PCI/PTM: Inherit Switch Downstream Port PTM settings from Upstream Port

Except for Endpoints, we enable PTM at enumeration-time.  Previously we did
not account for the fact that Switch Downstream Ports are not permitted to
have a PTM capability; their PTM behavior is controlled by the Upstream
Port (PCIe r5.0, sec 7.9.16).  Since Downstream Ports don't have a PTM
capability, we did not mark them as "ptm_enabled", which meant that
pci_enable_ptm() on an Endpoint failed because there was no PTM path to it.

Mark Downstream Ports as "ptm_enabled" if their Upstream Port has PTM
enabled.

Fixes: eec097d4 ("PCI: Add pci_enable_ptm() for drivers to enable PTM on endpoints")
Reported-by: Aditya Paluri <Venkata.AdityaPaluri@synopsys.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/pcie/ptm.c

@@ -39,10 +39,6 @@ void pci_ptm_init(struct pci_dev *dev)
 	if (!pci_is_pcie(dev))
 		return;
 
-	pos = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_PTM);
-	if (!pos)
-		return;
-
 	/*
 	 * Enable PTM only on interior devices (root ports, switch ports,
 	 * etc.) on the assumption that it causes no link traffic until an
@@ -52,6 +48,23 @@ void pci_ptm_init(struct pci_dev *dev)
 	     pci_pcie_type(dev) == PCI_EXP_TYPE_RC_END))
 		return;
 
+	/*
+	 * Switch Downstream Ports are not permitted to have a PTM
+	 * capability; their PTM behavior is controlled by the Upstream
+	 * Port (PCIe r5.0, sec 7.9.16).
+	 */
+	ups = pci_upstream_bridge(dev);
+	if (pci_pcie_type(dev) == PCI_EXP_TYPE_DOWNSTREAM &&
+	    ups && ups->ptm_enabled) {
+		dev->ptm_granularity = ups->ptm_granularity;
+		dev->ptm_enabled = 1;
+		return;
+	}
+
+	pos = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_PTM);
+	if (!pos)
+		return;
+
 	pci_read_config_dword(dev, pos + PCI_PTM_CAP, &cap);
 	local_clock = (cap & PCI_PTM_GRANULARITY_MASK) >> 8;
 
@@ -61,7 +74,6 @@ void pci_ptm_init(struct pci_dev *dev)
 	 * the spec recommendation (PCIe r3.1, sec 7.32.3), select the
 	 * furthest upstream Time Source as the PTM Root.
 	 */
-	ups = pci_upstream_bridge(dev);
 	if (ups && ups->ptm_enabled) {
 		ctrl = PCI_PTM_CTRL_ENABLE;
 		if (ups->ptm_granularity == 0)