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96 lines
3.8 KiB
Plaintext
96 lines
3.8 KiB
Plaintext
PORTING LIBUSB TO OTHER PLATFORMS
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Introduction
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============
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This document is aimed at developers wishing to port libusb to unsupported
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platforms. I believe the libusb API is OS-independent, so by supporting
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multiple operating systems we pave the way for cross-platform USB device
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drivers.
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Implementation-wise, the basic idea is that you provide an interface to
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libusb's internal "backend" API, which performs the appropriate operations on
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your target platform.
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In terms of USB I/O, your backend provides functionality to submit
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asynchronous transfers (synchronous transfers are implemented in the higher
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layers, based on the async interface). Your backend must also provide
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functionality to cancel those transfers.
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Your backend must also provide an event handling function to "reap" ongoing
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transfers and process their results.
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The backend must also provide standard functions for other USB operations,
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e.g. setting configuration, obtaining descriptors, etc.
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File descriptors for I/O polling
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================================
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For libusb to work, your event handling function obviously needs to be called
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at various points in time. Your backend must provide a set of file descriptors
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which libusb and its users can pass to poll() or select() to determine when
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it is time to call the event handling function.
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On Linux, this is easy: the usbfs kernel interface exposes a file descriptor
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which can be passed to poll(). If something similar is not true for your
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platform, you can emulate this using an internal library thread to reap I/O as
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necessary, and a pipe() with the main library to raise events. The file
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descriptor of the pipe can then be provided to libusb as an event source.
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Interface semantics and documentation
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=====================================
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Documentation of the backend interface can be found in libusbi.h inside the
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usbi_os_backend structure definition.
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Your implementations of these functions will need to call various internal
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libusb functions, prefixed with "usbi_". Documentation for these functions
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can be found in the .c files where they are implemented.
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You probably want to skim over *all* the documentation before starting your
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implementation. For example, you probably need to allocate and store private
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OS-specific data for device handles, but the documentation for the mechanism
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for doing so is probably not the first thing you will see.
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The Linux backend acts as a good example - view it as a reference
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implementation which you should try to match the behaviour of.
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Getting started
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===============
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1. Modify configure.ac to detect your platform appropriately (see the OS_LINUX
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stuff for an example).
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2. Implement your backend in the libusb/os/ directory, modifying
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libusb/os/Makefile.am appropriately.
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3. Add preprocessor logic to the top of libusb/core.c to statically assign the
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right usbi_backend for your platform.
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4. Produce and test your implementation.
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5. Send your implementation to libusb-devel mailing list.
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Implementation difficulties? Questions?
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=======================================
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If you encounter difficulties porting libusb to your platform, please raise
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these issues on the libusb-devel mailing list. Where possible and sensible, I
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am interested in solving problems preventing libusb from operating on other
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platforms.
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The libusb-devel mailing list is also a good place to ask questions and
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make suggestions about the internal API. Hopefully we can produce some
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better documentation based on your questions and other input.
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You are encouraged to get involved in the process; if the library needs
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some infrastructure additions/modifications to better support your platform,
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you are encouraged to make such changes (in cleanly distinct patch
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submissions). Even if you do not make such changes yourself, please do raise
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the issues on the mailing list at the very minimum.
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