[PATCH] PCI: pciehp: Do not turn off slot if presence comes up after link

[Alexandru Gagniuc <mr.nuke.me@gmail.com>]

According to PCIe 3.0, the presence detect state is a logical OR of
in-band and out-of-band presence. With this, we'd expect the presence
state to always be asserted when the link comes up.

Not all hardware follows this, and it is possible for the presence to
come up after the link. In this case, the PCIe device would be
erroneously disabled and re-probed. It is possible to distinguish
between a delayed presence and a card swap by looking at the DLL state
changed bit -- The link has to come down if the card is removed.

Thus, for a device that is probed, present and has its link active, a
lack of a link state change event guarantees we have the same device,
and shutdown of is not needed.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
---

Following some discussion in
"PCI: hotplug: Erroneous removal of hotplug PCI devices" [1]
It became apparent that we can't fully rely presence detect changed
as an indicator to shutdown a device. And in PCIe 4.0 the "logical OR"
requirement is going away, so we can use a way to slightly  decouple
presence detect and link active.

I think the approach here is the simplest, and least likely to
interfere with other mis-behaving hardware (in the PCIe 3.0 sense).

[1] https://www.spinics.net/lists/linux-pci/msg79564.html

 drivers/pci/hotplug/pciehp_ctrl.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/pci/hotplug/pciehp_ctrl.c b/drivers/pci/hotplug/pciehp_ctrl.c
index 3f3df4c29f6e..ea0dd3e956be 100644
--- a/drivers/pci/hotplug/pciehp_ctrl.c
+++ b/drivers/pci/hotplug/pciehp_ctrl.c
@@ -220,13 +220,23 @@ void pciehp_handle_presence_or_link_change(struct controller *ctrl, u32 events)
 	/*
 	 * If the slot is on and presence or link has changed, turn it off.
 	 * Even if it's occupied again, we cannot assume the card is the same.
+	 * When the card is swapped, we also expect a change in link state,
+	 * without which, it's likely presence became high after link-active.
 	 */
 	mutex_lock(&ctrl->state_lock);
+	present = pciehp_card_present(ctrl);
+	link_active = pciehp_check_link_active(ctrl);
 	switch (ctrl->state) {
 	case BLINKINGOFF_STATE:
 		cancel_delayed_work(&ctrl->button_work);
 		/* fall through */
 	case ON_STATE:
+		if (present && link_active &&
+		   !(events & PCI_EXP_SLTSTA_DLLSC)) {
+			mutex_unlock(&ctrl->state_lock);
+			ctrl_warn(ctrl, "Hardware bug: Presence state came up after link");
+			return;
+		}
 		ctrl->state = POWEROFF_STATE;
 		mutex_unlock(&ctrl->state_lock);
 		if (events & PCI_EXP_SLTSTA_DLLSC)
-- 
2.19.2