usb: dwc3: add support for USB 2.0-only core configuration

Newer DWC3 controllers can be built for USB 2.0-only mode, where
most of the USB 3.0 circuitry is left out. To support this mode,
the driver must limit the speed programmed into the DCFG register
to Hi-Speed or lower.

Reads and writes to the PIPECTL register are left as-is, since
they should be no-ops in USB 2.0-only mode. Calls to phy_init()
etc. for the USB3 phy are also left as-is, since the no-op USB3
phy should be used for USB 2.0-only mode controllers.
Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/dwc3/core.c

@@ -386,6 +386,13 @@ static int dwc3_core_init(struct dwc3 *dwc)
 	}
 	dwc->revision = reg;
 
+	/* Handle USB2.0-only core configuration */
+	if (DWC3_GHWPARAMS3_SSPHY_IFC(dwc->hwparams.hwparams3) ==
+			DWC3_GHWPARAMS3_SSPHY_IFC_DIS) {
+		if (dwc->maximum_speed == USB_SPEED_SUPER)
+			dwc->maximum_speed = USB_SPEED_HIGH;
+	}
+
 	/* issue device SoftReset too */
 	timeout = jiffies + msecs_to_jiffies(500);
 	dwc3_writel(dwc->regs, DWC3_DCTL, DWC3_DCTL_CSFTRST);

drivers/usb/dwc3/core.h

@@ -191,6 +191,19 @@
 #define DWC3_GHWPARAMS1_PWROPT(n)	((n) << 24)
 #define DWC3_GHWPARAMS1_PWROPT_MASK	DWC3_GHWPARAMS1_PWROPT(3)
 
+/* Global HWPARAMS3 Register */
+#define DWC3_GHWPARAMS3_SSPHY_IFC(n)		((n) & 3)
+#define DWC3_GHWPARAMS3_SSPHY_IFC_DIS		0
+#define DWC3_GHWPARAMS3_SSPHY_IFC_ENA		1
+#define DWC3_GHWPARAMS3_HSPHY_IFC(n)		(((n) & (3 << 2)) >> 2)
+#define DWC3_GHWPARAMS3_HSPHY_IFC_DIS		0
+#define DWC3_GHWPARAMS3_HSPHY_IFC_UTMI		1
+#define DWC3_GHWPARAMS3_HSPHY_IFC_ULPI		2
+#define DWC3_GHWPARAMS3_HSPHY_IFC_UTMI_ULPI	3
+#define DWC3_GHWPARAMS3_FSPHY_IFC(n)		(((n) & (3 << 4)) >> 4)
+#define DWC3_GHWPARAMS3_FSPHY_IFC_DIS		0
+#define DWC3_GHWPARAMS3_FSPHY_IFC_ENA		1
+
 /* Global HWPARAMS4 Register */
 #define DWC3_GHWPARAMS4_HIBER_SCRATCHBUFS(n)	(((n) & (0x0f << 13)) >> 13)
 #define DWC3_MAX_HIBER_SCRATCHBUFS		15