usb: common: introduce of_usb_get_maximum_speed()

this helper will be used for controllers which
want to work at a lower speed even though they
support higher USB transfer rates.

One such case is Texas Instruments' AM437x
SoC where it uses a USB3 controller without
a USB3 PHY, rendering the controller USB2-only.
Signed-off-by: Felipe Balbi <balbi@ti.com>

Documentation/devicetree/bindings/usb/generic.txt

+Generic USB Properties
+
+Optional properties:
+ - maximum-speed: tells USB controllers we want to work up to a certain
+			speed. Valid arguments are "super-speed", "high-speed",
+			"full-speed" and "low-speed". In case this isn't passed
+			via DT, USB controllers should default to their maximum
+			HW capability.
+ - dr_mode: tells Dual-Role USB controllers that we want to work on a
+			particular mode. Valid arguments are "host",
+			"peripheral" and "otg". In case this attribute isn't
+			passed via DT, USB DRD controllers should default to
+			OTG.
+
+This is an attribute to a USB controller such as:
+
+dwc3@4a030000 {
+	compatible = "synopsys,dwc3";
+	reg = <0x4a030000 0xcfff>;
+	interrupts = <0 92 4>
+	usb-phy = <&usb2_phy>, <&usb3,phy>;
+	maximum-speed = "super-speed";
+	dr_mode = "otg";
+};

drivers/usb/usb-common.c

@@ -43,20 +43,20 @@ const char *usb_otg_state_string(enum usb_otg_state state)
 }
 EXPORT_SYMBOL_GPL(usb_otg_state_string);
 
-const char *usb_speed_string(enum usb_device_speed speed)
-{
-	static const char *const names[] = {
+static const char *const speed_names[] = {
 	[USB_SPEED_UNKNOWN] = "UNKNOWN",
 	[USB_SPEED_LOW] = "low-speed",
 	[USB_SPEED_FULL] = "full-speed",
 	[USB_SPEED_HIGH] = "high-speed",
 	[USB_SPEED_WIRELESS] = "wireless",
 	[USB_SPEED_SUPER] = "super-speed",
-	};
+};
 
-	if (speed < 0 || speed >= ARRAY_SIZE(names))
+const char *usb_speed_string(enum usb_device_speed speed)
+{
+	if (speed < 0 || speed >= ARRAY_SIZE(speed_names))
 		speed = USB_SPEED_UNKNOWN;
-	return names[speed];
+	return speed_names[speed];
 }
 EXPORT_SYMBOL_GPL(usb_speed_string);
 
@@ -112,6 +112,33 @@ enum usb_dr_mode of_usb_get_dr_mode(struct device_node *np)
 	return USB_DR_MODE_UNKNOWN;
 }
 EXPORT_SYMBOL_GPL(of_usb_get_dr_mode);
+
+/**
+ * of_usb_get_maximum_speed - Get maximum requested speed for a given USB
+ * controller.
+ * @np: Pointer to the given device_node
+ *
+ * The function gets the maximum speed string from property "maximum-speed",
+ * and returns the corresponding enum usb_device_speed.
+ */
+enum usb_device_speed of_usb_get_maximum_speed(struct device_node *np)
+{
+	const char *maximum_speed;
+	int err;
+	int i;
+
+	err = of_property_read_string(np, "maximum-speed", &maximum_speed);
+	if (err < 0)
+		return USB_SPEED_UNKNOWN;
+
+	for (i = 0; i < ARRAY_SIZE(speed_names); i++)
+		if (strcmp(maximum_speed, speed_names[i]) == 0)
+			return i;
+
+	return USB_SPEED_UNKNOWN;
+}
+EXPORT_SYMBOL_GPL(of_usb_get_maximum_speed);
+
 #endif
 
 MODULE_LICENSE("GPL");

include/linux/usb/of.h

@@ -7,16 +7,24 @@
 #ifndef __LINUX_USB_OF_H
 #define __LINUX_USB_OF_H
 
+#include <linux/usb/ch9.h>
 #include <linux/usb/otg.h>
 #include <linux/usb/phy.h>
 
 #if IS_ENABLED(CONFIG_OF)
 enum usb_dr_mode of_usb_get_dr_mode(struct device_node *np);
+enum usb_device_speed of_usb_get_maximum_speed(struct device_node *np);
 #else
 static inline enum usb_dr_mode of_usb_get_dr_mode(struct device_node *np)
 {
 	return USB_DR_MODE_UNKNOWN;
 }
+
+static inline enum usb_device_speed
+of_usb_get_maximum_speed(struct device_node *np)
+{
+	return USB_SPEED_UNKNOWN;
+}
 #endif
 
 #if IS_ENABLED(CONFIG_OF) && IS_ENABLED(CONFIG_USB_PHY)