PORTING - platform/external/libusb - Gitiles

 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.
	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.