staging: dwc2: don't issue traffic to LS devices in FS mode

I fell over the problem reported in
https://github.com/raspberrypi/linux/pull/390:

"Issuing low-speed packets when the root port is in full-speed mode
 causes the root port to stop responding. Explicitly fail when
 enqueuing URBs to a LS endpoint on a FS bus."

with my dwc2 testing in NetBSD, so I adapted the change to dwc2.
Signed-off-by: Nick Hudson <skrll@netbsd.org>
[paulz: fixed up the patch to compile under Linux, and tested it]
Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/dwc2/hcd.c

@@ -355,6 +355,7 @@ static int dwc2_hcd_urb_enqueue(struct dwc2_hsotg *hsotg,
 	unsigned long flags;
 	u32 intr_mask;
 	int retval;
+	int dev_speed;
 
 	if (!hsotg->flags.b.port_connect_status) {
 		/* No longer connected */
@@ -362,6 +363,19 @@ static int dwc2_hcd_urb_enqueue(struct dwc2_hsotg *hsotg,
 		return -ENODEV;
 	}
 
+	dev_speed = dwc2_host_get_speed(hsotg, urb->priv);
+
+	/* Some configurations cannot support LS traffic on a FS root port */
+	if ((dev_speed == USB_SPEED_LOW) &&
+	    (hsotg->hw_params.fs_phy_type == GHWCFG2_FS_PHY_TYPE_DEDICATED) &&
+	    (hsotg->hw_params.hs_phy_type == GHWCFG2_HS_PHY_TYPE_UTMI)) {
+		u32 hprt0 = readl(hsotg->regs + HPRT0);
+		u32 prtspd = (hprt0 & HPRT0_SPD_MASK) >> HPRT0_SPD_SHIFT;
+
+		if (prtspd == HPRT0_SPD_FULL_SPEED)
+			return -ENODEV;
+	}
+
 	qtd = kzalloc(sizeof(*qtd), mem_flags);
 	if (!qtd)
 		return -ENOMEM;