usb: dwc3: workaround: metastability state on Run/Stop bit

All revisions prior to 2.20a have a known issue which
causes metastability state on Run/Stop bit if we
configure the core to work on any of the USB2-only
speeds.

The suggested workaround is just to never configure the
core to anything other than SuperSpeed.
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/dwc3/gadget.c

@@ -1356,7 +1356,24 @@ static int dwc3_gadget_start(struct usb_gadget *g,
 
 	reg = dwc3_readl(dwc->regs, DWC3_DCFG);
 	reg &= ~(DWC3_DCFG_SPEED_MASK);
-	reg |= dwc->maximum_speed;
+
+	/**
+	 * WORKAROUND: DWC3 revision < 2.20a have an issue
+	 * which would cause metastability state on Run/Stop
+	 * bit if we try to force the IP to USB2-only mode.
+	 *
+	 * Because of that, we cannot configure the IP to any
+	 * speed other than the SuperSpeed
+	 *
+	 * Refers to:
+	 *
+	 * STAR#9000525659: Clock Domain Crossing on DCTL in
+	 * USB 2.0 Mode
+	 */
+	if (dwc->revision < DWC3_REVISION_220A)
+		reg |= DWC3_DCFG_SUPERSPEED;
+	else
+		reg |= dwc->maximum_speed;
 	dwc3_writel(dwc->regs, DWC3_DCFG, reg);
 
 	dwc->start_config_issued = false;