2 * Copyright 2007 Haiku Inc. All rights reserved.
3 * Distributed under the terms of the MIT License.
6 * Niels Sascha Reedijk, niels.reedijk@gmail.com
11 \page usb_modules Writing drivers for USB devices
13 The introduction of USB standardized the way many devices connected to a
14 whole range of different computers and operating systems. It introduced a
15 standard that was capable of getting rid of all the legacy systems, such as
16 the LPT, the PS/2 and serial ports. The plug and play nature of the standard
17 was revolutionary at the time of its introduction, and it changed the way
18 which operating systems interacted with devices.
20 With the grand standard that USB has become, Haiku has an implementation
21 of it. It supports both the USB 1.1 and USB 2.0 specifications, and when
22 Haiku R1 is released, it will support the three host controller standards:
23 UHCI, OHCI and EHCI. The modularized design of Haiku's USB stack also paves
24 the way for easy implementation of any future specifications, such as
27 \section usb_modules_scope The Scope of this Document
29 This document is written for driver developers that need to interact with
30 USB devices. The USB specification standardizes the communication between
31 the host controller and the devices, and how devices should transfer data,
32 but it does not prescribe a standard environment that Operating Systems
33 should provide to the driver interfaces. As such, every operating system has
34 its own interface for drivers, and so does Haiku.
36 This document will point driver developers to relevant parts of the USB
37 module API and give a general impression of the workings of the USB stack.
38 This document will not give information on the basics of writing drivers, or
39 on how to use modules. Have a look elsewhere in this documentation for that.
40 This document also assumes a basic knowledge of the USB specification, and on
41 how you are supposed to interact with devices. See \ref usb_modules_resources
42 for tutorials on the web if you are looking for a basic introduction on
43 communication with the USB protocol.
45 \section usb_modules_structure Structure of the Stack
47 This section will outline how Haiku's USB stack is structured, and how you
48 can interact with this stack.
50 The goal of the USB stack is to provide a few basic features for drivers
51 interacting with USB devices. It is important that the stack maintains a
52 continually updated device grid, so that the driver modules are always
53 aware of the latest status. The stack should also facilitate communication
54 between drivers and the devices, by abstracting the actual transferring of
55 bits via the host controller hardware in the computer. The stack therefore
56 should implement a intuitive interface to give driver developers access to
57 all features and possibilities the USB specification offers, and at the same
58 time it should abstract many of the small requirements and peculiarities of
61 The stack internally can be divided into two parts. The first part is the
62 core module. This module, called \c usb_busmanager, performs all the
63 operations required by the USB specification. For example, it performs the
64 necessary low-level initialization when new devices are connected, or all the
65 requirements when it comes to performing transfers. The core module also
66 is the module that provides the abstractions to driver developers. The other
67 part of the USB stack are the individual modules that control the different
68 host controllers. Haiku supports the three types in existence: UHCI, OHCI
69 and EHCI. These modules perform the communication between the core module
70 and the hardware. As driver developer, you won't have to interact with these
71 modules: the core module provides all the abstractions you need.
73 Thus, as a driver developer you are interfacing with the \c usb_busmanager
74 module. On Haiku, this module implements two API's. The \c v2 API, identical
75 to the API offered by BeOS R5, can be found in the \c USB2.h file. However,
76 for new drivers, or for ports, the recommended API is the \c v3 API, defined
77 in the USB3.h file. This API is identical to the one provided by Zeta. The
78 \c v2 API should be considered to be deprecated.
80 \section usb_modules_registration Initial Steps: Driver Registration
82 In order to be able to start using the USB stack to communicate with your
83 devices, you will need to perform some actions. This section will outline
84 those actions and will point you to their appropriate locations.
86 \note The code examples are based on the \c usb_hid driver written by
87 Jerome Duval. Have a look at this driver for a complete working
90 The following example gives an overview of the requirements to open the
91 USB module, and to start your driver registration in order to receive
92 connect and disconnect events.
