haiku/docs/user/drivers/usb_modules.dox

/*
 * Copyright 2007 Haiku Inc. All rights reserved.
 * Distributed under the terms of the MIT License.
 *
 * Authors:
 *		Niels Sascha Reedijk, niels.reedijk@gmail.com
 */


/*!
	\page usb_modules Writing drivers for USB devices

	The introduction of USB standardized the way many devices connected to a
	whole range of different computers and operating systems. It introduced a
	standard that was capable of getting rid of all the legacy systems, such as
	the LPT, the PS/2 and serial ports. The plug and play nature of the standard
	was revolutionary at the time of its introduction, and it changed the way
	which operating systems interacted with devices.

	With the grand standard that USB has become, Haiku has an implementation
	of it. It supports both the USB 1.1 and USB 2.0 specifications, and when
	Haiku R1 is released, it will support the three host controller standards:
	UHCI, OHCI and EHCI. The modularized design of Haiku's USB stack also paves
	the way for easy implementation of any future specifications, such as
	Wireless USB.

	\section usb_modules_scope The Scope of this Document

	This document is written for driver developers that need to interact with
	USB devices. The USB specification standardizes the communication between
	the host controller and the devices, and how devices should transfer data,
	but it does not prescribe a standard environment that Operating Systems
	should provide to the driver interfaces. As such, every operating system has
	its own interface for drivers, and so does Haiku.

	This document will point driver developers to relevant parts of the USB
	module API and give a general impression of the workings of the USB stack.
	This document will not give information on the basics of writing drivers, or
	on how to use modules. Have a look elsewhere in this documentation for that.
	This document also assumes a basic knowledge of the USB specification, and on
	how you are supposed to interact with devices. See \ref usb_modules_resources
	for tutorials on the web if you are looking for a basic introduction on
	communication with the USB protocol.

	\section usb_modules_structure Structure of the Stack

	This section will outline how Haiku's USB stack is structured, and how you
	can interact with this stack.

	The goal of the USB stack is to provide a few basic features for drivers
	interacting with USB devices. It is important that the stack maintains a
	continually updated device grid, so that the driver modules are always
	aware of the latest status. The stack should also facilitate communication
	between drivers and the devices, by abstracting the actual transferring of
	bits via the host controller hardware in the computer. The stack therefore
	should implement a intuitive interface to give driver developers access to
	all features and possibilities the USB specification offers, and at the same
	time it should abstract many of the small requirements and peculiarities of
	that specification.

	The stack internally can be divided into two parts. The first part is the
	core module. This module, called \c usb_busmanager, performs all the
	operations required by the USB specification. For example, it performs the
	necessary low-level initialization when new devices are connected, or all the
	requirements when it comes to performing transfers. The core module also
	is the module that provides the abstractions to driver developers. The other
	part of the USB stack are the individual modules that control the different
	host controllers. Haiku supports the three types in existence: UHCI, OHCI
	and EHCI. These modules perform the communication between the core module
	and the hardware. As driver developer, you won't have to interact with these
	modules: the core module provides all the abstractions you need.

	Thus, as a driver developer you are interfacing with the \c usb_busmanager
	module. On Haiku, this module implements two API's. The \c v2 API, identical
	to the API offered by BeOS R5, can be found in the \c USB2.h file. However,
	for new drivers, or for ports, the recommended API is the \c v3 API, defined
	in the USB3.h file. This API is identical to the one provided by Zeta. The
	\c v2 API should be considered to be deprecated.

	\section usb_modules_registration Initial Steps: Driver Registration

	In order to be able to start using the USB stack to communicate with your
	devices, you will need to perform some actions. This section will outline
	those actions and will point you to their appropriate locations. 

	\note The code examples are based on the \c usb_hid driver written by
		Jerome Duval. Have a look at this driver for a complete working
		example.

	The following example gives an overview of the requirements to open the 
	USB module, and to start your driver registration in order to receive
	connect and disconnect events. 

