device's driver probe or later.

Ths acpi_pci_bridge_d3() case is different, though, AFAICS, because it
is invoked in the pci_pm_init() path, via pci_bridge_d3_possible(),
and that gets called from pci_device_add() *before* calling
device_add().

Mika, is that why acpi_pci_find_companion() gets callled from
acpi_pci_bridge_d3()?