Linux USB DRD Dual-Role Solution

RTL8730E USB is a Dual-Role Device (DRD) controller that can operate as either a USB host or a USB device. The operating role is selected through the dr_mode device tree property together with the role-switching method described below.

OTG

When dr_mode = "otg", both the host and device roles are initialized at driver startup. In a standard USB OTG design, the role is determined automatically by the OTG ID pin of the Micro-AB / Type-C connector: grounding the ID pin selects host mode, while a floating ID pin selects device mode.

DRD

On QFN100, the role is selected by software through the Realtek USB PHY force_mode mechanism instead of the OTG ID pin. It is controlled via the sysfs parameter /sys/module/phy_rtk_usb/parameters/usb_force_mode:

  • 1: Force USB host mode

  • 2: Force USB device mode

This feature is enabled by the device tree property rtk,usb-force-role-en.

Note

When QFN100 operates in OTG mode, specify the role manually via force_mode.

When to use:

  • Device mode: After completing insmod and ConfigFS configuration, set the following before enabling the UDC (see How to Enable and Disable Gadget):

    echo 2 > /sys/module/phy_rtk_usb/parameters/usb_force_mode

  • Host mode: After completing insmod, set:

    echo 1 > /sys/module/phy_rtk_usb/parameters/usb_force_mode