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   2<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
   3        "" []>
   5<book id="USBDeviceDriver">
   6 <bookinfo>
   7  <title>Writing USB Device Drivers</title>
   9  <authorgroup>
  10   <author>
  11    <firstname>Greg</firstname>
  12    <surname>Kroah-Hartman</surname>
  13    <affiliation>
  14     <address>
  15      <email></email>
  16     </address>
  17    </affiliation>
  18   </author>
  19  </authorgroup>
  21  <copyright>
  22   <year>2001-2002</year>
  23   <holder>Greg Kroah-Hartman</holder>
  24  </copyright>
  26  <legalnotice>
  27   <para>
  28     This documentation is free software; you can redistribute
  29     it and/or modify it under the terms of the GNU General Public
  30     License as published by the Free Software Foundation; either
  31     version 2 of the License, or (at your option) any later
  32     version.
  33   </para>
  35   <para>
  36     This program is distributed in the hope that it will be
  37     useful, but WITHOUT ANY WARRANTY; without even the implied
  39     See the GNU General Public License for more details.
  40   </para>
  42   <para>
  43     You should have received a copy of the GNU General Public
  44     License along with this program; if not, write to the Free
  45     Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
  46     MA 02111-1307 USA
  47   </para>
  49   <para>
  50     For more details see the file COPYING in the source
  51     distribution of Linux.
  52   </para>
  54   <para>
  55     This documentation is based on an article published in 
  56     Linux Journal Magazine, October 2001, Issue 90.
  57   </para>
  58  </legalnotice>
  59 </bookinfo>
  63  <chapter id="intro">
  64      <title>Introduction</title>
  65  <para>
  66      The Linux USB subsystem has grown from supporting only two different
  67      types of devices in the 2.2.7 kernel (mice and keyboards), to over 20
  68      different types of devices in the 2.4 kernel. Linux currently supports
  69      almost all USB class devices (standard types of devices like keyboards,
  70      mice, modems, printers and speakers) and an ever-growing number of
  71      vendor-specific devices (such as USB to serial converters, digital
  72      cameras, Ethernet devices and MP3 players). For a full list of the
  73      different USB devices currently supported, see Resources.
  74  </para>
  75  <para>
  76      The remaining kinds of USB devices that do not have support on Linux are
  77      almost all vendor-specific devices. Each vendor decides to implement a
  78      custom protocol to talk to their device, so a custom driver usually needs
  79      to be created. Some vendors are open with their USB protocols and help
  80      with the creation of Linux drivers, while others do not publish them, and
  81      developers are forced to reverse-engineer. See Resources for some links
  82      to handy reverse-engineering tools.
  83  </para>
  84  <para>
  85      Because each different protocol causes a new driver to be created, I have
  86      written a generic USB driver skeleton, modeled after the pci-skeleton.c
  87      file in the kernel source tree upon which many PCI network drivers have
  88      been based. This USB skeleton can be found at drivers/usb/usb-skeleton.c
  89      in the kernel source tree. In this article I will walk through the basics
  90      of the skeleton driver, explaining the different pieces and what needs to
  91      be done to customize it to your specific device.
  92  </para>
  93  </chapter>
  95  <chapter id="basics">
  96      <title>Linux USB Basics</title>
  97  <para>
  98      If you are going to write a Linux USB driver, please become familiar with
  99      the USB protocol specification. It can be found, along with many other
 100      useful documents, at the USB home page (see Resources). An excellent
 101      introduction to the Linux USB subsystem can be found at the USB Working
 102      Devices List (see Resources). It explains how the Linux USB subsystem is
 103      structured and introduces the reader to the concept of USB urbs
 104      (USB Request Blocks), which are essential to USB drivers.
 105  </para>
 106  <para>
 107      The first thing a Linux USB driver needs to do is register itself with
 108      the Linux USB subsystem, giving it some information about which devices
 109      the driver supports and which functions to call when a device supported
 110      by the driver is inserted or removed from the system. All of this
 111      information is passed to the USB subsystem in the usb_driver structure.
