--- linux/drivers/usb/host/pci-quirks.c.orig	2015-04-06 18:39:45.000000000 -0400
+++ linux/drivers/usb/host/pci-quirks.c	2015-04-12 15:38:52.764807435 -0400
@@ -902,6 +902,20 @@
 		return;
 	}
 
+	/* The Dell XPS 13 BIOS configures xHCI for us, so leave it alone here.
+	 * In particular, the left-side USB3 port cannot operate reliably at
+	 * SuperSpeed (bad design of an internal ribbon cable).  There is
+	 * a BIOS setting ("USB debug") which the user can "Enable" to treat
+	 * the port as a USB2 port rather than USB3 (sets USB3_PSSEN=0xd).
+	 * So leave leave the SuperSpeed config as-found here, but still switch
+	 * both ports over to the xHCI controller (more efficient).
+	 */
+	if (xhci_pdev->subsystem_vendor == PCI_VENDOR_ID_DELL &&
+	    xhci_pdev->subsystem_device == 0x058b) {
+		pci_write_config_dword(xhci_pdev, USB_INTEL_XUSB2PR, 0xf);
+		return;
+	}
+
 	/* Read USB3PRM, the USB 3.0 Port Routing Mask Register
 	 * Indicate the ports that can be changed from OS.
 	 */