[PATCH 4.2 097/124] usb: dwc3: gadget: let us set lower max_speed

[Greg Kroah-Hartman]

4.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ben McCauley <ben.mccauley@xxxxxxxxxx>

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.

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

Signed-off-by: Ben McCauley <ben.mccauley@xxxxxxxxxx>
Signed-off-by: Felipe Balbi <balbi@xxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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

--- a/drivers/usb/dwc3/gadget.c
+++ b/drivers/usb/dwc3/gadget.c
@@ -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.
 	 */


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