[PATCH 5.10.y] PCI/ACPI: Fix runtime PM ref imbalance on Hot-Plug Capable ports

[Sasha Levin <sashal@kernel.org>]

From: Lukas Wunner <lukas@wunner.de>

[ Upstream commit 6cff20ce3b92ffbf2fc5eb9e5a030b3672aa414a ]

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 <bigon@bigon.be>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220216
Reported-by: Mario 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: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael@kernel.org>
Cc: stable@vger.kernel.org # v4.18+
Link: https://patch.msgid.link/fe5dcc3b2e62ee1df7905d746bde161eb1b3291c.1752390101.git.lukas@wunner.de
[ Adjust surrounding documentation changes ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/pci/pci-acpi.c | 4 +---
 drivers/pci/pci.c      | 8 ++++++--
 drivers/pci/probe.c    | 2 +-
 include/linux/pci.h    | 1 +
 4 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/drivers/pci/pci-acpi.c b/drivers/pci/pci-acpi.c
index bda45c524187..f8afce7ba73c 100644
--- a/drivers/pci/pci-acpi.c
+++ b/drivers/pci/pci-acpi.c
@@ -791,13 +791,11 @@ 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)
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 31bcda363cbb..15618b87bc4b 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -2860,8 +2860,12 @@ 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 recent enough PCIe ports and Thunderbolt.
+ * 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)
 {
diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 7f3d10957eca..0b1ef4f2c90d 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -1582,7 +1582,7 @@ 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)
diff --git a/include/linux/pci.h b/include/linux/pci.h
index d3d84eb466f0..8360b87ca4d5 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -439,6 +439,7 @@ struct pci_dev {
 	unsigned int	is_virtfn:1;
 	unsigned int	reset_fn: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 */
 	/*
-- 
2.50.1