95 // Global variables and constants
96 usb_module_info *gUsb;
97 const char *kDriverName = "usb_hid";
99 static usb_support_descriptor sSupportedDevices[1] = {
100 { USB_HID_DEVICE_CLASS, 0, 0, 0, 0 },
103 // Prototype for the hooks that are called when devices are added or removed
104 static status_t hid_device_added(const usb_device *dev, void **cookie);
105 static status_t hid_device_removed(void *cookie);
107 static usb_notify_hooks sNotifyHooks = {
112 // Driver initialization, called by the kernel when the driver is loaded
116 if (get_module(B_USB_MODULE_NAME, (module_info **)&gUsb) != B_OK)
119 gUsb->register_driver(kDriverName, sSupportedDevices,
121 gUsb->install_notify(kDriverName, &sNotifyHooks);
127 Basically, this boils down to three steps. The first step is to acquire the
128 usb_module_info module. This struct contains a set of function pointers that
129 you use to communicate with the stack. You can retrieve it like you would
130 retrieve any other module.
132 As soon as you have done that you can start registering your driver in the
133 stack. What you do is you pass a unique identifier to identify your driver,
134 zero or more \link usb_support_descriptor support descriptors \endlink
135 to provide the stack with information on which devices you support, and the
136 number of support descriptors you provided. The stack is very flexible with
137 what patterns it accepts, so even the most complex driver will be able to
138 pass its credentials. Have a look at the \c usb_support_descriptor struct
139 and the \c usb_module_info::register_driver() call for all the details.
141 The last step in initialization is to provide the stack with notification
142 hooks. These are functions in your driver that the stack should call as soon
143 as a device is attached or removed. Please perform this call after your
144 internal driver data structures are initialized, because as soon as you
145 perform this call, the usb stack will start searching for already attached
146 devices that match the credentials. Have a look at
147 \c usb_module_info::install_notify() and the structure \c usb_notify_hooks
148 for the details on the signatures of your hooks.
150 \section usb_modules_changes Handling Device Changes
152 The USB stack will notify you of device connects and disconnects when they
153 occur. You will receive notifications as soon as you have supplied the hooks
154 to the stack, using \c usb_module_info::install_notify() . This section will
155 explain some of the details when it comes to handling device changes.
157 When a device is added, your supplied usb_notify_hooks::device_added() hook
158 will be called if its credentials matches one of your support descriptors.
159 Because the stack runs through all the registered drivers, it could be that
160 two or more drivers operate on the same device. The stack does not provide
161 a locking mechanism to prevent two conflicting drivers to get in each others
162 way. It is up to the device maker to have supplied such a mechanism.
164 \note In reality, it is very likely that your device will match at least one
165 other driver, because Haiku supplies the \c usb_raw driver. This driver
166 provides userland access to the usb devices and therefore it has a blank
167 support descriptor that matches everything. The \c usb_raw driver will
168 not conflict with your device interaction though (except when there is an
169 userland application that tries to meddle with your device).
171 If your driver is willing to accept the supplied device, and your
172 device_added() hook returns B_OK, the USB stack will ask the kernel to reload
173 your published devices, so that your device is visible in userspace in the
176 The other event that the stack reports, device disconnection, should be
177 handled by your \c usb_notify_hooks::device_removed() hook. Because "plug and
178 play" also means "unplug and leave", you should make sure your driver is
179 capable of cleaning up in the likely event that the user removes their
180 device, even during transfers. In your hook function, you have the ability to
181 do clean up whatever there is to clean up, however, make sure that you cancel
182 all the pending transfers. Use the usb_module_info::cancel_queued_transfers()
183 call for that end. Also, don't forget to free the cookie you supplied in your
186 \section usb_modules_standard Standard USB Operations
188 One of the many conveniences of the Haiku USB API is the fact that many of
189 the standard operations can be performed by simple function calls. As such,
190 you won't have to build many of the standard requests the USB specification
191 defines by hand. This section will outline all the different conveniences and
192 will point you to where to look if you do need something more advanced.
