drivers/usbhost: Support USB hubs on xHCI.

drivers/usbhost/Kconfig

@@ -832,6 +832,22 @@ config USBHOST_XHCI_MAX_DEVS
 	---help---
 		How many USB devices will be supported by xHCI driver.
 
+config USBHOST_XHCI_ENUM_RETRIES
+	int "xHCI enumeration attempts per port"
+	default 3
+	range 1 255
+	---help---
+		How many times to attempt enumeration of a newly connected device
+		before leaving the port alone until the device is unplugged.
+
+		A device whose descriptors cannot be read, or that no class driver
+		claims, fails enumeration every time.  Each failure marks the port
+		disconnected so the attempt repeats, so without a limit such a
+		device is retried for as long as it stays plugged in, logging and
+		taking a device slot on every pass.
+
+		The count is per root hub port and is cleared by a new connection.
+
 endif # USBHOST_XHCI
 
 menuconfig USBHOST_XHCI_PCI

drivers/usbhost/usbhost_hub.c

@@ -578,6 +578,17 @@ static inline int usbhost_hubdesc(FAR struct usbhost_class_s *hubclass)
   priv->pwrondelay  = (2 * hubdesc->pwrondelay);
   priv->ctrlcurrent = hubdesc->ctrlcurrent;
 
+  /* Publish what describes this hub as a hub, rather than as a device, on
+   * the port it occupies.  A host controller that has to be told about the
+   * hubs in a topology reads it from there when it sets up a device behind
+   * this one.  This runs before any downstream port is activated, so it is
+   * in place before there is anything behind it to set up.
+   */
+
+  hport->nports     = hubdesc->nports;
+  hport->ttt        = (hubchar & USBHUB_CHAR_TTTT_MASK) >>
+                       USBHUB_CHAR_TTTT_SHIFT;
+
   uinfo("Hub Descriptor:\n");
   uinfo("  bDescLength:         %d\n", hubdesc->len);
   uinfo("  bDescriptorType:     0x%02x\n", hubdesc->type);