usb: dwc3: xilinx: Skip resets and USB3 register settings for USB2.0 mode

It appears that the PIPE clock should not be selected when only USB 2.0
is being used in the design and no USB 3.0 reference clock is used.
Also, the core resets are not required if a USB3 PHY is not in use, and
will break things if USB3 is actually used but the PHY entry is not
listed in the device tree.

Skip core resets and register settings that are only required for
USB3 mode when no USB3 PHY is specified in the device tree.

Fixes: 84770f02 ("usb: dwc3: Add driver for Xilinx platforms")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Link: https://lore.kernel.org/r/20220126000253.1586760-2-robert.hancock@calian.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/dwc3/dwc3-xilinx.c

@@ -110,6 +110,18 @@ static int dwc3_xlnx_init_zynqmp(struct dwc3_xlnx *priv_data)
 		usb3_phy = NULL;
 	}
 
+	/*
+	 * The following core resets are not required unless a USB3 PHY
+	 * is used, and the subsequent register settings are not required
+	 * unless a core reset is performed (they should be set properly
+	 * by the first-stage boot loader, but may be reverted by a core
+	 * reset). They may also break the configuration if USB3 is actually
+	 * in use but the usb3-phy entry is missing from the device tree.
+	 * Therefore, skip these operations in this case.
+	 */
+	if (!usb3_phy)
+		goto skip_usb3_phy;
+
 	crst = devm_reset_control_get_exclusive(dev, "usb_crst");
 	if (IS_ERR(crst)) {
 		ret = PTR_ERR(crst);
@@ -188,6 +200,7 @@ static int dwc3_xlnx_init_zynqmp(struct dwc3_xlnx *priv_data)
 		goto err;
 	}
 
+skip_usb3_phy:
 	/*
 	 * This routes the USB DMA traffic to go through FPD path instead
 	 * of reaching DDR directly. This traffic routing is needed to