PCI: Make link retraining use RMW accessors for changing LNKCTL

Don't assume that the device is fully under the control of PCI core.  Use
RMW capability accessors in link retraining which do proper locking to
avoid losing concurrent updates to the register values.

Suggested-by: Lukas Wunner <lukas@wunner.de>
Fixes: 4ec7379 ("PCI: Work around Pericom PCIe-to-PCI bridge Retrain Link erratum")
Fixes: 7d715a6 ("PCI: add PCI Express ASPM support")
Link: https://lore.kernel.org/r/20230717120503.15276-3-ilpo.jarvinen@linux.intel.com
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: "Rafael J. Wysocki" <rafael@kernel.org>

Author: ij-intel

drivers/pci/pci.c

@@ -4927,7 +4927,6 @@ static int pcie_wait_for_link_status(struct pci_dev *pdev,
 int pcie_retrain_link(struct pci_dev *pdev, bool use_lt)
 {
 	int rc;
-	u16 lnkctl;
 
 	/*
 	 * Ensure the updated LNKCTL parameters are used during link
@@ -4939,17 +4938,14 @@ int pcie_retrain_link(struct pci_dev *pdev, bool use_lt)
 	if (rc)
 		return rc;
 
-	pcie_capability_read_word(pdev, PCI_EXP_LNKCTL, &lnkctl);
-	lnkctl |= PCI_EXP_LNKCTL_RL;
-	pcie_capability_write_word(pdev, PCI_EXP_LNKCTL, lnkctl);
+	pcie_capability_set_word(pdev, PCI_EXP_LNKCTL, PCI_EXP_LNKCTL_RL);
 	if (pdev->clear_retrain_link) {
 		/*
 		 * Due to an erratum in some devices the Retrain Link bit
 		 * needs to be cleared again manually to allow the link
 		 * training to succeed.
 		 */
-		lnkctl &= ~PCI_EXP_LNKCTL_RL;
-		pcie_capability_write_word(pdev, PCI_EXP_LNKCTL, lnkctl);
+		pcie_capability_clear_word(pdev, PCI_EXP_LNKCTL, PCI_EXP_LNKCTL_RL);
 	}
 
 	return pcie_wait_for_link_status(pdev, use_lt, !use_lt);