Re: [PATCH] PCI/ACPI: Fix runtime PM ref imbalance on hot-plug capable ports

[Mika Westerberg <mika.westerberg@linux.intel.com>]

On Mon, Jun 23, 2025 at 07:08:20PM +0200, Lukas Wunner wrote:
> pcie_portdrv_probe() and pcie_portdrv_remove() both call
> pci_bridge_d3_possible() to determine whether to use runtime power
> management.  The underlying assumption is that pci_bridge_d3_possible()
> always returns the same value because otherwise a runtime PM reference
> imbalance occurs.
> 
> That assumption falls apart if the device is inaccessible on ->remove()
> due to hot-unplug:  pci_bridge_d3_possible() calls pciehp_is_native(),
> which accesses Config Space to determine whether the device is Hot-Plug
> Capable.   An inaccessible device returns "all ones", which is converted
> to "all zeroes" by pcie_capability_read_dword().  Hence the device no
> longer seems Hot-Plug Capable on ->remove() even though it was on
> ->probe().
> 
> The resulting runtime PM ref imbalance causes errors such as:
> 
>   pcieport 0000:02:04.0: Runtime PM usage count underflow!
> 
> The Hot-Plug Capable bit is cached in pci_dev->is_hotplug_bridge.
> pci_bridge_d3_possible() only calls pciehp_is_native() if that flag is
> set.  Re-checking the bit in pciehp_is_native() is thus unnecessary.
> 
> However pciehp_is_native() is also called from hotplug_is_native().  Move
> the Config Space access to that function.  The function is only invoked
> from acpiphp_glue.c, so move it there instead of keeping it in a publicly
> visible header.
> 
> 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>
> Cc: stable@vger.kernel.org # v4.18+

Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>