\code
// Global variables and constants
usb_module_info *gUsb;
const char *kDriverName = "usb_hid";

static usb_support_descriptor sSupportedDevices[1] = {
    { USB_HID_DEVICE_CLASS, 0, 0, 0, 0 },
};

// Prototype for the hooks that are called when devices are added or removed
static status_t hid_device_added(const usb_device *dev, void **cookie);
static status_t hid_device_removed(void *cookie);

static usb_notify_hooks sNotifyHooks = {
	hid_device_added,
	hid_device_removed
};

// Driver initialization, called by the kernel when the driver is loaded
status_t 
init_driver(void)
{
    if (get_module(B_USB_MODULE_NAME, (module_info **)&gUsb) != B_OK)
        return B_ERROR;

    gUsb->register_driver(kDriverName, sSupportedDevices, 
        1, NULL);
    gUsb->install_notify(kDriverName, &sNotifyHooks);

    return B_OK;
}
\endcode

	Basically, this boils down to three steps. The first step is to acquire the
	usb_module_info module. This struct contains a set of function pointers that
	you use to communicate with the stack. You can retrieve it like you would
	retrieve any other module.

	As soon as you have done that you can start registering your driver in the
	stack. What you do is you pass a unique identifier to identify your driver,
	zero or more \link usb_support_descriptor support descriptors \endlink
	to provide the stack with information on which devices you support, and the
	number of support descriptors you provided. The stack is very flexible with
	what patterns it accepts, so even the most complex driver will be able to
	pass its credentials. Have a look at the \c usb_support_descriptor struct
	and the \c usb_module_info::register_driver() call for all the details.

	The last step in initialization is to provide the stack with notification
	hooks. These are functions in your driver that the stack should call as soon
	as a device is attached or removed. Please perform this call after your
	internal driver data structures are initialized, because as soon as you
	perform this call, the usb stack will start searching for already attached
	devices that match the credentials. Have a look at
	\c usb_module_info::install_notify() and the structure \c usb_notify_hooks
	for the details on the signatures of your hooks.

	\section usb_modules_changes Handling Device Changes

	The USB stack will notify you of device connects and disconnects when they
	occur. You will receive notifications as soon as you have supplied the hooks
	to the stack, using \c usb_module_info::install_notify() . This section will
	explain some of the details when it comes to handling device changes.

	When a device is added, your supplied usb_notify_hooks::device_added() hook
	will be called if its credentials matches one of your support descriptors.
	Because the stack runs through all the registered drivers, it could be that
	two or more drivers operate on the same device. The stack does not provide
	a locking mechanism to prevent two conflicting drivers to get in each others
	way. It is up to the device maker to have supplied such a mechanism.

	\note In reality, it is very likely that your device will match at least one
	other driver, because Haiku supplies the \c usb_raw driver. This driver
	provides userland access to the usb devices and therefore it has a blank
	support descriptor that matches everything. The \c usb_raw driver will
	not conflict with your device interaction though (except when there is an
	userland application that tries to meddle with your device).

	If your driver is willing to accept the supplied device, and your 
	device_added() hook returns B_OK, the USB stack will ask the kernel to reload
	your published devices, so that your device is visible in userspace in the
	\c /dev tree.

	The other event that the stack reports, device disconnection, should be
	handled by your \c usb_notify_hooks::device_removed() hook. Because "plug and
	play" also means "unplug and leave", you should make sure your driver is 
	capable of cleaning up in the likely event that the user removes their
	device, even during transfers. In your hook function, you have the ability to
	do clean up whatever there is to clean up, however, make sure that you cancel
	all the pending transfers. Use the usb_module_info::cancel_queued_transfers()
	call for that end. Also, don't forget to free the cookie you supplied in your
	device_added() hook. 

	\section usb_modules_standard Standard USB Operations

	One of the many conveniences of the Haiku USB API is the fact that many of
	the standard operations can be performed by simple function calls. As such,
	you won't have to build many of the standard requests the USB specification
	defines by hand. This section will outline all the different conveniences and
	will point you to where to look if you do need something more advanced.

