Android USB Gadget

The Android Linux kernel has an incredibly versatile USB gadget that is not present in mainline linux.

Using sysfs, you can configure a USB device that has a large number of interfaces, each exposing one particular function.

Each function (class) is implemented by a gadget driver. In the source code they are prefixed with "f_", so the serial function is called f_serial

Using sysfs to configure

Each USB device is exposed as a /sys/devices/virtual/android_usb/androidX interface where X corresponds to the logical number. Most devices these days have only one or two USB ports, so it is mostly android0 or android1 that you will see.

Below this sysfs directory ad a number of different files. They can only be modified/written to if the USB port is disabled by means of

echo 0 > sys/devices/virtual/android_usb/android0/enable

It will disconnect from the USB host at this point.

After modifying what you want to modify, you can use

echo 1 > sys/devices/virtual/android_usb/android0/enable

to re-enable the USB port. It will re-enumerate to the host at this point.


Allows you to set the USB Vendor ID


Allows you to set the USB Device ID


Allows you to set the USB Manufacturer string. You enter it in ASCII, it will be converted to a USB String automatically.


Allows you to set the USB Product string. You enter it in ASCII, it will be converted to a USB String automatically.


Allows you to configure which functions compose the interfaces of your USB device

The default on Quectel modems seems to be diag,serial,rmnet

A more generally useful setup for developers is something like adb,diag,serial,rmnet which adds ADB access.

You can also add further functions like MBIM, CCID, MTP, PTP, or even mass_storage. Please note that most of them require additional configuration to do anything useful.

The order of functions in the string you write to functions will determine the order of those interfaces in the interface descriptor.

Please note that some functions can themselves implement multiple interfaces. Even in the Quectel default you will see that the serial function offers three virtual serial ports: One for GPS and two for AT-Commands.

This is due to the fact that something like tty,smd,smd is written into the /sys/class/android_usb/androidX/f_serial/transports file (see below)

Gadgets / kernel code

'struct android_usb_function' implements a given function

ffs (function FS)

  • userspace process can implement USB functions
  • used e.g. by adbd

acm (CDC "astract control model")

  • USB-IF standardized version of serial-type ports
  • exposed via /dev/ttyGS* device


usb gadget driver to implement a rmnet gadget, f_rmnet.c

  • drivers/net/ethernet/msm/msm_rmnet_mhi.c
  • interface descriptor: class/subclass/proto = 0xff, numEP=3
    • stringdescriptor en-us, string 0 == "RmNet"
    • full speed descriptors (notify/in/out)
    • high speed descriptors (notify/in/out)
    • super speed descriptors (notify/in/out + comp)
  • rmnet has ports
    • control ports can map to SMD, QTI, HSIC, HSUART
      • SMD (Shared Memory Device) channel towards modem
      • QTI: ?
      • HSIC: High-speed Inter-IC USB variant (for chipsets with multiple ICs interconnected by HISC)
      • HSUART: High-speed UART
    • data ports can map to BAM2BAM, BAM_DMUX, BAM2BAM_IPA, HSIC, HSUART, ETHER
      • BAM2BAM is normal case? Replaced by BAM_DMUX in more recent kernels?
      • BAM2BAM_IPA via internet packet accelerator?
      • HSIC: High-speed Inter-IC USB variant
      • HSUART: High-speed UART
      • ETHER exposes the data pipe as usb_rmnet%d device on Linux (EC25 only)
  • frmnet_init_port() establishes connections


  • IF class/sub/proto = 0xff
  • only one endpoint (notify)
  • string0 (en-us) is "GPS"

ncm (CDC Netowrk (NCM) link function)

ecm_qc (CDC Ethernet)

usb_mbim (Mobile Broadband Interface Model)

audio (uac1?)


  • class/sub/proto = 0xff
  • num-ep 2
  • max-packet 512
  • some special assumption only if bInterfaceNumber==0

qdss (Qualcomm Debug Sub-System)

  • has QDSS DATA / QDSS CTRL string descriptor
  • somehow connects to BAM


Offers a f_serial/transports file that determines the number of virtual serial ports and the low-level transport to use for them

A maximum of four instances is supported, but one could change MAX_SERIAL_INSTANCES if more were needed.

  • class=0xff, sub/proto = 0
  • num_endpoints = 3
  • string descroptor "Generic Serial"
  • transports
    • tty (-> gserial, i.e. geenrates a /dev/ttyGSx device for use by local processes)
    • smd (shared memory [to baseband])
    • char_bridge (unknown)
    • hsic (a inter-chip version of USB in case of multi-chip modems)

ccid (chipcard reader)

  • /dev/ccid_ctrl
  • /dev/ccid_bulk
  • probably intended to be used with userspace progam translating USB CCID requests into QMI requests to access SIM card(s)


  • usb charger support















modem detection / EC20 re-using same usb device id / interface numbers

The idea here is to dynamically detect which interface number exposes a certain interface,
regardless what order / qty / interface number

  • ADB: 255/66/1, 2 EP
  • Diag: 255/255/255, 2 EP
  • TTY: 255/0/0, 3 EP, unrecognized descriptors
  • RMNET: 255/255/255, 3EP