linux-kernel - [PATCH 1/2] PCI: Clear LBMS on resume to avoid Target Speed quirk

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Date: Mon, 29 Jan 2024 13:27:09 +0200
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Bjorn Helgaas <bhelgaas@...gle.com>,
	linux-pci@...r.kernel.org,
	"Maciej W. Rozycki" <macro@...am.me.uk>,
	linux-kernel@...r.kernel.org
Cc: Mika Westerberg <mika.westerberg@...ux.intel.com>,
	Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Subject: [PATCH 1/2] PCI: Clear LBMS on resume to avoid Target Speed quirk

While a device is runtime suspended along with its PCIe hierarchy, the
device could get disconnected. Because of the suspend, the device
disconnection cannot be detected until portdrv/hotplug have resumed. On
runtime resume, pcie_wait_for_link_delay() is called:

  pci_pm_runtime_resume()
    pci_pm_bridge_power_up_actions()
      pci_bridge_wait_for_secondary_bus()
        pcie_wait_for_link_delay()

Because the device is already disconnected, this results in cascading
failures:

  1. pcie_wait_for_link_status() returns -ETIMEDOUT.

  2. After the commit a89c82249c37 ("PCI: Work around PCIe link
     training failures"), pcie_failed_link_retrain() spuriously detects
     this failure as a Link Retraining failure and attempts the Target
     Speed trick, which also fails.

  3. pci_bridge_wait_for_secondary_bus() then calls pci_dev_wait() which
     cannot succeed (but waits ~1 minute, delaying the resume).

The Target Speed trick (in step 2) is only used if LBMS bit (PCIe r6.1
sec 7.5.3.8) is set. For links that have been operational before
suspend, it is well possible that LBMS has been set at the bridge and
remains on. Thus, after resume, LBMS does not indicate the link needs
the Target Speed quirk. Clear LBMS on resume for bridges to avoid the
issue.

Fixes: a89c82249c37 ("PCI: Work around PCIe link training failures")
Reported-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
Tested-by: Mika Westerberg <mika.westerberg@...ux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
---
 drivers/pci/pci-driver.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
index 51ec9e7e784f..05a114962df3 100644
--- a/drivers/pci/pci-driver.c
+++ b/drivers/pci/pci-driver.c
@@ -574,6 +574,12 @@ static void pci_pm_bridge_power_up_actions(struct pci_dev *pci_dev)
 {
 	int ret;
 
+	/*
+	 * Clear LBMS on resume to avoid spuriously triggering Target Speed
+	 * quirk in pcie_failed_link_retrain().
+	 */
+	pcie_capability_write_word(pci_dev, PCI_EXP_LNKSTA, PCI_EXP_LNKSTA_LBMS);
+
 	ret = pci_bridge_wait_for_secondary_bus(pci_dev, "resume");
 	if (ret) {
 		/*
-- 
2.39.2