Patch "usb: dwc3: gadget: let us set lower max_speed" has been added to the 4.2-stable tree

This is a note to let you know that I've just added the patch titled

usb: dwc3: gadget: let us set lower max_speed

to the 4.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
usb-dwc3-gadget-let-us-set-lower-max_speed.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Ben McCauley <[email protected]>
Date: Mon, 16 Nov 2015 10:47:24 -0600
Subject: usb: dwc3: gadget: let us set lower max_speed

From: Ben McCauley <[email protected]>

commit b9e51b2b1fda19143f48d182ed7a2943f21e1ae4 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.

[ [email protected] : rewrote entire commit, including source code comment to make a lot clearer what the problem is ]

---
 drivers/usb/dwc3/gadget.c |   24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

@@ -2714,12 +2714,34 @@ 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)
+		dwc3_trace(trace_dwc3_gadget,
+				"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.
 	 */

Patches currently in stable-queue which might be from [email protected] are

queue-4.2/usb-dwc3-gadget-let-us-set-lower-max_speed.patch