	\subsection usb_modules_standard_descriptors Configurations, Interfaces and Descriptors

	Many standard USB operations have to do with configurations, interfaces and
	descriptors. All these operations are accessible by convenience functions.

	The device descriptor is one of the first things you will be interested in if
	you want to check out a device. The device descriptor can be retrieved quite
	easily using the \c usb_module_info::get_device_descriptor() call. The
	retrieved descriptor complies to the one dictated by the USB standard.

	Also important are configurations. Since every device has at least one
	configuration, you should be able to retrieve and manipulate configurations.
	You can use \c usb_module_info::get_nth_configuration() to get them. To set
	a configuration, you should use \c usb_module_info::set_configuration(). To
	get the active configuration, use \c usb_module_info::get_configuration().

	\attention By default, Haiku's stack will set the configuration at offset
	zero, which is according to the standard, the default configuration.
	Do not rely on that if you first get the device, that the currently active
	configuration is the default configuration though. Another driver might
	have manipulated this device already.

	Every configuration has associated interfaces. To make life easier, the stack
	automatically gets the interface descriptors (and their associated
	endpoints), and stores them in the \c usb_configuration_info structure. This
	structure has a member called \link usb_configuration_info::interface
	\c interface \endlink which is of the type \c usb_interface_list. That object
	containts all the interfaces, including a pointer to the interface that is
	currently active. Each interface is described as a \c usb_interface_info,
	which is a container for the interface, its associated endpoints and any
	unparsed descriptors. In order to change the active interface, you can use
	the stack's \c usb_module_info::set_alt_interface() call. 

	Endpoints, the basic units with which you can communicate, are stored as
	\c usb_endpoint_info structures. Each of these structures carries the actual
	endpoint descriptor, and the accompanying usb_pipe handle that you can use to
	actually send and receive data.

	The last point of interest are descriptors. As you have seen, Haiku caches
	all the relevant descriptors itself, however, you might want to retrieve any
	other type of descriptor that could be relevant for your device. The
	convenience function to use in such a case is the 
	\c usb_module_info::get_descriptor() call. This function takes all the
	parameters needed to build the actual descriptor, and performs the request
	over the default control pipe.

	\subsection usb_modules_standard_features Features

	Another one of the building blocks of USB are features. Every device should
	provide for a number of standard features, but the USB specification also
	leaves the option to using custom device specific features. Feature requests
	can be performed on devices, interfaces and pipes (which are tied to
	endpoints). 

	To set a feature, you can use the \c usb_module_info::set_feature() call. To
	clear a feature, use the \c usb_module_info::clear_feature() call. One of the
	most used feature calls is the call to clear a \c USB_FEATURE_ENDPOINT_HALT .

	\subsection usb_modules_standard_other Other Standard Calls

	To get the status of a device, an interface or an endpoint, you can use the
	\c usb_module_info::get_status() call.

	If you are using isochronous transfers, you can use the
	\c usb_module_info::set_pipe_policy() to set the properties of the
	isochronous pipe. 

	\section usb_modules_transfers Data Transfers

	Transfering data is one of the basic building blocks of the USB protocol.
	This section will demonstrate how to perform transfers via the four different
	protocols the USB stack offers.

	But first it is essential to show how to perform the transfers using the
	\c usb_module_info interface. The interface provides five \c queue_*
	functions, with the asterix being one of the following: \c bulk, \c bulk_v
	(bulk transfers using a vector), \c interrupt, \c isochronous or \c request
	(over the standard control pipe). These five functions work asynchronously,
	which means that your driver is called back from a different thread when your
	transfer is finished. 

	The five functions share some arguments. The first argument is always the
	pipe that is associated with the endpoint (except for control transfers,
	these only work on the device in general). All of the functions accept a data
	buffer, and the length of that   buffer. All of the functions require a
	\c #usb_callback_func, a function in your driver that can be called in case a
	transfer is finished. The functions also require a cookie that is provided to
	the callback function. 

