Re: [PATCH v2 1/3] PCI / ACPI: Use cached ACPI device state to get PCI device power state

[Mika Westerberg <mika.westerberg@linux.intel.com>]

On Thu, Jun 20, 2019 at 08:16:49AM -0500, Bjorn Helgaas wrote:
> On Thu, Jun 20, 2019 at 11:27:30AM +0300, Mika Westerberg wrote:
> > On Wed, Jun 19, 2019 at 04:28:01PM -0500, Bjorn Helgaas wrote:
> > > On Tue, Jun 18, 2019 at 07:18:56PM +0300, Mika Westerberg wrote:
> > > > Intel Ice Lake has an integrated Thunderbolt controller which
> > > > means that the PCIe topology is extended directly from the two
> > > > root ports (RP0 and RP1).
> > > 
> > > A PCIe topology is always extended directly from root ports,
> > > regardless of whether a Thunderbolt controller is integrated, so I
> > > guess I'm missing the point you're making.  It doesn't sound like
> > > this is anything specific to Thunderbolt?
> >
> > The point I'm trying to make here is to explain why this is problem
> > now and not with the previous discrete controllers. With the
> > previous there was only a single ACPI power resource for the root
> > port and the Thunderbolt host router was connected to that root
> > port. PCIe hierarchy was extended through downstream ports (not root
> > ports) of that controller (which includes PCIe switch).
> 
> Sounds like you're using "PCIe topology extension" to mean
> specifically something below a Thunderbolt controller, excluding a
> subtree below a root port.  I don't think the PCI core is aware of
> that distinction.

Right it is not.

> > Now the thing is part of the SoC so power management is different
> > and causes problems in Linux.
> 
> The SoC is a physical packaging issue that really doesn't enter into
> the specs directly.  I'm trying to get at the logical topology
> questions in terms of the PCIe and ACPI specs.
> 
> I assume we could dream up a non-Thunderbolt topology that would show
> the same problem?

Yes.

> > > > Power management is handled by ACPI power resources that are
> > > > shared between the root ports, Thunderbolt controller (NHI) and xHCI
> > > > controller.
> > > > 
> > > > The topology with the power resources (marked with []) looks like:
> > > > 
> > > >   Host bridge
> > > >     |
> > > >     +- RP0 ---\
> > > >     +- RP1 ---|--+--> [TBT]
> > > >     +- NHI --/   |
> > > >     |            |
> > > >     |            v
> > > >     +- xHCI --> [D3C]
> > > > 
> > > > Here TBT and D3C are the shared ACPI power resources. ACPI
> > > > _PR3() method returns either TBT or D3C or both.
> 
> I'm not very familiar with _PR3.  I guess this is under an ACPI object
> representing a PCI device, e.g., \_SB.PCI0.RP0._PR3?

Correct.

> > > > Say we runtime suspend first the root ports RP0 and RP1, then
> > > > NHI. Now since the TBT power resource is still on when the root
> > > > ports are runtime suspended their dev->current_state is set to
> > > > D3hot. When NHI is runtime suspended TBT is finally turned off
> > > > but state of the root ports remain to be D3hot.
> 
> So in this example we might have:
> 
>   _SB.PCI0.RP0._PR3: TBT
>   _SB.PCI0.RP1._PR3: TBT
>   _SB.PCI0.NHI._PR3: TBT

and also D3C.

> And when Linux figures out that everything depending on TBT is in
> D3hot, it evaluates TBT._OFF, which puts them all in D3cold?  And part
> of the problem is that they're now in D3cold (where config access
> doesn't work) but Linux still thinks they're in D3hot (where config
> access would work)?

Exactly.

> I feel like I'm missing something because I don't know how D3C is
> involved, since you didn't mention suspending xHCI.

That's another power resource so we will also have D3C turned off when
xHCI gets suspended but I did not want to complicate things too much in
the changelog.

> And I can't mentally match up the patch with the D3hot/D3cold state
> change (if indeed that's the problem).  If we were updating the path
> that evaluates _OFF so it changed the power state of all dependent
> devices, *that* would make a lot of sense to me because it sounds like
> that's where the physical change happens that makes things out of
> sync.

I did that in the first version [1] but Rafael pointed out that it is
racy one way or another [2].

[1] https://www.spinics.net/lists/linux-pci/msg83583.html
[2] https://www.spinics.net/lists/linux-pci/msg83600.html