PORTING LIBUSB TO OTHER PLATFORMS

Introduction

============

This document is aimed at developers wishing to port libusb to unsupported

platforms. I believe the libusb API is OS-independent, so by supporting

multiple operating systems we pave the way for cross-platform USB device

drivers.

Implementation-wise, the basic idea is that you provide an interface to

libusb's internal "backend" API, which performs the appropriate operations on

your target platform.

In terms of USB I/O, your backend provides functionality to submit

asynchronous transfers (synchronous transfers are implemented in the higher

layers, based on the async interface). Your backend must also provide

functionality to cancel those transfers.

Your backend must also provide an event handling function to "reap" ongoing

transfers and process their results.

The backend must also provide standard functions for other USB operations,

e.g. setting configuration, obtaining descriptors, etc.

File descriptors for I/O polling

================================

For libusb to work, your event handling function obviously needs to be called

at various points in time. Your backend must provide a set of file descriptors

which libusb and its users can pass to poll() or select() to determine when

it is time to call the event handling function.

On Linux, this is easy: the usbfs kernel interface exposes a file descriptor

which can be passed to poll(). If something similar is not true for your

platform, you can emulate this using an internal library thread to reap I/O as

necessary, and a pipe() with the main library to raise events. The file

descriptor of the pipe can then be provided to libusb as an event source.

Interface semantics and documentation

=====================================

Documentation of the backend interface can be found in libusbi.h inside the

usbi_os_backend structure definition.

Your implementations of these functions will need to call various internal

libusb functions, prefixed with "usbi_". Documentation for these functions

can be found in the .c files where they are implemented.

You probably want to skim over *all* the documentation before starting your

implementation. For example, you probably need to allocate and store private

OS-specific data for device handles, but the documentation for the mechanism

for doing so is probably not the first thing you will see.

The Linux backend acts as a good example - view it as a reference

implementation which you should try to match the behaviour of.

Getting started

===============

1. Modify configure.ac to detect your platform appropriately (see the OS_LINUX

stuff for an example).

2. Implement your backend in the libusb/os/ directory, modifying

libusb/os/Makefile.am appropriately.

3. Add preprocessor logic to the top of libusb/core.c to statically assign the

right usbi_backend for your platform.

4. Produce and test your implementation.

5. Send your implementation to libusb-devel mailing list.

Implementation difficulties? Questions?

=======================================

If you encounter difficulties porting libusb to your platform, please raise

these issues on the libusb-devel mailing list. Where possible and sensible, I

am interested in solving problems preventing libusb from operating on other

platforms.

The libusb-devel mailing list is also a good place to ask questions and

make suggestions about the internal API. Hopefully we can produce some

better documentation based on your questions and other input.

You are encouraged to get involved in the process; if the library needs

some infrastructure additions/modifications to better support your platform,

you are encouraged to make such changes (in cleanly distinct patch

submissions). Even if you do not make such changes yourself, please do raise

the issues on the mailing list at the very minimum.