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412 lines
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412 lines
17 KiB
<?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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|
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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, modelled 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 |
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(USB Request Blocks), which 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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|
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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 deregister |
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itself with the USB subsystem. This is done with the usb_deregister |
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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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|
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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. |
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</para> |
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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 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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|
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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 seen in |
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the 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_free_coherent (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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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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