Commit 6cff20ce authored by Lukas Wunner's avatar Lukas Wunner Committed by Bjorn Helgaas
Browse files

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: 5352a44a ("PCI: pciehp: Make pciehp_is_native() stricter")
Reported-by: default avatarLaurent Bigonville <bigon@bigon.be>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220216


Reported-by: default avatarMario Limonciello <mario.limonciello@amd.com>
Closes: https://lore.kernel.org/r/20250609020223.269407-3-superm1@kernel.org/
Link: https://lore.kernel.org/all/20250620025535.3425049-3-superm1@kernel.org/T/#u


Signed-off-by: default avatarLukas Wunner <lukas@wunner.de>
Signed-off-by: default avatarBjorn Helgaas <bhelgaas@google.com>
Acked-by: default avatarRafael J. Wysocki <rafael@kernel.org>
Cc: stable@vger.kernel.org # v4.18+
Link: https://patch.msgid.link/fe5dcc3b2e62ee1df7905d746bde161eb1b3291c.1752390101.git.lukas@wunner.de
parent 19272b37

drivers/pci/pci-acpi.c

+1 −3
Original line number Diff line number Diff line
@@ -816,13 +816,11 @@ int pci_acpi_program_hp_params(struct pci_dev *dev) 
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

+5 −1
Original line number Diff line number Diff line
@@ -3030,8 +3030,12 @@ static const struct dmi_system_id bridge_d3_blacklist[] = { 
 * 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

+1 −1
Original line number Diff line number Diff line
@@ -1678,7 +1678,7 @@ void set_pcie_hotplug_bridge(struct pci_dev *pdev) 


	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

+6 −0
Original line number Diff line number Diff line
@@ -328,6 +328,11 @@ struct rcec_ea; 
 *			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 */
@@ -451,6 +456,7 @@ struct pci_dev { 
	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 */

	/*