Date:   Mon, 27 May 2019 11:57:47 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Kai-Heng Feng <kai.heng.feng@...onical.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        linux-pci@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Mathias Nyman <mathias.nyman@...el.com>,
        linux-usb@...r.kernel.org
Subject: Re: [PATCH] PCI / PM: Don't runtime suspend when device only
 supports wakeup from D0

On Thu, May 23, 2019 at 12:39:23PM +0800, Kai-Heng Feng wrote:
> at 04:52, Bjorn Helgaas <helgaas@...nel.org> wrote:
> > On Wed, May 22, 2019 at 02:39:56PM -0400, Alan Stern wrote:
> > > On Wed, 22 May 2019, Bjorn Helgaas wrote:
> > > > On Wed, May 22, 2019 at 11:46:25PM +0800, Kai Heng Feng wrote:
> > > > > > On May 22, 2019, at 9:48 PM, Bjorn Helgaas <helgaas@...nel.org> wrote:
> > > > > > On Wed, May 22, 2019 at 11:42:14AM +0800, Kai Heng Feng wrote:
> > > > > > > at 6:23 AM, Bjorn Helgaas <helgaas@...nel.org> wrote:
> > > > > > > > On Wed, May 22, 2019 at 12:31:04AM +0800, Kai-Heng Feng wrote:
> > > > > > > > > There's an xHC device that doesn't wake when
> > > > > > > > > a USB device gets plugged
> > > > > > > > > to its USB port. The driver's own runtime
> > > > > > > > > suspend callback was called,
> > > > > > > > > PME signaling was enabled, but it stays at PCI D0.
> > > > 
> > > > > > ...
> > > > > > And I guess this patch basically means we wouldn't call
> > > > > > the driver's suspend callback if we're merely going to
> > > > > > stay at D0, so the driver would have no idea anything
> > > > > > happened.  That might match Documentation/power/pci.txt
> > > > > > better, because it suggests that the suspend callback is
> > > > > > related to putting a device in a low-power state, and D0
> > > > > > is not a low-power state.
> > > > > 
> > > > > Yes, the patch is to let the device stay at D0 and don’t run
> > > > > driver’s own runtime suspend routine.
> > > > > 
> > > > > I guess I’ll just proceed to send a V2 with updated commit message?
> > > > 
> > > > Now that I understand what "runtime suspended to D0" means, help me
> > > > understand what's actually wrong.
> > > 
> > > Kai's point is that the xhci-hcd driver thinks the device is now
> > > in runtime suspend, because the runtime_suspend method has been
> > > executed.  But in fact the device is still in D0, and as a
> > > result, PME signalling may not work correctly.
> > 
> > The device claims to be able to signal PME from D0 (this is from the lspci
> > in https://bugzilla.kernel.org/show_bug.cgi?id=203673):
> > 
> >   00:10.0 USB controller: Advanced Micro Devices, Inc. [AMD] FCH USB XHCI Controller (rev 20) (prog-if 30 [XHCI])
> >     Capabilities: [50] Power Management version 3
> >       Flags: PMEClk- DSI- D1- D2- AuxCurrent=0mA PME(D0+,D1-,D2-,D3hot+,D3cold+)
> > 
> > From the xHCI spec r1.0, sec 4.15.2.3, it looks like a connect
> > detected while in D0 should assert PME# if enabled (and WCE is
> > set).
> 
> I think section 4.15.2.3 is about S3 wake up, no S0 we are
> discussing here.

S0 and S3 are system-level ideas and have no meaning to an individual
PCI device.  The xHC is a PCI device and can't tell whether the system
as a whole is in S0 or S3.  If a PCI device claims to be able to
generate PME while in D0, that applies regardless of the system state.

xHCI r1.0, sec A.1 says "The host controller should be capable of
asserting PME# when in any supported device state."  In sec 4.19.2,
Figure 42 says PME# should be asserted whenever PMCSR.PME_En=1 and
WCE=1 and a connection is detected.