 112      The skeleton driver declares a usb_driver as:
 113  </para>
 114  <programlisting>
 115static struct usb_driver skel_driver = {
 116        .name        = "skeleton",
 117        .probe       = skel_probe,
 118        .disconnect  = skel_disconnect,
 119        .fops        = &amp;skel_fops,
 120        .minor       = USB_SKEL_MINOR_BASE,
 121        .id_table    = skel_table,
 123  </programlisting>
 124  <para>
 125      The variable name is a string that describes the driver. It is used in
 126      informational messages printed to the system log. The probe and
 127      disconnect function pointers are called when a device that matches the
 128      information provided in the id_table variable is either seen or removed.
 129  </para>
 130  <para>
 131      The fops and minor variables are optional. Most USB drivers hook into
 132      another kernel subsystem, such as the SCSI, network or TTY subsystem.
 133      These types of drivers register themselves with the other kernel
 134      subsystem, and any user-space interactions are provided through that
 135      interface. But for drivers that do not have a matching kernel subsystem,
 136      such as MP3 players or scanners, a method of interacting with user space
 137      is needed. The USB subsystem provides a way to register a minor device
 138      number and a set of file_operations function pointers that enable this
 139      user-space interaction. The skeleton driver needs this kind of interface,
 140      so it provides a minor starting number and a pointer to its
 141      file_operations functions.
 142  </para>
 143  <para>
 144      The USB driver is then registered with a call to usb_register, usually in
 145      the driver's init function, as shown here:
 146  </para>
 147  <programlisting>
 148static int __init usb_skel_init(void)
 150        int result;
 152        /* register this driver with the USB subsystem */
 153        result = usb_register(&amp;skel_driver);
 154        if (result &lt; 0) {
 155                err(&quot;usb_register failed for the &quot;__FILE__ &quot;driver.&quot;
 156                    &quot;Error number %d&quot;, result);
 157                return -1;
 158        }
 160        return 0;
 163  </programlisting>
 164  <para>
 165      When the driver is unloaded from the system, it needs to deregister
 166      itself with the USB subsystem. This is done with the usb_deregister
 167      function:
 168  </para>
 169  <programlisting>
 170static void __exit usb_skel_exit(void)
 172        /* deregister this driver with the USB subsystem */
 173        usb_deregister(&amp;skel_driver);
 176  </programlisting>
 177  <para>
 178     To enable the linux-hotplug system to load the driver automatically when
 179     the device is plugged in, you need to create a MODULE_DEVICE_TABLE. The
 180     following code tells the hotplug scripts that this module supports a
 181     single device with a specific vendor and product ID:
 182  </para>
 183  <programlisting>
 184/* table of devices that work with this driver */
 185static struct usb_device_id skel_table [] = {
 187        { }                      /* Terminating entry */
 189MODULE_DEVICE_TABLE (usb, skel_table);
 190  </programlisting>
 191  <para>
 192     There are other macros that can be used in describing a usb_device_id for
 193     drivers that support a whole class of USB drivers. See usb.h for more
 194     information on this.
 195  </para>
 196  </chapter>
 198  <chapter id="device">
 199      <title>Device operation</title>
 200  <para>
 201     When a device is plugged into the USB bus that matches the device ID
 202     pattern that your driver registered with the USB core, the probe function
 203     is called. The usb_device structure, interface number and the interface ID
 204     are passed to the function:
 205  </para>
 206  <programlisting>
 207static int skel_probe(struct usb_interface *interface,
 208    const struct usb_device_id *id)
 209  </programlisting>
 210  <para>
 211     The driver now needs to verify that this device is actually one that it
 212     can accept. If so, it returns 0.
 213     If not, or if any error occurs during initialization, an errorcode
 214     (such as <literal>-ENOMEM</literal> or <literal>-ENODEV</literal>)
 215     is returned from the probe function.
