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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
+
+<book id="USBDeviceDriver">
+ <bookinfo>
+  <title>Writing USB Device Drivers</title>
+  
+  <authorgroup>
+   <author>
+    <firstname>Greg</firstname>
+    <surname>Kroah-Hartman</surname>
+    <affiliation>
+     <address>
+      <email>greg@kroah.com</email>
+     </address>
+    </affiliation>
+   </author>
+  </authorgroup>
+
+  <copyright>
+   <year>2001-2002</year>
+   <holder>Greg Kroah-Hartman</holder>
+  </copyright>
+
+  <legalnotice>
+   <para>
+     This documentation is free software; you can redistribute
+     it and/or modify it under the terms of the GNU General Public
+     License as published by the Free Software Foundation; either
+     version 2 of the License, or (at your option) any later
+     version.
+   </para>
+      
+   <para>
+     This program is distributed in the hope that it will be
+     useful, but WITHOUT ANY WARRANTY; without even the implied
+     warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+     See the GNU General Public License for more details.
+   </para>
+      
+   <para>
+     You should have received a copy of the GNU General Public
+     License along with this program; if not, write to the Free
+     Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
+     MA 02111-1307 USA
+   </para>
+      
+   <para>
+     For more details see the file COPYING in the source
+     distribution of Linux.
+   </para>
+
+   <para>
+     This documentation is based on an article published in 
+     Linux Journal Magazine, October 2001, Issue 90.
+   </para>
+  </legalnotice>
+ </bookinfo>
+
+<toc></toc>
+
+  <chapter id="intro">
+      <title>Introduction</title>
+  <para>
+      The Linux USB subsystem has grown from supporting only two different
+      types of devices in the 2.2.7 kernel (mice and keyboards), to over 20
+      different types of devices in the 2.4 kernel. Linux currently supports
+      almost all USB class devices (standard types of devices like keyboards,
+      mice, modems, printers and speakers) and an ever-growing number of
+      vendor-specific devices (such as USB to serial converters, digital
+      cameras, Ethernet devices and MP3 players). For a full list of the
+      different USB devices currently supported, see Resources.
+  </para>
+  <para>
+      The remaining kinds of USB devices that do not have support on Linux are
+      almost all vendor-specific devices. Each vendor decides to implement a
+      custom protocol to talk to their device, so a custom driver usually needs
+      to be created. Some vendors are open with their USB protocols and help
+      with the creation of Linux drivers, while others do not publish them, and
+      developers are forced to reverse-engineer. See Resources for some links
+      to handy reverse-engineering tools.
+  </para>
+  <para>
+      Because each different protocol causes a new driver to be created, I have
+      written a generic USB driver skeleton, modeled after the pci-skeleton.c
+      file in the kernel source tree upon which many PCI network drivers have
+      been based. This USB skeleton can be found at drivers/usb/usb-skeleton.c
+      in the kernel source tree. In this article I will walk through the basics
+      of the skeleton driver, explaining the different pieces and what needs to
+      be done to customize it to your specific device.
+  </para>
+  </chapter>
+
+  <chapter id="basics">
+      <title>Linux USB Basics</title>
+  <para>
+      If you are going to write a Linux USB driver, please become familiar with
+      the USB protocol specification. It can be found, along with many other
+      useful documents, at the USB home page (see Resources). An excellent
+      introduction to the Linux USB subsystem can be found at the USB Working
+      Devices List (see Resources). It explains how the Linux USB subsystem is
+      structured and introduces the reader to the concept of USB urbs
+      (USB Request Blocks), which are essential to USB drivers.
+  </para>
+  <para>
+      The first thing a Linux USB driver needs to do is register itself with
+      the Linux USB subsystem, giving it some information about which devices
+      the driver supports and which functions to call when a device supported
+      by the driver is inserted or removed from the system. All of this
+      information is passed to the USB subsystem in the usb_driver structure.
