usb: xhci-pci: add support for hosts with zero USB3 ports

Add xhci support for PCI hosts that have zero USB3 ports.
Avoid creating a shared Host Controller Driver (HCD) when there is only
one root hub. Additionally, all references to 'xhci->shared_hcd' are now
checked before use.

Only xhci-pci.c requires modification to accommodate this change, as the
xhci core already supports configurations with zero USB3 ports. This
capability was introduced when xHCI Platform and MediaTek added support
for zero USB3 ports.

Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220181
Tested-by: Nick Nielsen <nick.kainielsen@free.fr>
Tested-by: grm1 <grm1@mailbox.org>
Signed-off-by: Niklas Neronin <niklas.neronin@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://lore.kernel.org/r/20250917210726.97100-4-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Epicurius

drivers/usb/host/xhci-pci.c

@@ -610,7 +610,7 @@ int xhci_pci_common_probe(struct pci_dev *dev, const struct pci_device_id *id)
 {
 	int retval;
 	struct xhci_hcd *xhci;
-	struct usb_hcd *hcd;
+	struct usb_hcd *hcd, *usb3_hcd;
 	struct reset_control *reset;
 
 	reset = devm_reset_control_get_optional_exclusive(&dev->dev, NULL);
@@ -636,26 +636,32 @@ int xhci_pci_common_probe(struct pci_dev *dev, const struct pci_device_id *id)
 	hcd = dev_get_drvdata(&dev->dev);
 	xhci = hcd_to_xhci(hcd);
 	xhci->reset = reset;
-	xhci->shared_hcd = usb_create_shared_hcd(&xhci_pci_hc_driver, &dev->dev,
-						 pci_name(dev), hcd);
-	if (!xhci->shared_hcd) {
-		retval = -ENOMEM;
-		goto dealloc_usb2_hcd;
-	}
 
-	retval = xhci_ext_cap_init(xhci);
-	if (retval)
-		goto put_usb3_hcd;
+	xhci->allow_single_roothub = 1;
+	if (!xhci_has_one_roothub(xhci)) {
+		xhci->shared_hcd = usb_create_shared_hcd(&xhci_pci_hc_driver, &dev->dev,
+							 pci_name(dev), hcd);
+		if (!xhci->shared_hcd) {
+			retval = -ENOMEM;
+			goto dealloc_usb2_hcd;
+		}
 
-	retval = usb_add_hcd(xhci->shared_hcd, dev->irq,
-			IRQF_SHARED);
-	if (retval)
-		goto put_usb3_hcd;
-	/* Roothub already marked as USB 3.0 speed */
+		retval = xhci_ext_cap_init(xhci);
+		if (retval)
+			goto put_usb3_hcd;
+
+		retval = usb_add_hcd(xhci->shared_hcd, dev->irq, IRQF_SHARED);
+		if (retval)
+			goto put_usb3_hcd;
+	} else {
+		retval = xhci_ext_cap_init(xhci);
+		if (retval)
+			goto dealloc_usb2_hcd;
+	}
 
-	if (!(xhci->quirks & XHCI_BROKEN_STREAMS) &&
-			HCC_MAX_PSA(xhci->hcc_params) >= 4)
-		xhci->shared_hcd->can_do_streams = 1;
+	usb3_hcd = xhci_get_usb3_hcd(xhci);
+	if (usb3_hcd && !(xhci->quirks & XHCI_BROKEN_STREAMS) && HCC_MAX_PSA(xhci->hcc_params) >= 4)
+		usb3_hcd->can_do_streams = 1;
 
 	/* USB-2 and USB-3 roothubs initialized, allow runtime pm suspend */
 	pm_runtime_put_noidle(&dev->dev);