a3a3395e48
This is the Documentation/ part of the big kfree cleanup patch. Remove pointless checks for NULL prior to calling kfree() in example code in Documentation/. Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
418 lines
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418 lines
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" []>
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<book id="USBDeviceDriver">
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<bookinfo>
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<title>Writing USB Device Drivers</title>
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<authorgroup>
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<author>
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<firstname>Greg</firstname>
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<surname>Kroah-Hartman</surname>
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<affiliation>
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<address>
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<email>greg@kroah.com</email>
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</address>
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</affiliation>
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</author>
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</authorgroup>
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<copyright>
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<year>2001-2002</year>
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<holder>Greg Kroah-Hartman</holder>
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</copyright>
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<legalnotice>
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<para>
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This documentation is free software; you can redistribute
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it and/or modify it under the terms of the GNU General Public
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License as published by the Free Software Foundation; either
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version 2 of the License, or (at your option) any later
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version.
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</para>
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<para>
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This program is distributed in the hope that it will be
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useful, but WITHOUT ANY WARRANTY; without even the implied
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warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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See the GNU General Public License for more details.
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</para>
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<para>
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You should have received a copy of the GNU General Public
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License along with this program; if not, write to the Free
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Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
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MA 02111-1307 USA
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</para>
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<para>
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For more details see the file COPYING in the source
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distribution of Linux.
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</para>
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<para>
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This documentation is based on an article published in
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Linux Journal Magazine, October 2001, Issue 90.
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</para>
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</legalnotice>
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</bookinfo>
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<toc></toc>
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<chapter id="intro">
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<title>Introduction</title>
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<para>
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The Linux USB subsystem has grown from supporting only two different
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types of devices in the 2.2.7 kernel (mice and keyboards), to over 20
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different types of devices in the 2.4 kernel. Linux currently supports
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almost all USB class devices (standard types of devices like keyboards,
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mice, modems, printers and speakers) and an ever-growing number of
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vendor-specific devices (such as USB to serial converters, digital
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cameras, Ethernet devices and MP3 players). For a full list of the
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different USB devices currently supported, see Resources.
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</para>
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<para>
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The remaining kinds of USB devices that do not have support on Linux are
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almost all vendor-specific devices. Each vendor decides to implement a
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custom protocol to talk to their device, so a custom driver usually needs
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to be created. Some vendors are open with their USB protocols and help
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with the creation of Linux drivers, while others do not publish them, and
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developers are forced to reverse-engineer. See Resources for some links
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to handy reverse-engineering tools.
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</para>
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<para>
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Because each different protocol causes a new driver to be created, I have
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written a generic USB driver skeleton, modeled after the pci-skeleton.c
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file in the kernel source tree upon which many PCI network drivers have
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been based. This USB skeleton can be found at drivers/usb/usb-skeleton.c
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in the kernel source tree. In this article I will walk through the basics
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of the skeleton driver, explaining the different pieces and what needs to
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be done to customize it to your specific device.
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</para>
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</chapter>
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<chapter id="basics">
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<title>Linux USB Basics</title>
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<para>
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If you are going to write a Linux USB driver, please become familiar with
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the USB protocol specification. It can be found, along with many other
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useful documents, at the USB home page (see Resources). An excellent
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introduction to the Linux USB subsystem can be found at the USB Working
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Devices List (see Resources). It explains how the Linux USB subsystem is
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structured and introduces the reader to the concept of USB urbs, which
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are essential to USB drivers.
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</para>
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<para>
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The first thing a Linux USB driver needs to do is register itself with
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the Linux USB subsystem, giving it some information about which devices
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the driver supports and which functions to call when a device supported
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by the driver is inserted or removed from the system. All of this
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information is passed to the USB subsystem in the usb_driver structure.
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The skeleton driver declares a usb_driver as:
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</para>
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<programlisting>
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static struct usb_driver skel_driver = {
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.name = "skeleton",
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.probe = skel_probe,
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.disconnect = skel_disconnect,
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.fops = &skel_fops,
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.minor = USB_SKEL_MINOR_BASE,
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.id_table = skel_table,
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};
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</programlisting>
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<para>
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The variable name is a string that describes the driver. It is used in
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informational messages printed to the system log. The probe and
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disconnect function pointers are called when a device that matches the
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information provided in the id_table variable is either seen or removed.
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</para>
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<para>
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The fops and minor variables are optional. Most USB drivers hook into
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another kernel subsystem, such as the SCSI, network or TTY subsystem.
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These types of drivers register themselves with the other kernel
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subsystem, and any user-space interactions are provided through that
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interface. But for drivers that do not have a matching kernel subsystem,
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such as MP3 players or scanners, a method of interacting with user space
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is needed. The USB subsystem provides a way to register a minor device
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number and a set of file_operations function pointers that enable this
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user-space interaction. The skeleton driver needs this kind of interface,
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so it provides a minor starting number and a pointer to its
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file_operations functions.