Figure 42 also shows that CSC (Connect Status Change) and related bits
feed into Port Status Change Event Generation.  So I assume the xhci
driver normally detects connect/disconnect via CSC, but the runtime
suspend method makes it use PME# instead?

And the way your patch works is by avoiding that xhci runtime suspend
method, so it *always* uses CSC and never uses PME#?  If that's the
case, we're just papering over a problem without really understanding
it.

I'm wondering if this platform has a firmware defect.  Here's my
thinking.  The xHC is a Root Complex Integrated Endpoint, so its PME
signaling is a little unusual.

The typical scenario is that a PCIe device is below a Root Port.  In
that case, it would send a PME Message upstream to the Root Port.  Per
PCIe r4.0, sec 6.1.6, when configured for native PME support (for ACPI
systems, I assume this means "when firmware has granted PME control to
the OS via _OSC"), the Root Port would generate a normal PCI INTx or
MSI interrupt:

  PCI Express-aware software can enable a mode where the Root Complex
  signals PME via an interrupt. When configured for native PME
  support, a Root Port receives the PME Message and sets the PME
  Status bit in its Root Status register. If software has set the PME
  Interrupt Enable bit in the Root Control register to 1b, the Root
  Port then generates an interrupt.

But on this platform the xHC is a Root Complex Integrated Endpoint, so
there is no Root Port upstream from it, and that mechanism can't be
used.  Per PCIe r4.0, sec 1.3.2.3, RCiEPs signal PME via "the same
mechanism as PCI systems" or via Root Complex Event Collectors:

  An RCiEP must signal PME and error conditions through the same
  mechanisms used on PCI systems. If a Root Complex Event Collector is
  implemented, an RCiEP may optionally signal PME and error conditions
  through a Root Complex Event Collector.

This platform has no Root Complex Event Collectors, so the xHC should
signal PME via the same mechanism as PCI systems, i.e., asserting a
PME# signal.  I think this means the OS cannot use native PCIe PME
control because it doesn't know what interrupt PME# is connected to.
The PCI Firmware Spec r3.2, sec 4.5.1 (also quoted in ACPI v6.2, sec
6.2.11.3), says:

  PCI Express Native Power Management Events control

  The firmware sets this bit to 1 to grant control over control over
  PCI Express native power management event interrupts (PMEs). If
  firmware allows the operating system control of this feature, then
  in the context of the _OSC method, it must ensure that all PMEs are
  routed to root port interrupts as described in the PCI Express Base
  Specification.

This platform cannot route all PMEs to Root Port interrupts because
the xHC RCiEP cannot report PME via a Root Port, so I think its _OSC
method should not grant control of PCIe Native Power Management Events
to the OS, and I think that would mean we have to use the ACPI
mechanism for PME on this platform.

Can you confirm or deny any of this line of reasoning?  I'm wondering
if there's something wrong with the platform's _OSC, so Linux thinks
it can use native PME, but that doesn't work for this device.

> It’s a platform in development so the name can’t be disclosed.

Please attach a complete dmesg log to the bugzilla.  You can remove
identifying details like the platform name, but I want to see the
results of the _OSC negotiation.

> > > According to Kai, PME signalling doesn't work in D0 -- or at
> > > least, it is _documented_ not to work in D0 -- even though it is
> > > enabled and the device claims to support it.
> > 
> > I didn't understand this part.  From a PCI perspective, PME
> > signaling while in D0 is an optional feature and should work if
> > the device advertises support for it.  If it doesn't work on this
> > device, we should have a quirk to indicate that.
> 
> The only document I can find is the "Device Working State D0” from
> Microsoft. It says:
> "As a best practice, the driver should configure the device to
> generate interrupts only when the device is in D0, and to generate
> wake signals only when the device is in a low-power Dx state.”

Without some details about what is common between Windows and Linux, I
don't think a Windows "best practice" for drivers is really applicable
to Linux.  The paragraph you quoted from talks about simplifying
interrupt and wake code paths, not hardware capabilities.

Bjorn

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