194 \subsection usb_modules_standard_descriptors Configurations, Interfaces and Descriptors
196 Many standard USB operations have to do with configurations, interfaces and
197 descriptors. All these operations are accessible by convenience functions.
199 The device descriptor is one of the first things you will be interested in if
200 you want to check out a device. The device descriptor can be retrieved quite
201 easily using the \c usb_module_info::get_device_descriptor() call. The
202 retrieved descriptor complies to the one dictated by the USB standard.
204 Also important are configurations. Since every device has at least one
205 configuration, you should be able to retrieve and manipulate configurations.
206 You can use \c usb_module_info::get_nth_configuration() to get them. To set
207 a configuration, you should use \c usb_module_info::set_configuration(). To
208 get the active configuration, use \c usb_module_info::get_configuration().
210 \attention By default, Haiku's stack will set the configuration at offset
211 zero, which is according to the standard, the default configuration.
212 Do not rely on that if you first get the device, that the currently active
213 configuration is the default configuration though. Another driver might
214 have manipulated this device already.
216 Every configuration has associated interfaces. To make life easier, the stack
217 automatically gets the interface descriptors (and their associated
218 endpoints), and stores them in the \c usb_configuration_info structure. This
219 structure has a member called \link usb_configuration_info::interface
220 \c interface \endlink which is of the type \c usb_interface_list. That object
221 containts all the interfaces, including a pointer to the interface that is
222 currently active. Each interface is described as a \c usb_interface_info,
223 which is a container for the interface, its associated endpoints and any
224 unparsed descriptors. In order to change the active interface, you can use
225 the stack's \c usb_module_info::set_alt_interface() call.
227 Endpoints, the basic units with which you can communicate, are stored as
228 \c usb_endpoint_info structures. Each of these structures carries the actual
229 endpoint descriptor, and the accompanying usb_pipe handle that you can use to
230 actually send and receive data.
232 The last point of interest are descriptors. As you have seen, Haiku caches
233 all the relevant descriptors itself, however, you might want to retrieve any
234 other type of descriptor that could be relevant for your device. The
235 convenience function to use in such a case is the
236 \c usb_module_info::get_descriptor() call. This function takes all the
237 parameters needed to build the actual descriptor, and performs the request
238 over the default control pipe.
240 \subsection usb_modules_standard_features Features
242 Another one of the building blocks of USB are features. Every device should
243 provide for a number of standard features, but the USB specification also
244 leaves the option to using custom device specific features. Feature requests
245 can be performed on devices, interfaces and pipes (which are tied to
248 To set a feature, you can use the \c usb_module_info::set_feature() call. To
249 clear a feature, use the \c usb_module_info::clear_feature() call. One of the
250 most used feature calls is the call to clear a \c USB_FEATURE_ENDPOINT_HALT .
252 \subsection usb_modules_standard_other Other Standard Calls
254 To get the status of a device, an interface or an endpoint, you can use the
255 \c usb_module_info::get_status() call.
257 If you are using isochronous transfers, you can use the
258 \c usb_module_info::set_pipe_policy() to set the properties of the
261 \section usb_modules_transfers Data Transfers
263 Transfering data is one of the basic building blocks of the USB protocol.
264 This section will demonstrate how to perform transfers via the four different
265 protocols the USB stack offers.
267 But first it is essential to show how to perform the transfers using the
268 \c usb_module_info interface. The interface provides five \c queue_*
269 functions, with the asterix being one of the following: \c bulk, \c bulk_v
270 (bulk transfers using a vector), \c interrupt, \c isochronous or \c request
271 (over the standard control pipe). These five functions work asynchronously,
272 which means that your driver is called back from a different thread when your
273 transfer is finished.
275 The five functions share some arguments. The first argument is always the
276 pipe that is associated with the endpoint (except for control transfers,
277 these only work on the device in general). All of the functions accept a data
278 buffer, and the length of that buffer. All of the functions require a
279 \c #usb_callback_func, a function in your driver that can be called in case a
280 transfer is finished. The functions also require a cookie that is provided to
281 the callback function.