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</para>
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<para>
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The USB driver is then registered with a call to usb_register, usually in
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the driver's init function, as shown here:
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</para>
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<programlisting>
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static int __init usb_skel_init(void)
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{
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int result;
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/* register this driver with the USB subsystem */
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result = usb_register(&skel_driver);
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if (result < 0) {
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err("usb_register failed for the "__FILE__ "driver."
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"Error number %d", result);
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return -1;
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}
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return 0;
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}
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module_init(usb_skel_init);
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</programlisting>
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<para>
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When the driver is unloaded from the system, it needs to unregister
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itself with the USB subsystem. This is done with the usb_unregister
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function:
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</para>
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<programlisting>
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static void __exit usb_skel_exit(void)
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{
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/* deregister this driver with the USB subsystem */
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usb_deregister(&skel_driver);
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}
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module_exit(usb_skel_exit);
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</programlisting>
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<para>
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To enable the linux-hotplug system to load the driver automatically when
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the device is plugged in, you need to create a MODULE_DEVICE_TABLE. The
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following code tells the hotplug scripts that this module supports a
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single device with a specific vendor and product ID:
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</para>
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<programlisting>
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/* table of devices that work with this driver */
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static struct usb_device_id skel_table [] = {
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{ USB_DEVICE(USB_SKEL_VENDOR_ID, USB_SKEL_PRODUCT_ID) },
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{ } /* Terminating entry */
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};
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MODULE_DEVICE_TABLE (usb, skel_table);
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</programlisting>
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<para>
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There are other macros that can be used in describing a usb_device_id for
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drivers that support a whole class of USB drivers. See usb.h for more
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information on this.
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</para>
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</chapter>
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<chapter id="device">
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<title>Device operation</title>
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<para>
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When a device is plugged into the USB bus that matches the device ID
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pattern that your driver registered with the USB core, the probe function
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is called. The usb_device structure, interface number and the interface ID
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are passed to the function:
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</para>
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<programlisting>
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static int skel_probe(struct usb_interface *interface,
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const struct usb_device_id *id)
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</programlisting>
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<para>
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The driver now needs to verify that this device is actually one that it
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can accept. If so, it returns 0.
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If not, or if any error occurs during initialization, an errorcode
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(such as <literal>-ENOMEM</literal> or <literal>-ENODEV</literal>)
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is returned from the probe function.
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</para>
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<para>
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In the skeleton driver, we determine what end points are marked as bulk-in
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and bulk-out. We create buffers to hold the data that will be sent and
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received from the device, and a USB urb to write data to the device is
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initialized.
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</para>
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<para>
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Conversely, when the device is removed from the USB bus, the disconnect
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function is called with the device pointer. The driver needs to clean any
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private data that has been allocated at this time and to shut down any
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pending urbs that are in the USB system. The driver also unregisters
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itself from the devfs subsystem with the call:
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</para>
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<programlisting>
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/* remove our devfs node */
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devfs_unregister(skel->devfs);
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</programlisting>
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<para>
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Now that the device is plugged into the system and the driver is bound to
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the device, any of the functions in the file_operations structure that
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were passed to the USB subsystem will be called from a user program trying
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to talk to the device. The first function called will be open, as the
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program tries to open the device for I/O. We increment our private usage
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count and save off a pointer to our internal structure in the file
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structure. This is done so that future calls to file operations will
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enable the driver to determine which device the user is addressing. All
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of this is done with the following code:
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</para>
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<programlisting>
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/* increment our usage count for the module */
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++skel->open_count;
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/* save our object in the file's private structure */
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file->private_data = dev;
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</programlisting>
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<para>
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After the open function is called, the read and write functions are called
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to receive and send data to the device. In the skel_write function, we
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receive a pointer to some data that the user wants to send to the device
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and the size of the data. The function determines how much data it can
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send to the device based on the size of the write urb it has created (this
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size depends on the size of the bulk out end point that the device has).
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Then it copies the data from user space to kernel space, points the urb to
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the data and submits the urb to the USB subsystem. This can be shown in
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he following code:
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</para>
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<programlisting>
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/* we can only write as much as 1 urb will hold */
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bytes_written = (count > skel->bulk_out_size) ? skel->bulk_out_size : count;
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/* copy the data from user space into our urb */
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copy_from_user(skel->write_urb->transfer_buffer, buffer, bytes_written);
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/* set up our urb */
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usb_fill_bulk_urb(skel->write_urb,
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skel->dev,
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usb_sndbulkpipe(skel->dev, skel->bulk_out_endpointAddr),
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skel->write_urb->transfer_buffer,
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bytes_written,
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skel_write_bulk_callback,
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skel);
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/* send the data out the bulk port */
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result = usb_submit_urb(skel->write_urb);
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if (result) {
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err("Failed submitting write urb, error %d", result);
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}
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</programlisting>
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<para>
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When the write urb is filled up with the proper information using the
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usb_fill_bulk_urb function, we point the urb's completion callback to call our
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own skel_write_bulk_callback function. This function is called when the
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urb is finished by the USB subsystem. The callback function is called in
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interrupt context, so caution must be taken not to do very much processing
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at that time. Our implementation of skel_write_bulk_callback merely
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reports if the urb was completed successfully or not and then returns.