+      The skeleton driver declares a usb_driver as:
+  </para>
+  <programlisting>
+static struct usb_driver skel_driver = {
+        .name        = "skeleton",
+        .probe       = skel_probe,
+        .disconnect  = skel_disconnect,
+        .fops        = &amp;skel_fops,
+        .minor       = USB_SKEL_MINOR_BASE,
+        .id_table    = skel_table,
+};
+  </programlisting>
+  <para>
+      The variable name is a string that describes the driver. It is used in
+      informational messages printed to the system log. The probe and
+      disconnect function pointers are called when a device that matches the
+      information provided in the id_table variable is either seen or removed.
+  </para>
+  <para>
+      The fops and minor variables are optional. Most USB drivers hook into
+      another kernel subsystem, such as the SCSI, network or TTY subsystem.
+      These types of drivers register themselves with the other kernel
+      subsystem, and any user-space interactions are provided through that
+      interface. But for drivers that do not have a matching kernel subsystem,
+      such as MP3 players or scanners, a method of interacting with user space
+      is needed. The USB subsystem provides a way to register a minor device
+      number and a set of file_operations function pointers that enable this
+      user-space interaction. The skeleton driver needs this kind of interface,
+      so it provides a minor starting number and a pointer to its
+      file_operations functions.
+  </para>
+  <para>
+      The USB driver is then registered with a call to usb_register, usually in
+      the driver's init function, as shown here:
+  </para>
+  <programlisting>
+static int __init usb_skel_init(void)
+{
+        int result;
+
+        /* register this driver with the USB subsystem */
+        result = usb_register(&amp;skel_driver);
+        if (result &lt; 0) {
+                err(&quot;usb_register failed for the &quot;__FILE__ &quot;driver.&quot;
+                    &quot;Error number %d&quot;, result);
+                return -1;
+        }
+
+        return 0;
+}
+module_init(usb_skel_init);
+  </programlisting>
+  <para>
+      When the driver is unloaded from the system, it needs to deregister
+      itself with the USB subsystem. This is done with the usb_deregister
+      function:
+  </para>
+  <programlisting>
+static void __exit usb_skel_exit(void)
+{
+        /* deregister this driver with the USB subsystem */
+        usb_deregister(&amp;skel_driver);
+}
+module_exit(usb_skel_exit);
+  </programlisting>
+  <para>
+     To enable the linux-hotplug system to load the driver automatically when
+     the device is plugged in, you need to create a MODULE_DEVICE_TABLE. The
+     following code tells the hotplug scripts that this module supports a
+     single device with a specific vendor and product ID:
+  </para>
+  <programlisting>
+/* table of devices that work with this driver */
+static struct usb_device_id skel_table [] = {
+        { USB_DEVICE(USB_SKEL_VENDOR_ID, USB_SKEL_PRODUCT_ID) },
+        { }                      /* Terminating entry */
+};
+MODULE_DEVICE_TABLE (usb, skel_table);
+  </programlisting>
+  <para>
+     There are other macros that can be used in describing a usb_device_id for
+     drivers that support a whole class of USB drivers. See usb.h for more
+     information on this.
+  </para>
+  </chapter>
+
+  <chapter id="device">
+      <title>Device operation</title>
+  <para>
+     When a device is plugged into the USB bus that matches the device ID
+     pattern that your driver registered with the USB core, the probe function
+     is called. The usb_device structure, interface number and the interface ID
+     are passed to the function:
+  </para>
+  <programlisting>
+static int skel_probe(struct usb_interface *interface,
+    const struct usb_device_id *id)
+  </programlisting>
+  <para>
+     The driver now needs to verify that this device is actually one that it
+     can accept. If so, it returns 0.
+     If not, or if any error occurs during initialization, an errorcode
+     (such as <literal>-ENOMEM</literal> or <literal>-ENODEV</literal>)
+     is returned from the probe function.
