PCI: pciehp: Add runtime PM support for PCIe hotplug ports

PCI: pciehp: Add runtime PM support for PCIe hotplug ports

Linux 4.8 added support for runtime suspending PCIe ports to D3hot with
commit 006d44e49a25 ("PCI: Add runtime PM support for PCIe ports"), but
excluded hotplug ports.  Those are now afforded runtime PM by the present
commit.

Hotplug ports require a few extra considerations:

- The configuration space of the port remains accessible in D3hot, so all
  the functions to read or modify the Slot Status and Slot Control
  registers need not be modified.  Even turning on slot power doesn't seem
  to require the port to be in D0, at least the PCIe spec doesn't say so
  and I confirmed that by testing with a Thunderbolt controller.

- However D0 is required to access devices on the secondary bus.  This
  happens in pciehp_check_link_status() and pciehp_configure_device() (both
  called from board_added()) and in pciehp_unconfigure_device() (called
  from remove_board()), so acquire a runtime PM ref for their invocation.

- The hotplug port stays active as long as it has active children.  If all
  hotplugged devices below the port runtime suspend, the port is allowed to
  runtime suspend as well.  Plug and unplug detection continues to work in
  D3hot.

- Hotplug interrupts are delivered in-band, so while the hotplug port
  itself is allowed to go to D3hot, its parent ports must stay in D0 for
  interrupts to come through.  Add a corresponding restriction to
  pci_dev_check_d3cold().

- Runtime PM may only be allowed if the hotplug port is handled natively by
  the OS.  On ACPI systems, the port may alternatively be handled by the
  firmware and things break if the OS puts the port into D3 behind the
  firmware's back:  E.g. Thunderbolt hotplug ports on non-Macs are handled
  by Intel's firmware in System Management Mode and the firmware is known
  to access devices on the port's secondary bus without checking first if
  the port is in D0: https://bugzilla.kernel.org/show_bug.cgi?id=53811

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
CC: Mika Westerberg <mika.westerberg@linux.intel.com>