linux/linux-5.18.11/Documentation/usb/gadget_serial.rst

===============================
Linux Gadget Serial Driver v2.0
===============================

11/20/2004

(updated 8-May-2008 for v2.3)


License and Disclaimer
----------------------
This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License as
published by the Free Software Foundation; either version 2 of
the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public
License along with this program; if not, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
MA 02111-1307 USA.

This document and the gadget serial driver itself are
Copyright (C) 2004 by Al Borchers (alborchers@steinerpoint.com).

If you have questions, problems, or suggestions for this driver
please contact Al Borchers at alborchers@steinerpoint.com.


Prerequisites
-------------
Versions of the gadget serial driver are available for the
2.4 Linux kernels, but this document assumes you are using
version 2.3 or later of the gadget serial driver in a 2.6
Linux kernel.

This document assumes that you are familiar with Linux and
Windows and know how to configure and build Linux kernels, run
standard utilities, use minicom and HyperTerminal, and work with
USB and serial devices.  It also assumes you configure the Linux
gadget and usb drivers as modules.

With version 2.3 of the driver, major and minor device nodes are
no longer statically defined.  Your Linux based system should mount
sysfs in /sys, and use "mdev" (in Busybox) or "udev" to make the
/dev nodes matching the sysfs /sys/class/tty files.



Overview
--------
The gadget serial driver is a Linux USB gadget driver, a USB device
side driver.  It runs on a Linux system that has USB device side
hardware; for example, a PDA, an embedded Linux system, or a PC
with a USB development card.

The gadget serial driver talks over USB to either a CDC ACM driver
or a generic USB serial driver running on a host PC::

   Host
   --------------------------------------
  | Host-Side   CDC ACM       USB Host   |
  | Operating |   or        | Controller |   USB
  | System    | Generic USB | Driver     |--------
  | (Linux or | Serial      | and        |        |
  | Windows)    Driver        USB Stack  |        |
   --------------------------------------         |
                                                  |
                                                  |
                                                  |
   Gadget                                         |
   --------------------------------------         |
  | Gadget                   USB Periph. |        |
  | Device-Side |  Gadget  | Controller  |        |
  | Linux       |  Serial  | Driver      |--------
  | Operating   |  Driver  | and         |
  | System                   USB Stack   |
   --------------------------------------

On the device-side Linux system, the gadget serial driver looks
like a serial device.

On the host-side system, the gadget serial device looks like a
CDC ACM compliant class device or a simple vendor specific device
with bulk in and bulk out endpoints, and it is treated similarly
to other serial devices.

The host side driver can potentially be any ACM compliant driver
or any driver that can talk to a device with a simple bulk in/out
interface.  Gadget serial has been tested with the Linux ACM driver,
the Windows usbser.sys ACM driver, and the Linux USB generic serial
driver.

With the gadget serial driver and the host side ACM or generic
serial driver running, you should be able to communicate between
the host and the gadget side systems as if they were connected by a
serial cable.

The gadget serial driver only provides simple unreliable data
communication.  It does not yet handle flow control or many other
features of normal serial devices.


Installing the Gadget Serial Driver
-----------------------------------
To use the gadget serial driver you must configure the Linux gadget
side kernel for "Support for USB Gadgets", for a "USB Peripheral
Controller" (for example, net2280), and for the "Serial Gadget"
driver.  All this are listed under "USB Gadget Support" when
configuring the kernel.  Then rebuild and install the kernel or
modules.

Then you must load the gadget serial driver.  To load it as an
ACM device (recommended for interoperability), do this::

  modprobe g_serial

To load it as a vendor specific bulk in/out device, do this::

  modprobe g_serial use_acm=0

This will also automatically load the underlying gadget peripheral
controller driver.  This must be done each time you reboot the gadget
side Linux system.  You can add this to the start up scripts, if
desired.

Your system should use mdev (from busybox) or udev to make the
device nodes.  After this gadget driver has been set up you should
then see a /dev/ttyGS0 node::

  # ls -l /dev/ttyGS0 | cat
  crw-rw----    1 root     root     253,   0 May  8 14:10 /dev/ttyGS0
  #

Note that the major number (253, above) is system-specific.  If
you need to create /dev nodes by hand, the right numbers to use
will be in the /sys/class/tty/ttyGS0/dev file.

When you link this gadget driver early, perhaps even statically,
you may want to set up an /etc/inittab entry to run "getty" on it.
The /dev/ttyGS0 line should work like most any other serial port.


If gadget serial is loaded as an ACM device you will want to use
either the Windows or Linux ACM driver on the host side.  If gadget
serial is loaded as a bulk in/out device, you will want to use the
Linux generic serial driver on the host side.  Follow the appropriate
instructions below to install the host side driver.


Installing the Windows Host ACM Driver
--------------------------------------
To use the Windows ACM driver you must have the "linux-cdc-acm.inf"
file (provided along this document) which supports all recent versions
of Windows.

When the gadget serial driver is loaded and the USB device connected
to the Windows host with a USB cable, Windows should recognize the
gadget serial device and ask for a driver.  Tell Windows to find the
driver in the folder that contains the "linux-cdc-acm.inf" file.

For example, on Windows XP, when the gadget serial device is first
plugged in, the "Found New Hardware Wizard" starts up.  Select
"Install from a list or specific location (Advanced)", then on the
next screen select "Include this location in the search" and enter the
path or browse to the folder containing the "linux-cdc-acm.inf" file.
Windows will complain that the Gadget Serial driver has not passed
Windows Logo testing, but select "Continue anyway" and finish the
driver installation.

