From 6c35a1ac3da63a7fe5b18b435a5a9d6b9fd3990a Mon Sep 17 00:00:00 2001
From: Lukas Wunner <lukas@wunner.de>
Date: Thu, 19 Jul 2018 17:27:49 -0500
Subject: [PATCH] PCI: pciehp: Tolerate initially unstable link
Git-commit: 6c35a1ac3da63a7fe5b18b435a5a9d6b9fd3990a
Patch-mainline: v4.19-rc1
References: FATE#326145
When a device is hotplugged, Presence Detect and Link Up events often do
not occur simultaneously, but with a lag of a few milliseconds. Only
the first event received is relevant, the other one can be disregarded.
Moreover, Stefan Roese reports that on certain platforms, Link State and
Presence Detect may flap for up to 100 ms before stabilizing, suggesting
that such events should be disregarded for at least this long:
https://lkml.kernel.org/r/20180130084121.18653-1-sr@denx.de
On slot enablement, pciehp_check_link_status() waits for 100 ms per
PCIe r4.0, sec 6.7.3.3, then probes the hotplugged device's vendor
register for up to 1 second.
If this succeeds, the link is definitely up, so ignore any Presence
Detect or Link State events that occurred up to this point.
pciehp_check_link_status() then checks the Link Training bit in the
Link Status register. This is the final opportunity to detect
inaccessibility of the device and abort slot enablement. Any link
or presence change that occurs afterwards will cause the slot to be
disabled again immediately after attempting to enable it.
The astute reviewer may appreciate that achieving this behavior would be
more complicated had pciehp not just been converted to enable/disable
the slot exclusively from the IRQ thread: When the slot is enabled via
sysfs, each link or presence flap would otherwise cause the IRQ thread
to run and it would have to sense that those events are belonging to a
concurrent slot enablement operation and disregard them. It would be
much more difficult than this mere 3 line change.
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Stefan Roese <sr@denx.de>
Acked-by: Takashi Iwai <tiwai@suse.de>
---
drivers/pci/hotplug/pciehp_hpc.c | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
index 7f1a29cd6a17..6b8350a3875c 100644
--- a/drivers/pci/hotplug/pciehp_hpc.c
+++ b/drivers/pci/hotplug/pciehp_hpc.c
@@ -270,6 +270,11 @@ int pciehp_check_link_status(struct controller *ctrl)
found = pci_bus_check_dev(ctrl->pcie->port->subordinate,
PCI_DEVFN(0, 0));
+ /* ignore link or presence changes up to this point */
+ if (found)
+ atomic_and(~(PCI_EXP_SLTSTA_DLLSC | PCI_EXP_SLTSTA_PDC),
+ &ctrl->pending_events);
+
pcie_capability_read_word(pdev, PCI_EXP_LNKSTA, &lnk_status);
ctrl_dbg(ctrl, "%s: lnk_status = %x\n", __func__, lnk_status);
if ((lnk_status & PCI_EXP_LNKSTA_LT) ||
--
2.19.2