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Date:   Mon, 16 Aug 2021 12:07:32 -0500
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Shanker R Donthineni <sdonthineni@nvidia.com>
Cc:     Amey Narkhede <ameynarkhede03@gmail.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        alex.williamson@redhat.com,
        Raphael Norwitz <raphael.norwitz@nutanix.com>,
        linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
        kw@linux.com, Sinan Kaya <okaya@kernel.org>,
        Len Brown <lenb@kernel.org>,
        "Rafael J . Wysocki" <rjw@rjwysocki.net>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Mika Westerberg <mika.westerberg@linux.intel.com>
Subject: Re: [PATCH v15 7/9] PCI: Setup ACPI fwnode early and at the same
 time with OF
Message-ID: <20210816170732.GA2927693@bjorn-Precision-5520>
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On Sat, Aug 14, 2021 at 11:16:11AM -0500, Shanker R Donthineni wrote:
> Hi Bjorn,
> 
> On 8/13/21 11:10 PM, Bjorn Helgaas wrote:
> >>>> diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
> >>>> index eaddbf701759..dae021322b3f 100644
> >>>> --- a/drivers/pci/pci-acpi.c
> >>>> +++ b/drivers/pci/pci-acpi.c
> >>>> @@ -952,7 +952,6 @@ static bool acpi_pci_bridge_d3(struct pci_dev *dev)
> >>>>               return false;
> >>>>
> >>>>       /* Assume D3 support if the bridge is power-manageable by ACPI. */
> >>>> -     pci_set_acpi_fwnode(dev);
> >>>>       adev = ACPI_COMPANION(&dev->dev);
> >>> I *think* the Root Port code farther down in this function is also now
> >>> unnecessary:
> >>>
> >>>   acpi_pci_bridge_d3(...)
> >>>   {
> >>>     ...
> >>>     root = pcie_find_root_port(dev);
> >>>     adev = ACPI_COMPANION(&root->dev);
> >>>     if (root == dev) {
> >>>       /*
> >>>        * 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);
> >>>     }
> >>>
> >>> Since we're now setting the ACPI_COMPANION for every pci_dev long
> >>> before we get here, I think this could now be simplified to something
> >>> like this:
> >>>
> >>>   acpi_pci_bridge_d3(...)
> >>>   {
> >>>     if (!dev->is_hotplug_bridge)
> >>>       return false;
> >>>
> >>>     adev = ACPI_COMPANION(&dev->dev);
> >>>     if (adev && acpi_device_power_manageable(adev))
> >>>       return true;
> >>>
> >>>     root = pcie_find_root_port(dev);
> >>>     if (!root)
> >>>       return false;
> >>>
> >>>     adev = ACPI_COMPANION(&root->dev);
> >>>     if (!adev)
> >>>       return false;
> >>>
> >>>     rc = acpi_dev_get_property(dev, "HotPlugSupportInD3",
> >>>                                ACPI_TYPE_INTEGER, &val);
> >>>     if (rc < 0)
> >>>       return false;
> >>>
> >>>     return val == 1;
> >>>   }
> >> Agree, thanks for your suggestion. Yes, it can be simplified too.
> >> Can I do something like this using the unified device property API?
> >>
> >> static bool acpi_pci_bridge_d3(struct pci_dev *dev)
> >> {
> >>         struct acpi_device *adev;
> >>         struct pci_dev *root;
> >>         u8 val;
> >>
> >>         if (!dev->is_hotplug_bridge)
> >>                 return false;
> >>
> >>         adev = ACPI_COMPANION(&dev->dev);
> >>         if (adev && acpi_device_power_manageable(adev))
> >>                 return true;
> >>
> >>         root = pcie_find_root_port(dev);
> >>         if (!root)
> >>                 return false;
> >>
> >>         if (device_property_read_u8(&root->dev, "HotPlugSupportInD3", &val))
> >>                 return false;
> > I guess that might be OK.
> >
> > TBH I don't really like the device_property_read_u8() thing because
> > (1) we know this is an ACPI property and I don't see a reason to use
> > an "generic" interface that doesn't buy us anything, and (2) the
> > connection to the source of the data (a _DSD method) is really, really
> > hard to find.
> >
> > Admittedly, it's still pretty hard to connect acpi_dev_get_property()
> > with "_DSD".  The only real clue is the comment about "Look for a
> > special _DSD property ..."
> >
> Does it satisfy you if I change the comment and still use device_property API?
> 
> static bool acpi_pci_bridge_d3(struct pci_dev *dev)
> {
> ??????? struct pci_dev *rpdev;
> ??????? u8 val;
> 
> ??????? if (!dev->is_hotplug_bridge)
> ??????????????? return false;
> 
> ??????? /* Assume D3 support if the bridge is power-manageable by ACPI. */
> ??????? if (acpi_pci_power_manageable(dev))
> ??????????????? return true;
> 
> ??????? /*
> ???????? * Look for 'HotPlugSupportInD3' property for the root port and if
> ???????? * it is set we know the hierarchy behind it supports D3 just fine.
> ???????? */
> ??????? rpdev = pcie_find_root_port(dev);
> ??????? if (!rpdev)
> ??????????????? return false;
> 
> ??????? if (device_property_read_u8(&rpdev->dev, "HotPlugSupportInD3", &val))
> ??????????????? return false;
> 
> ??????? return val == 1;
> }
> 
> If not, I'll do changes like this.

I guess either one is fine.  But I think we should extend the comment
and commit log to make it clear that device_property_read_u8() and
acpi_dev_get_property() are ultimately looking for a _DSD.  I should
have asked for this when we merged 26ad34d510a8 ("PCI / ACPI:
Whitelist D3 for more PCIe hotplug ports") in the first place.

If we expect that power management *should* be enabled for a bridge,
and we observe that it *isn't* enabled, it is unreasonably difficult
to figure out from the code what is missing in the firmware, namely,
the _DSD laid out in the commit log for 26ad34d510a8.

> static bool acpi_pci_bridge_d3(struct pci_dev *dev)
> {
> ??????? const union acpi_object *obj;
> ??????? struct acpi_device *adev;
> ??????? struct pci_dev *rpdev;
> 
> 
> ??????? if (!dev->is_hotplug_bridge)
> ??????????????? return false;
> 
> ??????? /* Assume D3 support if the bridge is power-manageable by ACPI. */
> ??????? if (acpi_pci_power_manageable(dev))
> ??????????????? return true;
> 
> ??????? /*
> ???????? * Look for 'HotPlugSupportInD3' property for the root port and if
> ???????? * it is set we know the hierarchy behind it supports D3 just fine.
> ???????? */
> ??????? rpdev = pcie_find_root_port(dev);
> ??????? if (!rpdev)
> ??????????????? return false;
> 
> ??????? adev = ACPI_COMPANION(&rpdev->dev);
> ??????? if (!adev)
> ??????????????? return false;
> 
> ?????? if (acpi_dev_get_property(adev, "HotPlugSupportInD3",
> ?????????????????????????????????? ACPI_TYPE_INTEGER, &obj) < 0)
> ??????????????? return false;
> 
> ??????? return obj->integer.value == 1;
> }
> 
>