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</para>
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<para>
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The read function works a bit differently from the write function in that
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we do not use an urb to transfer data from the device to the driver.
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Instead we call the usb_bulk_msg function, which can be used to send or
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receive data from a device without having to create urbs and handle
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urb completion callback functions. We call the usb_bulk_msg function,
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giving it a buffer into which to place any data received from the device
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and a timeout value. If the timeout period expires without receiving any
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data from the device, the function will fail and return an error message.
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This can be shown with the following code:
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</para>
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<programlisting>
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/* do an immediate bulk read to get data from the device */
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retval = usb_bulk_msg (skel->dev,
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usb_rcvbulkpipe (skel->dev,
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skel->bulk_in_endpointAddr),
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skel->bulk_in_buffer,
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skel->bulk_in_size,
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&count, HZ*10);
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/* if the read was successful, copy the data to user space */
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if (!retval) {
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if (copy_to_user (buffer, skel->bulk_in_buffer, count))
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retval = -EFAULT;
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else
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retval = count;
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}
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</programlisting>
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<para>
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The usb_bulk_msg function can be very useful for doing single reads or
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writes to a device; however, if you need to read or write constantly to a
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device, it is recommended to set up your own urbs and submit them to the
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USB subsystem.
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</para>
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<para>
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When the user program releases the file handle that it has been using to
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talk to the device, the release function in the driver is called. In this
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function we decrement our private usage count and wait for possible
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pending writes:
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</para>
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<programlisting>
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/* decrement our usage count for the device */
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--skel->open_count;
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</programlisting>
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<para>
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One of the more difficult problems that USB drivers must be able to handle
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smoothly is the fact that the USB device may be removed from the system at
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any point in time, even if a program is currently talking to it. It needs
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to be able to shut down any current reads and writes and notify the
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user-space programs that the device is no longer there. The following
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code (function <function>skel_delete</function>)
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is an example of how to do this: </para>
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<programlisting>
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static inline void skel_delete (struct usb_skel *dev)
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{
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kfree (dev->bulk_in_buffer);
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if (dev->bulk_out_buffer != NULL)
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usb_buffer_free (dev->udev, dev->bulk_out_size,
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dev->bulk_out_buffer,
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dev->write_urb->transfer_dma);
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if (dev->write_urb != NULL)
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usb_free_urb (dev->write_urb);
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kfree (dev);
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}
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</programlisting>
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<para>
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If a program currently has an open handle to the device, we reset the flag
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<literal>device_present</literal>. For
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every read, write, release and other functions that expect a device to be
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present, the driver first checks this flag to see if the device is
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still present. If not, it releases that the device has disappeared, and a
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-ENODEV error is returned to the user-space program. When the release
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function is eventually called, it determines if there is no device
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and if not, it does the cleanup that the skel_disconnect
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function normally does if there are no open files on the device (see
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Listing 5).
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</para>
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</chapter>
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<chapter id="iso">
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<title>Isochronous Data</title>
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<para>
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This usb-skeleton driver does not have any examples of interrupt or
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isochronous data being sent to or from the device. Interrupt data is sent
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almost exactly as bulk data is, with a few minor exceptions. Isochronous
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data works differently with continuous streams of data being sent to or
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from the device. The audio and video camera drivers are very good examples
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of drivers that handle isochronous data and will be useful if you also
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need to do this.
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</para>
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</chapter>
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<chapter id="Conclusion">
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<title>Conclusion</title>
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<para>
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Writing Linux USB device drivers is not a difficult task as the
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usb-skeleton driver shows. This driver, combined with the other current
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USB drivers, should provide enough examples to help a beginning author
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create a working driver in a minimal amount of time. The linux-usb-devel
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mailing list archives also contain a lot of helpful information.
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</para>
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</chapter>
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<chapter id="resources">
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<title>Resources</title>
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<para>
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The Linux USB Project: <ulink url="http://www.linux-usb.org">http://www.linux-usb.org/</ulink>
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</para>
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<para>
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Linux Hotplug Project: <ulink url="http://linux-hotplug.sourceforge.net">http://linux-hotplug.sourceforge.net/</ulink>
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</para>
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<para>
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Linux USB Working Devices List: <ulink url="http://www.qbik.ch/usb/devices">http://www.qbik.ch/usb/devices/</ulink>
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</para>
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<para>
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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>
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</para>
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<para>
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Programming Guide for Linux USB Device Drivers: <ulink url="http://usb.cs.tum.edu/usbdoc">http://usb.cs.tum.edu/usbdoc</ulink>
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</para>
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<para>
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USB Home Page: <ulink url="http://www.usb.org">http://www.usb.org</ulink>
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</para>
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</chapter>
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</book>
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