[PATCH] PCI/PM: Skip the suspend_noirq config save if runtime-suspended

[Navon John Lukose <navonjohnlukose@gmail.com>]

pci_pm_suspend_noirq() saves config space unconditionally when the driver
of the device has no PM callbacks, which includes unbound devices.  If
such a device is runtime-suspended, the bridge above it may be in a
low-power state with the link down.  Depending on the platform, the
config reads then either hang the CPU or return all ones, which overwrite
the snapshot taken at runtime suspend and are written back to the device
on resume.

Skip the save if the device is runtime-suspended, as pci_pm_freeze()
does, since pci_pm_runtime_suspend() has already saved the config space.
Use pm_runtime_status_suspended(), because runtime PM is disabled by the
noirq phase and pm_runtime_suspended() would always be false.

Fixes: 931ff68a5a53 ("PCI PM: Restore config spaces of all devices during early resume")
Cc: stable@vger.kernel.org # v6.19+
Signed-off-by: Navon John Lukose <navonjohnlukose@gmail.com>
---
Found on a Lenovo Yoga 83KF (Arrow Lake-H).  A driverless O2 Micro SD
reader at 57:00.0 with power/control=auto lets its root port 00:1c.0
runtime-suspend to D3hot.  The port then swallows the ECAM reads in
pci_save_state(), and the forward-progress watchdog raises a fatal machine
check at the ECAM load in pci_mmcfg_read().

The same port returns all ones for CF8/CFC reads.  A local quirk hid the
reader's extended config space so the save did only those reads, and it
was tried once on each kernel.  The unpatched kernel still died, and the
patched one resumed with no "restore config" writes for the reader.

!pci_dev->state_saved would only work for one cycle per boot, because
pci_restore_state() clears it and pci_pm_runtime_suspend() does not run
again for a device left in RPM_SUSPENDED.

Stable starts at v6.19 because the fix relies on a2f1e22390ac2 ("PCI/ERR:
Ensure error recoverability at all times").  Older trees still return
early from pci_restore_state() when state_saved is false, so a backport
there also needs "pci_dev->state_saved = true;" on the skip path.

Tested on 7.2.5 with the root port in D3hot, three suspend/resume cycles
in one boot.  The unpatched kernel dies on the first.  W=1 and sparse
clean.

 drivers/pci/pci-driver.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
index e16aa59dd..bdc8bad57 100644
--- a/drivers/pci/pci-driver.c
+++ b/drivers/pci/pci-driver.c
@@ -915,7 +915,15 @@ static int pci_pm_suspend_noirq(struct device *dev)
 		return pci_legacy_suspend_late(dev);
 
 	if (!pm) {
-		pci_save_state(pci_dev);
+		/*
+		 * The bridge above a runtime-suspended device may be in a
+		 * low-power state with the link down, which makes the device's
+		 * config space inaccessible.  pci_pm_runtime_suspend() has
+		 * saved it already.
+		 */
+		if (!pm_runtime_status_suspended(dev))
+			pci_save_state(pci_dev);
+
 		goto set_unknown;
 	}
 
-- 
2.55.0