usb: dwc3: gadget: let us set lower max_speed

commit b9e51b2b upstream.

In some SoCs, dwc3 is implemented as a USB2.0 only
core, meaning that it can't ever achieve SuperSpeed.

Currect driver always sets gadget.max_speed to
USB_SPEED_SUPER unconditionally. This can causes
issues to some Host stacks where the host will issue
a GetBOS() request and we will reply with a BOS
containing Superspeed Capability Descriptor.

At least Windows seems to be upset by this fact and
prints a warning that we should connect $this device
to another port.

[ balbi@ti.com : rewrote entire commit, including
source code comment to make a lot clearer what the
problem is ]
Signed-off-by: Ben McCauley <ben.mccauley@garmin.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
[ luis: backported to 3.16:
  - used dev_vdbg() instead of dwc3_trace() ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/usb/dwc3/gadget.c

@@ -2786,11 +2786,32 @@ int dwc3_gadget_init(struct dwc3 *dwc)
 	}
 
 	dwc->gadget.ops			= &dwc3_gadget_ops;
-	dwc->gadget.max_speed		= USB_SPEED_SUPER;
 	dwc->gadget.speed		= USB_SPEED_UNKNOWN;
 	dwc->gadget.sg_supported	= true;
 	dwc->gadget.name		= "dwc3-gadget";
 
+	/*
+	 * FIXME We might be setting max_speed to <SUPER, however versions
+	 * <2.20a of dwc3 have an issue with metastability (documented
+	 * elsewhere in this driver) which tells us we can't set max speed to
+	 * anything lower than SUPER.
+	 *
+	 * Because gadget.max_speed is only used by composite.c and function
+	 * drivers (i.e. it won't go into dwc3's registers) we are allowing this
+	 * to happen so we avoid sending SuperSpeed Capability descriptor
+	 * together with our BOS descriptor as that could confuse host into
+	 * thinking we can handle super speed.
+	 *
+	 * Note that, in fact, we won't even support GetBOS requests when speed
+	 * is less than super speed because we don't have means, yet, to tell
+	 * composite.c that we are USB 2.0 + LPM ECN.
+	 */
+	if (dwc->revision < DWC3_REVISION_220A)
+		dev_vdbg(dwc->dev, "Changing max_speed on rev %08x\n",
+			 dwc->revision);
+
+	dwc->gadget.max_speed		= dwc->maximum_speed;
+
 	/*
 	 * Per databook, DWC3 needs buffer size to be aligned to MaxPacketSize
 	 * on ep out.