Re: [PATCH] PCI: Do not use pcie_get_speed_cap() to determine when to start waiting

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

On Thu, May 07, 2020 at 07:24:37AM -0500, Bjorn Helgaas wrote:
> On Thu, May 7, 2020 at 6:45 AM Mika Westerberg
> <mika.westerberg@linux.intel.com> wrote:
> >
> > On Wed, May 06, 2020 at 05:42:28PM -0500, Bjorn Helgaas wrote:
> > > On Thu, Apr 16, 2020 at 11:32:45AM +0300, Mika Westerberg wrote:
> > > > Kai-Heng Feng reported that it takes long time (>1s) to resume
> > > > Thunderbolt connected PCIe devices from both runtime suspend and system
> > > > sleep (s2idle).
> > > >
> > > > These PCIe downstream ports the second link capability (PCI_EXP_LNKCAP2)
> > > > announces support for speeds > 5 GT/s but it is then capped by the
> > > > second link control (PCI_EXP_LNKCTL2) register to 2.5 GT/s. This
> > > > possiblity was not considered in pci_bridge_wait_for_secondary_bus() so
> > > > it ended up waiting for 1100 ms as these ports do not support active
> > > > link layer reporting either.
> > > >
> > > > PCIe spec 5.0 section 6.6.1 mandates that we must wait minimum of 100 ms
> > > > before sending configuration request to the device below, if the port
> > > > does not support speeds > 5 GT/s, and if it does we first need to wait
> > > > for the data link layer to become active before waiting for that 100 ms.
> > > >
> > > > PCIe spec 5.0 section 7.5.3.6 further says that all downstream ports
> > > > that support speeds > 5 GT/s must support active link layer reporting so
> > > > instead of looking for the speed we can check for the active link layer
> > > > reporting capability and determine how to wait based on that (as they go
> > > > hand in hand).
> > >
> > > I can't quite tell what the defect is here.
> > >
> > > I assume you're talking about this text from sec 6.6.1:
> > >
> > >   - With a Downstream Port that does not support Link speeds greater
> > >     than 5.0 GT/s, software must wait a minimum of 100 ms before
> > >     sending a Configuration Request to the device immediately below
> > >     that Port.
> > >
> > >   - With a Downstream Port that supports Link speeds greater than 5.0
> > >     GT/s, software must wait a minimum of 100 ms after Link training
> > >     completes before sending a Configuration Request to the device
> > >     immediately below that Port. Software can determine when Link
> > >     training completes by polling the Data Link Layer Link Active bit
> > >     or by setting up an associated interrupt (see Section 6.7.3.3 ).
> > >
> > > I don't understand what Link Control 2 has to do with this.  The spec
> > > talks about ports *supporting* certain link speeds, which sounds to me
> > > like the Link Capabilities.  It doesn't say anything about the current
> > > or target link speed.
> >
> > PCIe 5.0 page 764 recommends using Supported Link Speeds Vector in Link
> > Capabilities 2 register and that's what pcie_get_speed_cap() is doing.
> >
> > However, we can avoid figuring the speed altogether by checking the
> > dev->link_active_reporting instead because that needs to be implemented
> > by all Downstream Ports that supports speeds > 5 GT/s (PCIe 5.0 page 735).
> 
> I understand that part.  But the code as-is matches sec 6.6.1, which
> makes it easy to understand.  Checking link_active_reporting instead
> makes it harder to understand because it makes it one step removed
> from 6.6.1.  And link_active_reporting must be set for ports that
> support > 5 GT/s, but it must *also* be set in some hotplug cases, so
> that further complicates the connection between it and 6.6.1.
> 
> And apparently there's an actual defect, and I don't understand what
> that is yet.  It sounds like we agree that pcie_get_speed_cap() is
> doing the right thing.  Is there something in
> pci_bridge_wait_for_secondary_bus() that doesn't match sec 6.6.1?

The defect is that some downstream PCIe ports on a system Kai-Heng is
using have Supported Link Speeds Vector with > 5GT/s and it does not
support active link reporting so the currect code ends up waiting 1100 ms
slowing down resume time.

The Target Link Speed of this port (in Link Control 2) has the speed
capped to 2.5 GT/s.