Documentation/DocBook/writing_usb_driver.tmpl

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   2 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
   3         "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
   4
   5 <book id="USBDeviceDriver">
   6  <bookinfo>
   7   <title>Writing USB Device Drivers</title>
   8
   9   <authorgroup>
  10    <author>
  11     <firstname>Greg</firstname>
  12     <surname>Kroah-Hartman</surname>
  13     <affiliation>
  14      <address>
  15       <email>greg@kroah.com</email>
  16      </address>
  17     </affiliation>
  18    </author>
  19   </authorgroup>
  20
  21   <copyright>
  22    <year>2001-2002</year>
  23    <holder>Greg Kroah-Hartman</holder>
  24   </copyright>
  25
  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>
  34
  35    <para>
  36      This program is distributed in the hope that it will be
  37      useful, but WITHOUT ANY WARRANTY; without even the implied
  38      warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  39      See the GNU General Public License for more details.
  40    </para>
  41
  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>
  48
  49    <para>
  50      For more details see the file COPYING in the source
  51      distribution of Linux.
  52    </para>
  53
  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>
  60
  61 <toc></toc>
  62
  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, modelled 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>
  94
  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>
 115 static 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,
 122 };
 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>
 148 static int __init usb_skel_init(void)
 149 {
 150         int result;
 151
 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         }
 159
 160         return 0;
 161 }
 162 module_init(usb_skel_init);
 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>
 170 static void __exit usb_skel_exit(void)
 171 {
 172         /* deregister this driver with the USB subsystem */
 173         usb_deregister(&amp;skel_driver);
 174 }
 175 module_exit(usb_skel_exit);
 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 */
 185 static struct usb_device_id skel_table [] = {
 186         { USB_DEVICE(USB_SKEL_VENDOR_ID, USB_SKEL_PRODUCT_ID) },
 187         { }                      /* Terminating entry */
 188 };
 189 MODULE_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>
 197
 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>
 207 static 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 */
 242 ++skel->open_count;
 243
 244 /* save our object in the file's private structure */
 245 file->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 */
 260 bytes_written = (count > skel->bulk_out_size) ? skel->bulk_out_size : count;
 261
 262 /* copy the data from user space into our urb */
 263 copy_from_user(skel->write_urb->transfer_buffer, buffer, bytes_written);
 264
 265 /* set up our urb */
 266 usb_fill_bulk_urb(skel->write_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);
 273
 274 /* send the data out the bulk port */
 275 result = usb_submit_urb(skel->write_urb);
 276 if (result) {
 277         err(&quot;Failed submitting write urb, error %d&quot;, result);
 278 }
 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 */
 302 retval = 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 */
 309 if (!retval) {
 310         if (copy_to_user (buffer, skel->bulk_in_buffer, count))
 311                 retval = -EFAULT;
 312         else
 313                 retval = count;
 314 }
 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 */
 330 --skel->open_count;
 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>
 341 static inline void skel_delete (struct usb_skel *dev)
 342 {
 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);
 350 }
 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>
 365
 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>
 378
 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>
 389
 390   <chapter id="resources">
 391       <title>Resources</title>
 392   <para>
 393      The Linux USB Project: <ulink url="http://www.linux-usb.org">http://www.linux-usb.org/</ulink>
 394   </para>
 395   <para>
 396      Linux Hotplug Project: <ulink url="http://linux-hotplug.sourceforge.net">http://linux-hotplug.sourceforge.net/</ulink>
 397   </para>
 398   <para>
 399      Linux USB Working Devices List: <ulink url="http://www.qbik.ch/usb/devices">http://www.qbik.ch/usb/devices/</ulink>
 400   </para>
 401   <para>
 402      linux-usb-devel Mailing List Archives: <ulink url="http://marc.theaimsgroup.com/?l=linux-usb-devel">http://marc.theaimsgroup.com/?l=linux-usb-devel</ulink>
 403   </para>
 404   <para>
 405      Programming Guide for Linux USB Device Drivers: <ulink url="http://usb.cs.tum.edu/usbdoc">http://usb.cs.tum.edu/usbdoc</ulink>
 406   </para>
 407   <para>
 408      USB Home Page: <ulink url="http://www.usb.org">http://www.usb.org</ulink>
 409   </para>
 410   </chapter>
 411
 412 </book>