+  </para>
+  <para>
+     In the skeleton driver, we determine what end points are marked as bulk-in
+     and bulk-out. We create buffers to hold the data that will be sent and
+     received from the device, and a USB urb to write data to the device is
+     initialized.
+  </para>
+  <para>
+     Conversely, when the device is removed from the USB bus, the disconnect
+     function is called with the device pointer. The driver needs to clean any
+     private data that has been allocated at this time and to shut down any
+     pending urbs that are in the USB system.
+  </para>
+  <para>
+     Now that the device is plugged into the system and the driver is bound to
+     the device, any of the functions in the file_operations structure that
+     were passed to the USB subsystem will be called from a user program trying
+     to talk to the device. The first function called will be open, as the
+     program tries to open the device for I/O. We increment our private usage
+     count and save a pointer to our internal structure in the file
+     structure. This is done so that future calls to file operations will
+     enable the driver to determine which device the user is addressing.  All
+     of this is done with the following code:
+  </para>
+  <programlisting>
+/* increment our usage count for the module */
+++skel->open_count;
+
+/* save our object in the file's private structure */
+file->private_data = dev;
+  </programlisting>
+  <para>
+     After the open function is called, the read and write functions are called
+     to receive and send data to the device. In the skel_write function, we
+     receive a pointer to some data that the user wants to send to the device
+     and the size of the data. The function determines how much data it can
+     send to the device based on the size of the write urb it has created (this
+     size depends on the size of the bulk out end point that the device has).
+     Then it copies the data from user space to kernel space, points the urb to
+     the data and submits the urb to the USB subsystem.  This can be seen in
+     the following code:
+  </para>
+  <programlisting>
+/* we can only write as much as 1 urb will hold */
+bytes_written = (count > skel->bulk_out_size) ? skel->bulk_out_size : count;
+
+/* copy the data from user space into our urb */
+copy_from_user(skel->write_urb->transfer_buffer, buffer, bytes_written);
+
+/* set up our urb */
+usb_fill_bulk_urb(skel->write_urb,
+                  skel->dev,
+                  usb_sndbulkpipe(skel->dev, skel->bulk_out_endpointAddr),
+                  skel->write_urb->transfer_buffer,
+                  bytes_written,
+                  skel_write_bulk_callback,
+                  skel);
+
+/* send the data out the bulk port */
+result = usb_submit_urb(skel->write_urb);
+if (result) {
+        err(&quot;Failed submitting write urb, error %d&quot;, result);
+}
+  </programlisting>
+  <para>
+     When the write urb is filled up with the proper information using the
+     usb_fill_bulk_urb function, we point the urb's completion callback to call our
+     own skel_write_bulk_callback function. This function is called when the
+     urb is finished by the USB subsystem. The callback function is called in
+     interrupt context, so caution must be taken not to do very much processing
+     at that time. Our implementation of skel_write_bulk_callback merely
+     reports if the urb was completed successfully or not and then returns.
+  </para>
+  <para>
+     The read function works a bit differently from the write function in that
+     we do not use an urb to transfer data from the device to the driver.
+     Instead we call the usb_bulk_msg function, which can be used to send or
+     receive data from a device without having to create urbs and handle
+     urb completion callback functions. We call the usb_bulk_msg function,
+     giving it a buffer into which to place any data received from the device
+     and a timeout value. If the timeout period expires without receiving any
+     data from the device, the function will fail and return an error message.
+     This can be shown with the following code:
+  </para>
+  <programlisting>
+/* do an immediate bulk read to get data from the device */
+retval = usb_bulk_msg (skel->dev,
+                       usb_rcvbulkpipe (skel->dev,
+                       skel->bulk_in_endpointAddr),
+                       skel->bulk_in_buffer,
+                       skel->bulk_in_size,
+                       &amp;count, HZ*10);
+/* if the read was successful, copy the data to user space */
+if (!retval) {
+        if (copy_to_user (buffer, skel->bulk_in_buffer, count))
+                retval = -EFAULT;
+        else
+                retval = count;
+}
+  </programlisting>
+  <para>
+     The usb_bulk_msg function can be very useful for doing single reads or
+     writes to a device; however, if you need to read or write constantly to a
+     device, it is recommended to set up your own urbs and submit them to the
+     USB subsystem.
