Revert 129069 - Import libusb 1.0.9-rc3 into third_party.

third_party/libusb/PORTING

-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.
-