Linux 4.8 added support for runtime suspending PCIe ports to D3hot with
commit 006d44e49a ("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>
68db9bc814