 216  </para>
 217  <para>
 218     In the skeleton driver, we determine what end points are marked as bulk-in
 219     and bulk-out. We create buffers to hold the data that will be sent and
 220     received from the device, and a USB urb to write data to the device is
 221     initialized.
 222  </para>
 223  <para>
 224     Conversely, when the device is removed from the USB bus, the disconnect
 225     function is called with the device pointer. The driver needs to clean any
 226     private data that has been allocated at this time and to shut down any
 227     pending urbs that are in the USB system.
 228  </para>
 229  <para>
 230     Now that the device is plugged into the system and the driver is bound to
 231     the device, any of the functions in the file_operations structure that
 232     were passed to the USB subsystem will be called from a user program trying
 233     to talk to the device. The first function called will be open, as the
 234     program tries to open the device for I/O. We increment our private usage
 235     count and save a pointer to our internal structure in the file
 236     structure. This is done so that future calls to file operations will
 237     enable the driver to determine which device the user is addressing.  All
 238     of this is done with the following code:
 239  </para>
 240  <programlisting>
 241/* increment our usage count for the module */
 244/* save our object in the file's private structure */
 245file->private_data = dev;
 246  </programlisting>
 247  <para>
 248     After the open function is called, the read and write functions are called
 249     to receive and send data to the device. In the skel_write function, we
 250     receive a pointer to some data that the user wants to send to the device
 251     and the size of the data. The function determines how much data it can
 252     send to the device based on the size of the write urb it has created (this
 253     size depends on the size of the bulk out end point that the device has).
 254     Then it copies the data from user space to kernel space, points the urb to
 255     the data and submits the urb to the USB subsystem.  This can be seen in
 256     the following code:
 257  </para>
 258  <programlisting>
 259/* we can only write as much as 1 urb will hold */
 260bytes_written = (count > skel->bulk_out_size) ? skel->bulk_out_size : count;
 262/* copy the data from user space into our urb */
 263copy_from_user(skel->write_urb->transfer_buffer, buffer, bytes_written);
 265/* set up our urb */
 267                  skel->dev,
 268                  usb_sndbulkpipe(skel->dev, skel->bulk_out_endpointAddr),
 269                  skel->write_urb->transfer_buffer,
 270                  bytes_written,
 271                  skel_write_bulk_callback,
 272                  skel);
 274/* send the data out the bulk port */
 275result = usb_submit_urb(skel->write_urb);
 276if (result) {
 277        err(&quot;Failed submitting write urb, error %d&quot;, result);
 279  </programlisting>
 280  <para>
 281     When the write urb is filled up with the proper information using the
 282     usb_fill_bulk_urb function, we point the urb's completion callback to call our
 283     own skel_write_bulk_callback function. This function is called when the
 284     urb is finished by the USB subsystem. The callback function is called in
 285     interrupt context, so caution must be taken not to do very much processing
 286     at that time. Our implementation of skel_write_bulk_callback merely
 287     reports if the urb was completed successfully or not and then returns.
 288  </para>
 289  <para>
 290     The read function works a bit differently from the write function in that
 291     we do not use an urb to transfer data from the device to the driver.
 292     Instead we call the usb_bulk_msg function, which can be used to send or
 293     receive data from a device without having to create urbs and handle
 294     urb completion callback functions. We call the usb_bulk_msg function,
 295     giving it a buffer into which to place any data received from the device
 296     and a timeout value. If the timeout period expires without receiving any
 297     data from the device, the function will fail and return an error message.
