path: root/Documentation/DocBook/writing_usb_driver.tmpl
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authorLinus Torvalds <>2005-04-16 15:20:36 -0700
committerLinus Torvalds <>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/DocBook/writing_usb_driver.tmpl
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+ "" []>
+<book id="USBDeviceDriver">
+ <bookinfo>
+ <title>Writing USB Device Drivers</title>
+ <authorgroup>
+ <author>
+ <firstname>Greg</firstname>
+ <surname>Kroah-Hartman</surname>
+ <affiliation>
+ <address>
+ <email></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
+ 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>
+ <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, 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,
+ .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;
+ </programlisting>
+ <para>
+ When the driver is unloaded from the system, it needs to unregister
+ itself with the USB subsystem. This is done with the usb_unregister
+ function:
+ </para>
+ <programlisting>
+static void __exit usb_skel_exit(void)
+ /* deregister this driver with the USB subsystem */
+ usb_deregister(&amp;skel_driver);
+ </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 [] = {
+ { } /* 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. The driver also unregisters
+ itself from the devfs subsystem with the call:
+ </para>
+ <programlisting>
+/* remove our devfs node */
+ </programlisting>
+ <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 off 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 */
+/* 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 shown in
+ he 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 */
+ 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 */
+ </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)
+ if (dev->bulk_in_buffer != NULL)
+ kfree (dev->bulk_in_buffer);
+ if (dev->bulk_out_buffer != NULL)
+ usb_buffer_free (dev->udev, dev->bulk_out_size,
+ dev->bulk_out_buffer,
+ dev->write_urb->transfer_dma);
+ if (dev->write_urb != NULL)
+ 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=""></ulink>
+ </para>
+ <para>
+ Linux Hotplug Project: <ulink url=""></ulink>
+ </para>
+ <para>
+ Linux USB Working Devices List: <ulink url=""></ulink>
+ </para>
+ <para>
+ linux-usb-devel Mailing List Archives: <ulink url=""></ulink>
+ </para>
+ <para>
+ Programming Guide for Linux USB Device Drivers: <ulink url=""></ulink>
+ </para>
+ <para>
+ USB Home Page: <ulink url=""></ulink>
+ </para>
+ </chapter>