PCI/ACPI: Fix runtime PM ref imbalance on Hot-Plug Capable ports

pci_bridge_d3_possible() is called from both pcie_portdrv_probe() and
pcie_portdrv_remove() to determine whether runtime power management shall
be enabled (on probe) or disabled (on remove) on a PCIe port.

The underlying assumption is that pci_bridge_d3_possible() always returns
the same value, else a runtime PM reference imbalance would occur. That
assumption is not given if the PCIe port is inaccessible on remove due to
hot-unplug: pci_bridge_d3_possible() calls pciehp_is_native(), which
accesses Config Space to determine whether the port is Hot-Plug Capable.
An inaccessible port returns "all ones", which is converted to "all
zeroes" by pcie_capability_read_dword(). Hence the port no longer seems
Hot-Plug Capable on remove even though it was on probe.

The resulting runtime PM ref imbalance causes warning messages such as:

pcieport 0000:02:04.0: Runtime PM usage count underflow!

Avoid the Config Space access (and thus the runtime PM ref imbalance) by
caching the Hot-Plug Capable bit in struct pci_dev.

The struct already contains an "is_hotplug_bridge" flag, which however is
not only set on Hot-Plug Capable PCIe ports, but also Conventional PCI
Hot-Plug bridges and ACPI slots. The flag identifies bridges which are
allocated additional MMIO and bus number resources to allow for hierarchy
expansion.

The kernel is somewhat sloppily using "is_hotplug_bridge" in a number of
places to identify Hot-Plug Capable PCIe ports, even though the flag
encompasses other devices. Subsequent commits replace these occurrences
with the new flag to clearly delineate Hot-Plug Capable PCIe ports from
other kinds of hotplug bridges.

Document the existing "is_hotplug_bridge" and the new "is_pciehp" flag
and document the (non-obvious) requirement that pci_bridge_d3_possible()
always returns the same value across the entire lifetime of a bridge,
including its hot-removal.

Fixes: 5352a44a561d ("PCI: pciehp: Make pciehp_is_native() stricter")
Reported-by: Laurent Bigonville <[e-mail]>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220216
Reported-by: Mario Limonciello <[e-mail]>
Closes: https://lore.kernel.org/r/[e-mail]/
Link: https://lore.kernel.org/all/[e-mail]/T/#u
Signed-off-by: Lukas Wunner <[e-mail]>
Signed-off-by: Bjorn Helgaas <[e-mail]>
Acked-by: Rafael J. Wysocki <[e-mail]>
Cc: [e-mail] # v4.18+
Link: https://patch.msgid.link/[e-mail]

drivers/pci/pci-acpi.c

···
 bool pciehp_is_native(struct pci_dev *bridge)
 {
 	const struct pci_host_bridge *host;
-	u32 slot_cap;
 
 	if (!IS_ENABLED(CONFIG_HOTPLUG_PCI_PCIE))
 		return false;
 
-	pcie_capability_read_dword(bridge, PCI_EXP_SLTCAP, &slot_cap);
-	if (!(slot_cap & PCI_EXP_SLTCAP_HPC))
+	if (!bridge->is_pciehp)
 		return false;
 
 	if (pcie_ports_native)

drivers/pci/pci.c

···
  * pci_bridge_d3_possible - Is it possible to put the bridge into D3
  * @bridge: Bridge to check
  *
- * This function checks if it is possible to move the bridge to D3.
  * Currently we only allow D3 for some PCIe ports and for Thunderbolt.
+ *
+ * Return: Whether it is possible to move the bridge to D3.
+ *
+ * The return value is guaranteed to be constant across the entire lifetime
+ * of the bridge, including its hot-removal.
  */
 bool pci_bridge_d3_possible(struct pci_dev *bridge)
 {

drivers/pci/probe.c

···
 
 	pcie_capability_read_dword(pdev, PCI_EXP_SLTCAP, &reg32);
 	if (reg32 & PCI_EXP_SLTCAP_HPC)
-		pdev->is_hotplug_bridge = 1;
+		pdev->is_hotplug_bridge = pdev->is_pciehp = 1;
 }
 
 static void set_pcie_thunderbolt(struct pci_dev *dev)

include/linux/pci.h

···
  *			determined (e.g., for Root Complex Integrated
  *			Endpoints without the relevant Capability
  *			Registers).
+ * @is_hotplug_bridge:	Hotplug bridge of any kind (e.g. PCIe Hot-Plug Capable,
+ *			Conventional PCI Hot-Plug, ACPI slot).
+ *			Such bridges are allocated additional MMIO and bus
+ *			number resources to allow for hierarchy expansion.
+ * @is_pciehp:		PCIe Hot-Plug Capable bridge.
  */
 struct pci_dev {
 	struct list_head bus_list;	/* Node in per-bus list */
···
 	unsigned int	is_physfn:1;
 	unsigned int	is_virtfn:1;
 	unsigned int	is_hotplug_bridge:1;
+	unsigned int	is_pciehp:1;
 	unsigned int	shpc_managed:1;		/* SHPC owned by shpchp */
 	unsigned int	is_thunderbolt:1;	/* Thunderbolt controller */
 	/*