PCI/PM: Keep the saved config space of an inaccessible device

When a PCI device stops responding while the system is suspending, pci_save_state() stores all ones over its saved config space, and pci_restore_state() writes that back on resume once the device responds again. For a bridge, that means bus numbers ff/ff/ff and Secondary Bus Reset asserted, which cuts off everything below it. Resume can also wait about 65 seconds for a device behind a dead bridge, because an all-ones Link Status reads as an active link. I ran into both on a MacBookPro14,3.

Patch 1 makes pci_save_state() keep the previous snapshot of a device that is not accessible, instead of overwriting it with all ones. Patch 2 stops the link wait code from taking an all-ones Link Status for an active link.

Patch 2 does not depend on patch 1 and is unchanged from v2, so it can be applied on its own, as you offered on v2.

Why patch 1 changed
-------------------
On v2 I described [1] what returning -EIO from pci_save_state() does to some drivers: stmmac_pci_plat_suspend(), intel_eth_pci_suspend(), atl1e, atl2 and amd_mp2 abort system suspend, and libata and others trigger the "State of device not saved" warning. I asked how you would like those drivers handled, and said I would hold v3 until then.

The Thunderbolt work exposed the same PCI problem in another path. I have sent a Thunderbolt patch [2] that calls the SXFP ACPI method on Apple systems with Alpine Ridge before system sleep. SXFP also powers down the xHCI on the side of the controller with nothing connected. The automated review of that patch pointed out that if this happens before the xHCI saves its state, pci_save_state() stores all ones and resume writes them back. I found the same by reading the code; I have not run that case without this series.

On v2 you wrote that skipping pci_save_state() when the device is not accessible seemed worthwhile. That preserves a usable saved state, but does not make the Thunderbolt controller resume. The Thunderbolt part now has its own patch in drivers/thunderbolt [2]. This version only does the skipping: pci_save_state() keeps the previous snapshot, marks the state as saved and returns 0, as it does today. Callers see no change, and there is nothing for the PM core to warn about. The device gets the config space it had when it was last accessible back on resume. This makes v2 patch 1 (usb: hcd-pci) unnecessary, so it is dropped. The values written back can be older than the last ones the device had; patch 1 describes that.

If you would still rather have pci_save_state() report the failure, I'd like to know how you want the drivers above handled.

[1] https://lore.kernel.org/all/[email protected]/
[2] https://lore.kernel.org/all/[email protected]/

Changes since v2:
- 1/2 (was 2/3): keep the previous snapshot and return 0 instead of returning -EIO, with the check once at the start of pci_save_state() as you suggested. On v2 I said I would make pci_dev_save_and_disable() use the return value of pci_save_state(); with this version that no longer works, so it keeps its own check (see the notes in patch 1).
- Dropped v2 1/3 (usb: hcd-pci).
- 2/2 (was 3/3): no code change. Added Darrell's Tested-by from v2.
- Based on pci/next as of 2026-10-15 (732c4f7d2c04).

v2: https://lore.kernel.org/all/[email protected]/
v1: https://lore.kernel.org/all/[email protected]/

Testing:
I built patch 1 into a v6.18.49 kernel with patch 2, a backport of e18d1abc3bff and other local changes for the MacBookPro14,3, including the next version of the Thunderbolt patch above, with the changes Mika asked for in his review. The USB HCD patch was not included. I tested it on that machine with 30 second suspend/resume cycles and a USB 3 disk on the Type-C ports, which sit behind the two Thunderbolt controllers:

- Five cycles: three with the disk on one side or the other, one removing the disk while the system was suspended, and one with nothing connected. All resumed, and both Thunderbolt controllers came back. The disk was read after each cycle in which it stayed connected, and after the other two it worked when plugged in again (on both sides after the first, on one after the second). In these cycles the xHCI saved its state before SXFP powered it down, so the new check in pci_save_state() did not trigger. The order of the two is not fixed, as both suspend asynchronously.
- To hit the case above, I also called SXFP earlier, in the PCI suspend phase, through a test-only PCI quirk that is otherwise disabled. In 3 of 3 cycles the xHCI on the empty side (each side at least once) was reported as "Device config space inaccessible; unable to save state", the snapshot from before was written back on resume instead of all ones, and the xHCI came back. A disk plugged into that xHCI afterwards worked at USB 3 speed, on both sides. In all of them the snapshot was one taken earlier while the xHCI was accessible.

There was no kernel WARNING and no "State of device not saved" during the whole boot. The series itself was built on pci/next with W=1. I have no SR-IOV hardware, so the VF case is untested, and the machine never reaches the pcie_wait_for_link_status() change.

Thanks to Bjorn for the review and the simpler check, to Farhan for the pointer to the 2025 discussion, and to Darrell Gum for testing v2 on a second MacBookPro14,3.

Francisco Beltrán Millalén (2):
PCI/PM: Keep the saved config space of an inaccessible device
PCI: Do not mistake an absent device for an active link

 drivers/pci/pci.c | 29 ++++++++++++++++++++++++-----
 1 file changed, 24 insertions(+), 5 deletions(-)