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>

3 files changed, 70 insertions, 11 deletions

diff --git a/Documentation/devicetree/bindings/usb/generic.txt b/Documentation/devicetree/bindings/usb/generic.txt
new file mode 100644
index 000000000000..477d5bb5e51c
--- /dev/null
+++ b/Documentation/devicetree/bindings/usb/generic.txt
@@ -0,0 +1,24 @@
+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";
+};
diff --git a/drivers/usb/usb-common.c b/drivers/usb/usb-common.c
index 675384dabfe9..d771870a819e 100644
--- a/drivers/usb/usb-common.c
+++ b/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);
 
+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",
+};
+
 const char *usb_speed_string(enum usb_device_speed speed)
 {
-        static const char *const 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))
+        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");
diff --git a/include/linux/usb/of.h b/include/linux/usb/of.h
index a0ef405368b8..5a7cb9ebf0ef 100644
--- a/include/linux/usb/of.h
+++ b/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)