diff options
Diffstat (limited to 'Documentation/DocBook/writing_usb_driver.tmpl')
| -rw-r--r-- | Documentation/DocBook/writing_usb_driver.tmpl | 419 |
1 files changed, 419 insertions, 0 deletions
diff --git a/Documentation/DocBook/writing_usb_driver.tmpl b/Documentation/DocBook/writing_usb_driver.tmpl new file mode 100644 index 000000000000..51f3bfb6fb6e --- /dev/null +++ b/Documentation/DocBook/writing_usb_driver.tmpl @@ -0,0 +1,419 @@ +<?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, 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 = &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(&skel_driver); + if (result < 0) { + err("usb_register failed for the "__FILE__ "driver." + "Error number %d", result); + return -1; + } + + return 0; +} +module_init(usb_skel_init); + </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(&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. The driver also unregisters + itself from the devfs subsystem with the call: + </para> + <programlisting> +/* remove our devfs node */ +devfs_unregister(skel->devfs); + </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 */ +++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 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 */ +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("Failed submitting write urb, error %d", 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, + &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) +{ + 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="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> |
