Currently Linux does not follow PCIe spec regarding the required delays
after reset. A concrete example is a Thunderbolt add-in-card that
consists of a PCIe switch and two PCIe endpoints:

  +-1b.0-[01-6b]----00.0-[02-6b]--+-00.0-[03]----00.0 TBT controller
                                  +-01.0-[04-36]-- DS hotplug port
                                  +-02.0-[37]----00.0 xHCI controller
                                  \-04.0-[38-6b]-- DS hotplug port

The root port (1b.0) and the PCIe switch downstream ports are all PCIe
gen3 so they support 8GT/s link speeds.

We wait for the PCIe hierarchy to enter D3cold (runtime):

  pcieport 0000:00:1b.0: power state changed by ACPI to D3cold

When it wakes up from D3cold, according to the PCIe 4.0 section 5.8 the
PCIe switch is put to reset and its power is re-applied. This means that
we must follow the rules in PCIe 4.0 section 6.6.1.

For the PCIe gen3 ports we are dealing with here, the following applies:

  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).

Translating this into the above topology we would need to do this (DLLLA
stands for Data Link Layer Link Active):

  pcieport 0000:00:1b.0: wait for 100ms after DLLLA is set before access to 0000:01:00.0
  pcieport 0000:02:00.0: wait for 100ms after DLLLA is set before access to 0000:03:00.0
  pcieport 0000:02:02.0: wait for 100ms after DLLLA is set before access to 0000:37:00.0

I've instrumented the kernel with additional logging so we can see the
actual delays the kernel performs:

  pcieport 0000:00:1b.0: power state changed by ACPI to D0
  pcieport 0000:00:1b.0: waiting for D3cold delay of 100 ms
  pcieport 0000:00:1b.0: waking up bus
  pcieport 0000:00:1b.0: waiting for D3hot delay of 10 ms
  pcieport 0000:00:1b.0: restoring config space at offset 0x2c (was 0x60, writing 0x60)
  ...
  pcieport 0000:00:1b.0: PME# disabled
  pcieport 0000:01:00.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
  ...
  pcieport 0000:01:00.0: PME# disabled
  pcieport 0000:02:00.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
  ...
  pcieport 0000:02:00.0: PME# disabled
  pcieport 0000:02:01.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
  ...
  pcieport 0000:02:01.0: restoring config space at offset 0x4 (was 0x100000, writing 0x100407)
  pcieport 0000:02:01.0: PME# disabled
  pcieport 0000:02:02.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
  ...
  pcieport 0000:02:02.0: PME# disabled
  pcieport 0000:02:04.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
  ...
  pcieport 0000:02:04.0: PME# disabled
  pcieport 0000:02:01.0: PME# enabled
  pcieport 0000:02:01.0: waiting for D3hot delay of 10 ms
  pcieport 0000:02:04.0: PME# enabled
  pcieport 0000:02:04.0: waiting for D3hot delay of 10 ms
  thunderbolt 0000:03:00.0: restoring config space at offset 0x14 (was 0x0, writing 0x8a040000)
  ...
  thunderbolt 0000:03:00.0: PME# disabled
  xhci_hcd 0000:37:00.0: restoring config space at offset 0x10 (was 0x0, writing 0x73f00000)
  ...
  xhci_hcd 0000:37:00.0: PME# disabled

For the switch upstream port (01:00.0) we wait for 100ms but not taking
into account the DLLLA requirement. We then wait 10ms for D3hot -> D0
transition of the root port and the two downstream hotplug ports. This
means that we deviate from what the spec requires.

Performing the same check for system sleep (s2idle) transitions we can
see following when resuming from s2idle:

  pcieport 0000:00:1b.0: power state changed by ACPI to D0
  pcieport 0000:00:1b.0: restoring config space at offset 0x2c (was 0x60, writing 0x60)
  ...
  pcieport 0000:01:00.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
  ...
  pcieport 0000:02:02.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
  pcieport 0000:02:02.0: restoring config space at offset 0x2c (was 0x0, writing 0x0)
  pcieport 0000:02:01.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
  pcieport 0000:02:04.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
  pcieport 0000:02:02.0: restoring config space at offset 0x28 (was 0x0, writing 0x0)
  pcieport 0000:02:00.0: restoring config space at offset 0x3c (was 0x1ff, writing 0x201ff)
  pcieport 0000:02:02.0: restoring config space at offset 0x24 (was 0x10001, writing 0x1fff1)
  pcieport 0000:02:01.0: restoring config space at offset 0x2c (was 0x0, writing 0x60)
  pcieport 0000:02:02.0: restoring config space at offset 0x20 (was 0x0, writing 0x73f073f0)
  pcieport 0000:02:04.0: restoring config space at offset 0x2c (was 0x0, writing 0x60)
  pcieport 0000:02:01.0: restoring config space at offset 0x28 (was 0x0, writing 0x60)
  pcieport 0000:02:00.0: restoring config space at offset 0x2c (was 0x0, writing 0x0)
  pcieport 0000:02:02.0: restoring config space at offset 0x1c (was 0x101, writing 0x1f1)
  pcieport 0000:02:04.0: restoring config space at offset 0x28 (was 0x0, writing 0x60)
  pcieport 0000:02:01.0: restoring config space at offset 0x24 (was 0x10001, writing 0x1ff10001)
  pcieport 0000:02:00.0: restoring config space at offset 0x28 (was 0x0, writing 0x0)
  pcieport 0000:02:02.0: restoring config space at offset 0x18 (was 0x0, writing 0x373702)
  pcieport 0000:02:04.0: restoring config space at offset 0x24 (was 0x10001, writing 0x49f12001)
  pcieport 0000:02:01.0: restoring config space at offset 0x20 (was 0x0, writing 0x73e05c00)
  pcieport 0000:02:00.0: restoring config space at offset 0x24 (was 0x10001, writing 0x1fff1)
  pcieport 0000:02:04.0: restoring config space at offset 0x20 (was 0x0, writing 0x89f07400)
  pcieport 0000:02:01.0: restoring config space at offset 0x1c (was 0x101, writing 0x5151)
  pcieport 0000:02:00.0: restoring config space at offset 0x20 (was 0x0, writing 0x8a008a00)
  pcieport 0000:02:02.0: restoring config space at offset 0xc (was 0x10000, writing 0x10020)
  pcieport 0000:02:04.0: restoring config space at offset 0x1c (was 0x101, writing 0x6161)
  pcieport 0000:02:01.0: restoring config space at offset 0x18 (was 0x0, writing 0x360402)
  pcieport 0000:02:00.0: restoring config space at offset 0x1c (was 0x101, writing 0x1f1)
  pcieport 0000:02:04.0: restoring config space at offset 0x18 (was 0x0, writing 0x6b3802)
  pcieport 0000:02:02.0: restoring config space at offset 0x4 (was 0x100000, writing 0x100407)
  pcieport 0000:02:00.0: restoring config space at offset 0x18 (was 0x0, writing 0x30302)
  pcieport 0000:02:01.0: restoring config space at offset 0xc (was 0x10000, writing 0x10020)
  pcieport 0000:02:04.0: restoring config space at offset 0xc (was 0x10000, writing 0x10020)
  pcieport 0000:02:00.0: restoring config space at offset 0xc (was 0x10000, writing 0x10020)
  pcieport 0000:02:01.0: restoring config space at offset 0x4 (was 0x100000, writing 0x100407)
  pcieport 0000:02:04.0: restoring config space at offset 0x4 (was 0x100000, writing 0x100407)
  pcieport 0000:02:00.0: restoring config space at offset 0x4 (was 0x100000, writing 0x100407)
  xhci_hcd 0000:37:00.0: restoring config space at offset 0x10 (was 0x0, writing 0x73f00000)
  ...
  thunderbolt 0000:03:00.0: restoring config space at offset 0x14 (was 0x0, writing 0x8a040000)

This is even worse. None of the mandatory delays are performed. If this
would be S3 instead of s2idle then according to PCI FW spec 3.2 section
4.6.8.  there is a specific _DSM that allows the OS to skip the delays
but this platform does not provide the _DSM and does not go to S3 anyway
so no firmware is involved that could already handle these delays.

In this particular Intel Coffee Lake platform these delays are not
actually needed because there is an additional delay as part of the ACPI
power resource that is used to turn on power to the hierarchy but since
that additional delay is not required by any of standards (PCIe, ACPI)
it is not present in the Intel Ice Lake, for example where missing the
mandatory delays causes pciehp to start tearing down the stack too early
(links are not yet trained).

For this reason, change the PCIe portdrv PM resume hooks so that they
perform the mandatory delays before the downstream component gets
resumed. We perform the delays before port services are resumed because
otherwise pciehp might find that the link is not up (even if it is just
training) and tears-down the hierarchy.
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>