usb: dwc3: core: set force_gen1 bit in USB31 devices if max speed is SS

Currently for dwc3_usb31 controller, if maximum_speed is limited to
super-speed in DT, then device mode is limited to SS, but host mode
still works in SSP.

The documentation for max-speed property is as follows:

"Tells USB controllers we want to work up to a certain speed.
Incase  this isn't passed via DT, USB controllers should default to
their maximum HW capability."

It doesn't specify that the property is only for device mode.
There are cases where we need to limit the host's maximum speed to
SuperSpeed only. Use this property for host mode to contrain host's
speed to SuperSpeed.
Signed-off-by: Krishna Kurapati <quic_kriskura@quicinc.com>
Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/20231219041559.15789-1-quic_kriskura@quicinc.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/dwc3/core.c

@@ -1330,6 +1330,18 @@ static int dwc3_core_init(struct dwc3 *dwc)
 
 	dwc3_config_threshold(dwc);
 
+	/*
+	 * Modify this for all supported Super Speed ports when
+	 * multiport support is added.
+	 */
+	if (hw_mode != DWC3_GHWPARAMS0_MODE_GADGET &&
+	    (DWC3_IP_IS(DWC31)) &&
+	    dwc->maximum_speed == USB_SPEED_SUPER) {
+		reg = dwc3_readl(dwc->regs, DWC3_LLUCTL);
+		reg |= DWC3_LLUCTL_FORCE_GEN1;
+		dwc3_writel(dwc->regs, DWC3_LLUCTL, reg);
+	}
+
 	return 0;
 
 err_power_off_phy:

drivers/usb/dwc3/core.h

@@ -172,6 +172,8 @@
 #define DWC3_OEVTEN		0xcc0C
 #define DWC3_OSTS		0xcc10
 
+#define DWC3_LLUCTL		0xd024
+
 /* Bit fields */
 
 /* Global SoC Bus Configuration INCRx Register 0 */
@@ -657,6 +659,9 @@
 #define DWC3_OSTS_VBUSVLD		BIT(1)
 #define DWC3_OSTS_CONIDSTS		BIT(0)
 
+/* Force Gen1 speed on Gen2 link */
+#define DWC3_LLUCTL_FORCE_GEN1		BIT(10)
+
 /* Structures */
 
 struct dwc3_trb;