 298     This can be shown with the following code:
 299  </para>
 300  <programlisting>
 301/* do an immediate bulk read to get data from the device */
 302retval = usb_bulk_msg (skel->dev,
 303                       usb_rcvbulkpipe (skel->dev,
 304                       skel->bulk_in_endpointAddr),
 305                       skel->bulk_in_buffer,
 306                       skel->bulk_in_size,
 307                       &amp;count, HZ*10);
 308/* if the read was successful, copy the data to user space */
 309if (!retval) {
 310        if (copy_to_user (buffer, skel->bulk_in_buffer, count))
 311                retval = -EFAULT;
 312        else
 313                retval = count;
 315  </programlisting>
 316  <para>
 317     The usb_bulk_msg function can be very useful for doing single reads or
 318     writes to a device; however, if you need to read or write constantly to a
 319     device, it is recommended to set up your own urbs and submit them to the
 320     USB subsystem.
 321  </para>
 322  <para>
 323     When the user program releases the file handle that it has been using to
 324     talk to the device, the release function in the driver is called. In this
 325     function we decrement our private usage count and wait for possible
 326     pending writes:
 327  </para>
 328  <programlisting>
 329/* decrement our usage count for the device */
 331  </programlisting>
 332  <para>
 333     One of the more difficult problems that USB drivers must be able to handle
 334     smoothly is the fact that the USB device may be removed from the system at
 335     any point in time, even if a program is currently talking to it. It needs
 336     to be able to shut down any current reads and writes and notify the
 337     user-space programs that the device is no longer there. The following
 338     code (function <function>skel_delete</function>)
 339     is an example of how to do this: </para>
 340  <programlisting>
 341static inline void skel_delete (struct usb_skel *dev)
 343    kfree (dev->bulk_in_buffer);
 344    if (dev->bulk_out_buffer != NULL)
 345        usb_free_coherent (dev->udev, dev->bulk_out_size,
 346            dev->bulk_out_buffer,
 347            dev->write_urb->transfer_dma);
 348    usb_free_urb (dev->write_urb);
 349    kfree (dev);
 351  </programlisting>
 352  <para>
 353     If a program currently has an open handle to the device, we reset the flag
 354     <literal>device_present</literal>. For
 355     every read, write, release and other functions that expect a device to be
 356     present, the driver first checks this flag to see if the device is
 357     still present. If not, it releases that the device has disappeared, and a
 358     -ENODEV error is returned to the user-space program. When the release
 359     function is eventually called, it determines if there is no device
 360     and if not, it does the cleanup that the skel_disconnect
 361     function normally does if there are no open files on the device (see
 362     Listing 5).
 363  </para>
 364  </chapter>
 366  <chapter id="iso">
 367      <title>Isochronous Data</title>
 368  <para>
 369     This usb-skeleton driver does not have any examples of interrupt or
 370     isochronous data being sent to or from the device. Interrupt data is sent
 371     almost exactly as bulk data is, with a few minor exceptions.  Isochronous
 372     data works differently with continuous streams of data being sent to or
 373     from the device. The audio and video camera drivers are very good examples
 374     of drivers that handle isochronous data and will be useful if you also
 375     need to do this.
 376  </para>
 377  </chapter>
 379  <chapter id="Conclusion">
 380      <title>Conclusion</title>
 381  <para>
 382     Writing Linux USB device drivers is not a difficult task as the
 383     usb-skeleton driver shows. This driver, combined with the other current
 384     USB drivers, should provide enough examples to help a beginning author
 385     create a working driver in a minimal amount of time. The linux-usb-devel
 386     mailing list archives also contain a lot of helpful information.
 387  </para>
 388  </chapter>
 390  <chapter id="resources">
 391      <title>Resources</title>
 392  <para>
 393     The Linux USB Project: <ulink url=""></ulink>
 394  </para>
 395  <para>
 396     Linux Hotplug Project: <ulink url=""></ulink>
 397  </para>
 398  <para>
 399     Linux USB Working Devices List: <ulink url=""></ulink>
 400  </para>
 401  <para>
 402     linux-usb-devel Mailing List Archives: <ulink url=""></ulink>
 403  </para>
 404  <para>
 405     Programming Guide for Linux USB Device Drivers: <ulink url=""></ulink>
 406  </para>
 407  <para>
 408     USB Home Page: <ulink url=""></ulink>
 409  </para>
 410  </chapter>
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