On Windows XP, in the "Device Manager" (under "Control Panel",
"System", "Hardware") expand the "Ports (COM & LPT)" entry and you
should see "Gadget Serial" listed as the driver for one of the COM
ports.

To uninstall the Windows XP driver for "Gadget Serial", right click
on the "Gadget Serial" entry in the "Device Manager" and select
"Uninstall".


Installing the Linux Host ACM Driver
------------------------------------
To use the Linux ACM driver you must configure the Linux host side
kernel for "Support for Host-side USB" and for "USB Modem (CDC ACM)
support".

Once the gadget serial driver is loaded and the USB device connected
to the Linux host with a USB cable, the host system should recognize
the gadget serial device.  For example, the command::

  cat /sys/kernel/debug/usb/devices

should show something like this:::

  T:  Bus=01 Lev=01 Prnt=01 Port=01 Cnt=02 Dev#=  5 Spd=480 MxCh= 0
  D:  Ver= 2.00 Cls=02(comm.) Sub=00 Prot=00 MxPS=64 #Cfgs=  1
  P:  Vendor=0525 ProdID=a4a7 Rev= 2.01
  S:  Manufacturer=Linux 2.6.8.1 with net2280
  S:  Product=Gadget Serial
  S:  SerialNumber=0
  C:* #Ifs= 2 Cfg#= 2 Atr=c0 MxPwr=  2mA
  I:  If#= 0 Alt= 0 #EPs= 1 Cls=02(comm.) Sub=02 Prot=01 Driver=acm
  E:  Ad=83(I) Atr=03(Int.) MxPS=   8 Ivl=32ms
  I:  If#= 1 Alt= 0 #EPs= 2 Cls=0a(data ) Sub=00 Prot=00 Driver=acm
  E:  Ad=81(I) Atr=02(Bulk) MxPS= 512 Ivl=0ms
  E:  Ad=02(O) Atr=02(Bulk) MxPS= 512 Ivl=0ms

If the host side Linux system is configured properly, the ACM driver
should be loaded automatically.  The command "lsmod" should show the
"acm" module is loaded.


Installing the Linux Host Generic USB Serial Driver
---------------------------------------------------
To use the Linux generic USB serial driver you must configure the
Linux host side kernel for "Support for Host-side USB", for "USB
Serial Converter support", and for the "USB Generic Serial Driver".

Once the gadget serial driver is loaded and the USB device connected
to the Linux host with a USB cable, the host system should recognize
the gadget serial device.  For example, the command::

  cat /sys/kernel/debug/usb/devices

should show something like this:::

  T:  Bus=01 Lev=01 Prnt=01 Port=01 Cnt=02 Dev#=  6 Spd=480 MxCh= 0
  D:  Ver= 2.00 Cls=ff(vend.) Sub=00 Prot=00 MxPS=64 #Cfgs=  1
  P:  Vendor=0525 ProdID=a4a6 Rev= 2.01
  S:  Manufacturer=Linux 2.6.8.1 with net2280
  S:  Product=Gadget Serial
  S:  SerialNumber=0
  C:* #Ifs= 1 Cfg#= 1 Atr=c0 MxPwr=  2mA
  I:  If#= 0 Alt= 0 #EPs= 2 Cls=0a(data ) Sub=00 Prot=00 Driver=serial
  E:  Ad=81(I) Atr=02(Bulk) MxPS= 512 Ivl=0ms
  E:  Ad=02(O) Atr=02(Bulk) MxPS= 512 Ivl=0ms

You must load the usbserial driver and explicitly set its parameters
to configure it to recognize the gadget serial device, like this::

  echo 0x0525 0xA4A6 >/sys/bus/usb-serial/drivers/generic/new_id

The legacy way is to use module parameters::

  modprobe usbserial vendor=0x0525 product=0xA4A6

If everything is working, usbserial will print a message in the
system log saying something like "Gadget Serial converter now
attached to ttyUSB0".


Testing with Minicom or HyperTerminal
-------------------------------------
Once the gadget serial driver and the host driver are both installed,
and a USB cable connects the gadget device to the host, you should
be able to communicate over USB between the gadget and host systems.
You can use minicom or HyperTerminal to try this out.

On the gadget side run "minicom -s" to configure a new minicom
session.  Under "Serial port setup" set "/dev/ttygserial" as the
"Serial Device".  Set baud rate, data bits, parity, and stop bits,
to 9600, 8, none, and 1--these settings mostly do not matter.
Under "Modem and dialing" erase all the modem and dialing strings.

On a Linux host running the ACM driver, configure minicom similarly
but use "/dev/ttyACM0" as the "Serial Device".  (If you have other
ACM devices connected, change the device name appropriately.)

On a Linux host running the USB generic serial driver, configure
minicom similarly, but use "/dev/ttyUSB0" as the "Serial Device".
(If you have other USB serial devices connected, change the device
name appropriately.)

On a Windows host configure a new HyperTerminal session to use the
COM port assigned to Gadget Serial.  The "Port Settings" will be
set automatically when HyperTerminal connects to the gadget serial
device, so you can leave them set to the default values--these
settings mostly do not matter.

With minicom configured and running on the gadget side and with
minicom or HyperTerminal configured and running on the host side,
you should be able to send data back and forth between the gadget
side and host side systems.  Anything you type on the terminal
window on the gadget side should appear in the terminal window on
the host side and vice versa.