283 The working order is as follows: first you queue a transfer, then you handle
284 the result in the callback function when it's done. The callback function
285 will be called with a \a status argument, in which you can check whether or
286 not the transfer actually succeeded. See this \link #usb_callback_func
287 description \endlink for how your callback function should behave and what
288 kind of status there might have been.
290 Finally, before going into the different transfer types, a note on buffer
291 ownership. The usb stack keeps the internal buffers tidy, but the buffer you
292 provide to the \c queue_* functions are yours. You are responsible for
293 allocating and freeing them, and you may do with them whatever you like,
294 \e except between queueing your transfer and the callback. During that period
295 you should consider the USB stack the owner of the buffer.
297 \subsection usb_modules_transfers_control Control Requests
299 Control requests are done over the device wide control pipe which is provided
300 by every device. Haiku's stack has two functions that you can use to perform
301 custom requests (opposed to many of the \ref usb_modules_standard
302 "standard operations"). Control transfers are the only transfers that you can
303 perform synchronously as well as asynchronously. The functions you can use
304 are \c usb_module_info::send_request() for synchronous requests and
305 \c usb_module_info::queue_request() for asynchronous requests.
307 Many of the constants that you should use when performing can be found in
308 the USB_spec.h file which is automatically included if you include the main
309 USB header. Have a look of how to use these constants in the following
313 // Send a request that is defined by the standard of this class. We retrieve
314 // a report from the device on one of its interfaces.
315 // This request is specified by the HID specification.
317 status = usb->send_request(dev,
318 USB_REQTYPE_INTERFACE_IN | USB_REQTYPE_CLASS,
319 USB_REQUEST_HID_GET_REPORT, 0x0100 | report_id,
320 interfaceNumber, device->total_report_size,
321 device->buffer, &actual);
324 \warning Both the \link usb_module_info::send_request() \a send_request()
325 \endlink and \link usb_module_info::queue_request() \a queue_request()
326 \endlink functions can be used to perform standard usb requests. Avoid
327 low-level operations, because the stack needs to keep its internal
328 data structures consistent. If you need to perform one of the
329 \ref usb_modules_standard "standard operations", use the provided
330 convenience functions.
332 \subsection usb_modules_transfers_interrupt Interrupt
334 Interrupt transfers apply to endpoints that receive data, or that can be
335 polled in several instances of time. The intervals are determined by the
338 To schedule a transfer, use usb_module_info::queue_interrupt(). You only have
339 to supply a buffer, the stack schedule the transfer in such a way that it
340 will be performed within a certain timeframe. To create a continuous
341 interrupt system, you should queue the next transfer in the callback function
342 of the previous. The stack will make sure that the new transfer will be
343 performed exactly after the required interval.
345 \subsection usb_modules_transfers_bulk Bulk
347 Bulk transfers are very similar to control transfers. They will be performed
348 as soon as possible without stalling other transfers, and they transfer data.
349 Bulk transfers are designed to transfer up to large amounts of data as
350 efficiently as possible. Performing bulk transfers isn't difficult, you
351 merely supply a buffer and the endpoint that should execute the request, and
354 Bulk transfers come in two flavours. The first is
355 usb_module_info::queue_bulk(), which takes a standard data buffer. The second
356 flavour is the usb_module_info::queue_bulk_v() function, which is designed to
357 operate on (an array of) POSIX vectors. These functions only differ in the
358 buffer they accept, they function in exactly the same way.
360 \subsection usb_modules_transfers_isochronous Isochronous
362 Isochronous transfers are not implemented on Haiku yet. As soon as they are,
363 this section should contain information on how to queue them.
365 \section usb_modules_cleanup Cleaning Up
367 This section describes how to gracefully leave the stack after your driver is
368 requested to shut down.
370 There are truely only two simple actions to perform. The first is to
371 uninstall your notification hooks, using
372 \c usb_module_info::uninstall_notify(). The second action is to 'put' the
379 usb->uninstall_notify(kDriverName);
380 put_module(B_USB_MODULE_NAME);
384 \section usb_modules_resources More Resources
386 This section should list more resources on the web.