	The working order is as follows: first you queue a transfer, then you handle
	the result in the callback function when it's done. The callback function
	will be called with a \a status argument, in which you can check whether or
	not the transfer actually succeeded. See this \link #usb_callback_func 
	description \endlink for how your callback function should behave and what
	kind of status there might have been.

	Finally, before going into the different transfer types, a note on buffer
	ownership. The usb stack keeps the internal buffers tidy, but the buffer you
	provide to the \c queue_* functions are yours. You are responsible for
	allocating and freeing them, and you may do with them whatever you like,
	\e except between queueing your transfer and the callback. During that period
	you should consider the USB stack the owner of the buffer.

	\subsection usb_modules_transfers_control Control Requests

	Control requests are done over the device wide control pipe which is provided
	by every device. Haiku's stack has two functions that you can use to perform
	custom requests (opposed to many of the \ref usb_modules_standard
	"standard operations"). Control transfers are the only transfers that you can
	perform synchronously as well as asynchronously. The functions you can use
	are \c usb_module_info::send_request() for synchronous requests and
	\c usb_module_info::queue_request() for asynchronous requests. 

	Many of the constants that you should use when performing can be found in
	the USB_spec.h file which is automatically included if you include the main
	USB header. Have a look of how to use these constants in the following
	example:

\code
    // Send a request that is defined by the standard of this class. We retrieve
    // a report from the device on one of its interfaces.
    // This request is specified by the HID specification.

    status = usb->send_request(dev,
        USB_REQTYPE_INTERFACE_IN | USB_REQTYPE_CLASS,
        USB_REQUEST_HID_GET_REPORT, 0x0100 | report_id,
        interfaceNumber, device->total_report_size,
        device->buffer, &actual);
\endcode

	\warning Both the \link usb_module_info::send_request() \a send_request()
	\endlink and \link usb_module_info::queue_request() \a queue_request()
	\endlink functions can be used to perform standard usb requests. Avoid
	low-level operations, because the stack needs to keep its internal
	data structures consistent. If you need to perform one of the 
	\ref usb_modules_standard "standard operations", use the provided
	convenience functions.

	\subsection usb_modules_transfers_interrupt Interrupt

	Interrupt transfers apply to endpoints that receive data, or that can be
	polled in several instances of time. The intervals are determined by the
	endpoint descriptor.

	To schedule a transfer, use usb_module_info::queue_interrupt(). You only have
	to supply a buffer, the stack schedule the transfer in such a way that it
	will be performed within a certain timeframe. To create a continuous
	interrupt system, you should queue the next transfer in the callback function
	of the previous. The stack will make sure that the new transfer will be
	performed exactly after the required interval. 

	\subsection usb_modules_transfers_bulk Bulk

	Bulk transfers are very similar to control transfers. They will be performed
	as soon as possible without stalling other transfers, and they transfer data.
	Bulk transfers are designed to transfer up to large amounts of data as
	efficiently as possible. Performing bulk transfers isn't difficult, you
	merely supply a buffer and the endpoint that should execute the request, and
	you're set. 

	Bulk transfers come in two flavours. The first is 
	usb_module_info::queue_bulk(), which takes a standard data buffer. The second
	flavour is the usb_module_info::queue_bulk_v() function, which is designed to
	operate on (an array of) POSIX vectors. These functions only differ in the
	buffer they accept, they function in exactly the same way.

	\subsection usb_modules_transfers_isochronous Isochronous

	Isochronous transfers are not implemented on Haiku yet. As soon as they are,
	this section should contain information on how to queue them.

	\section usb_modules_cleanup Cleaning Up

	This section describes how to gracefully leave the stack after your driver is
	requested to shut down.

	There are truely only two simple actions to perform. The first is to
	uninstall your notification hooks, using 
	\c usb_module_info::uninstall_notify(). The second action is to 'put' the
	module.

\code
void
uninit_driver(void)
{
    usb->uninstall_notify(kDriverName);
    put_module(B_USB_MODULE_NAME);
}
\endcode

	\section usb_modules_resources More Resources

	This section should list more resources on the web.
*/