Date:   Fri, 11 Nov 2022 12:58:28 -0600
From:   "Limonciello, Mario" <mario.limonciello@....com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Mehta Sanju <Sanju.Mehta@....com>,
        Lukas Wunner <lukas@...ner.de>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        linux-acpi@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5] PCI/ACPI: PCI/ACPI: Validate devices with power
 resources support D3

On 11/11/2022 11:41, Bjorn Helgaas wrote:
> On Mon, Oct 31, 2022 at 05:33:55PM -0500, Mario Limonciello wrote:
>> Firmware typically advertises that ACPI devices that represent PCIe
>> devices can support D3 by a combination of the value returned by
>> _S0W as well as the HotPlugSupportInD3 _DSD [1].
>>
>> `acpi_pci_bridge_d3` looks for this combination but also contains
>> an assumption that if an ACPI device contains power resources the PCIe
>> device it's associated with can support D3.  This was introduced
>> from commit c6e331312ebf ("PCI/ACPI: Whitelist hotplug ports for
>> D3 if power managed by ACPI").
>>
>> Some firmware configurations for "AMD Pink Sardine" do not support
>> wake from D3 in _S0W for the ACPI device representing the PCIe root
>> port used for tunneling. The PCIe device will still be opted into
>> runtime PM in the kernel [2] because of the logic within
>> `acpi_pci_bridge_d3`. This currently happens because the ACPI
>> device contains power resources.
>>
>> When the thunderbolt driver is loaded two device links are created:
>> * USB4 router <- PCIe root port for tunneling
>> * USB4 router <- XHCI PCIe device
>>
>> These device links are created because the ACPI devices declare the
>> `usb4-host-interface` _DSD [3]. For both links the USB4 router is the
>> supplier and these other devices are the consumers.
>> Here is a demonstration of this topology that occurs:
>>
>> |
>> ├─ 00:03.1
>> |       | "PCIe root port used for tunneling"
>> |       | ACPI Path: \_SB_.PCI0.GP11
>> |       | ACPI Power Resources: Yes
> 
> I guess this means it has _PR0 and/or _PS0?  (same for devices below)

Yeah.

> 
>> |       | ACPI _S0W return value: 0
>> |       | Device Links: supplier:pci:0000:c4:00.5
>> |       └─ PCIe Power state: D0
> 
> What bus does 00:03.1 lead to?  I guess it doesn't lead to any of the
> devices below?

By default - nothing.  The topology was drawn correctly.

When you plug in a USB4 device with PCIe the newly enumerated devices 
hang off this bus.

> 
>> └─ 00:08.3
>>          | ACPI Path: \_SB_.PCI0.GP19
>>          ├─ PCIe Power state: D0
> 
> I guess 00:08.3 is a Root Port leading to bus c4?
> 

Right.

>>          ├─ c4:00.3
>>          |       | "XHCI PCIe device used for tunneling"
>>          |       | ACPI Path: \_SB_.PCI0.GP19.XHC3
>>          |       | ACPI Power Resources: Yes
>>          |       | ACPI _S0W return value: 4
>>          |       | Device Links: supplier:pci:0000:c4:00.5
>>          |       └─ PCIe Power state: D3cold
>>          └─ c4:00.5
>>                  | "USB4 Router"
>>                  | ACPI Path: \_SB_.PCI0.GP19.NHI0
>>                  | ACPI Power Resources: Yes
>>                  | ACPI _S0W return value: 4
>>                  | Device Links: consumer:pci:0000:00:03.1 consumer:pci:0000:c4:00.3
>>                  └─ PCIe Power state: D3cold
> 
> I'm reading the excellent Documentation/driver-api/device_link.rst and
> trying to match up with this topology.  This is a case where 00:03.1
> is a consumer of c4:00.5.  The driver core automatically enforces that
> children are suspended before parents, but since 00:03.1 is an aunt
> (not a child) of c4:00.5, the device link enforces the requirement
> that 00:03.1 be suspended before c4:00.5.  Right?

Yup!

> 
> Is c4:00.5 an NHI?

Yes.

> 
> The PCI power states shown above are the failure case?  c4:00.5
> *should* remain in D0 as long as 00:03.1 is in D0, but was instead put
> in D3cold?

Yes.

> 
>> Currently runtime PM is allowed for all of these devices.
> 
> This is because they all have _PR0 and/or _PS0?  (Diagram doesn't show
> that for 00:08.3, but I assume it must?)

For the root port used for tunneling it's because 
pci_bridge_d3_possible() returns TRUE.
This returns true because platform_pci_bridge(d3) return TRUE.

For the NHI it's because thunderbolt.ko sets this for all NHIs (see 
drivers/thunderbolt/nhi.c).

> 
> And I guess they all must have dev->is_hotplug_bridge set?

The PCIe root port for tunneling does; yes.

> 
>> This means that
>> when all consumers are idle long enough, they will runtime suspend to
>> enter their deepest allowed sleep state. Once all consumers are in their
>> deepest allowed sleep state the suppliers will enter the deepest sleep
>> state as well.
>>
>> * The PCIe root port for tunneling doesn't support waking from D3hot or
>>    D3cold so although runtime suspended it stays in D0.
>> * The XHCI PCIe device supports wakeup from D3cold so it runtime suspends
>>    to D3cold.
>> * Both consumers are in their deepest state and the USB4 router supports
>>    wakeup from D3cold, so it runtime suspends into this state.
>>
>> At least for AMD's case the hardware designer's expectation is the USB4
>> router should have also remained in D0 since the PCIe root port for
>> tunneling remained in D0 and a device link exists between the two devices.
>> The current Linux behavior is undefined.
> 
> Is the requirement that the supplier can never be in a lower-power
> state than the consumer?
> 
> I guess that's *not* actually a requirement even though that's the
> effect of this patch in this situation.  If it *were* that simple, we
> would just check the supplier and consumer power states instead of
> looking at all these ACPI properties.

Yeah I think that's too broad of a generalization.

I don't know how realistic this is but for example what if the supplier 
supported D3 but the consumer supported D2?

> 
>> Instead of making the assertion that the device can support D3 (and thus
>> runtime PM) solely from the presence of ACPI power resources, move the check
>> to later on in the function, which will have validated that the ACPI device
>> supports wake from D3hot or D3cold.
>>
>> This fix prevents the USB4 router being put into D3 when the firmware
>> says that ACPI device representing the PCIe root port for tunneling can't
>> handle it while still allowing ACPI devices that don't have the
>> HotplugSupportInD3 _DSD to also enter D3 if they have power resources that
>> can wake from D3.
> 
> So I guess this changes what acpi_pci_bridge_d3() returns for 00:03.1?
> Previously it returned "true" because it has _PR0/_PS0 (so
> acpi_pci_power_manageable() is true), but now it will return "false"
> because _S0W says it can only wake from D0?
> 

Exactly.

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