+  </para>
+  <para>
+     When the user program releases the file handle that it has been using to
+     talk to the device, the release function in the driver is called. In this
+     function we decrement our private usage count and wait for possible
+     pending writes:
+  </para>
+  <programlisting>
+/* decrement our usage count for the device */
+--skel->open_count;
+  </programlisting>
+  <para>
+     One of the more difficult problems that USB drivers must be able to handle
+     smoothly is the fact that the USB device may be removed from the system at
+     any point in time, even if a program is currently talking to it. It needs
+     to be able to shut down any current reads and writes and notify the
+     user-space programs that the device is no longer there. The following
+     code (function <function>skel_delete</function>)
+     is an example of how to do this: </para>
+  <programlisting>
+static inline void skel_delete (struct usb_skel *dev)
+{
+    kfree (dev->bulk_in_buffer);
+    if (dev->bulk_out_buffer != NULL)
+        usb_free_coherent (dev->udev, dev->bulk_out_size,
+            dev->bulk_out_buffer,
+            dev->write_urb->transfer_dma);
+    usb_free_urb (dev->write_urb);
+    kfree (dev);
+}
+  </programlisting>
+  <para>
+     If a program currently has an open handle to the device, we reset the flag
+     <literal>device_present</literal>. For
+     every read, write, release and other functions that expect a device to be
+     present, the driver first checks this flag to see if the device is
+     still present. If not, it releases that the device has disappeared, and a
+     -ENODEV error is returned to the user-space program. When the release
+     function is eventually called, it determines if there is no device
+     and if not, it does the cleanup that the skel_disconnect
+     function normally does if there are no open files on the device (see
+     Listing 5).
+  </para>
+  </chapter>
+
+  <chapter id="iso">
+      <title>Isochronous Data</title>
+  <para>
+     This usb-skeleton driver does not have any examples of interrupt or
+     isochronous data being sent to or from the device. Interrupt data is sent
+     almost exactly as bulk data is, with a few minor exceptions.  Isochronous
+     data works differently with continuous streams of data being sent to or
+     from the device. The audio and video camera drivers are very good examples
+     of drivers that handle isochronous data and will be useful if you also
+     need to do this.
+  </para>
+  </chapter>
+  
+  <chapter id="Conclusion">
+      <title>Conclusion</title>
+  <para>
+     Writing Linux USB device drivers is not a difficult task as the
+     usb-skeleton driver shows. This driver, combined with the other current
+     USB drivers, should provide enough examples to help a beginning author
+     create a working driver in a minimal amount of time. The linux-usb-devel
+     mailing list archives also contain a lot of helpful information.
+  </para>
+  </chapter>
+
+  <chapter id="resources">
+      <title>Resources</title>
+  <para>
+     The Linux USB Project: <ulink url="http://www.linux-usb.org">http://www.linux-usb.org/</ulink>
+  </para>
+  <para>
+     Linux Hotplug Project: <ulink url="http://linux-hotplug.sourceforge.net">http://linux-hotplug.sourceforge.net/</ulink>
+  </para>
+  <para>
+     Linux USB Working Devices List: <ulink url="http://www.qbik.ch/usb/devices">http://www.qbik.ch/usb/devices/</ulink>
+  </para>
+  <para>
+     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>
+  </para>
+  <para>
+     Programming Guide for Linux USB Device Drivers: <ulink url="http://usb.cs.tum.edu/usbdoc">http://usb.cs.tum.edu/usbdoc</ulink>
+  </para>
+  <para>
+     USB Home Page: <ulink url="http://www.usb.org">http://www.usb.org</ulink>
+  </para>
+  </chapter>
+
+</book>