[PATCH RFC v2 2/4] 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 is not needed.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
---
 drivers/pci/hotplug/pciehp_ctrl.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/drivers/pci/hotplug/pciehp_ctrl.c b/drivers/pci/hotplug/pciehp_ctrl.c
index 3f3df4c29f6e..28965995ebb9 100644
--- a/drivers/pci/hotplug/pciehp_ctrl.c
+++ b/drivers/pci/hotplug/pciehp_ctrl.c
@@ -213,6 +213,21 @@ void pciehp_handle_disable_request(struct controller *ctrl)
 	ctrl->request_result = pciehp_disable_slot(ctrl, SAFE_REMOVAL);
 }
 
+static bool is_delayed_presence_up_event(struct controller *ctrl, u32 events)
+{
+	bool present, link_active;
+
+	if (!ctrl->inband_presence_disabled)
+		return false;
+
+	present = pciehp_card_present(ctrl);
+	link_active = pciehp_check_link_active(ctrl);
+
+	if (!present || !link_active || events & PCI_EXP_SLTSTA_DLLSC)
+		return false;
+
+	return true;
+}
 void pciehp_handle_presence_or_link_change(struct controller *ctrl, u32 events)
 {
 	bool present, link_active;
@@ -220,13 +235,22 @@ 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 (is_delayed_presence_up_event(ctrl, events)) {
+			mutex_unlock(&ctrl->state_lock);
+			ctrl_dbg(ctrl, "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