1.. _writing-usb-driver: 2 3========================== 4Writing USB Device Drivers 5========================== 6 7:Author: Greg Kroah-Hartman 8 9Introduction 10============ 11 12The Linux USB subsystem has grown from supporting only two different 13types of devices in the 2.2.7 kernel (mice and keyboards), to over 20 14different types of devices in the 2.4 kernel. Linux currently supports 15almost all USB class devices (standard types of devices like keyboards, 16mice, modems, printers and speakers) and an ever-growing number of 17vendor-specific devices (such as USB to serial converters, digital 18cameras, Ethernet devices and MP3 players). For a full list of the 19different USB devices currently supported, see Resources. 20 21The remaining kinds of USB devices that do not have support on Linux are 22almost all vendor-specific devices. Each vendor decides to implement a 23custom protocol to talk to their device, so a custom driver usually 24needs to be created. Some vendors are open with their USB protocols and 25help with the creation of Linux drivers, while others do not publish 26them, and developers are forced to reverse-engineer. See Resources for 27some links to handy reverse-engineering tools. 28 29Because each different protocol causes a new driver to be created, I 30have written a generic USB driver skeleton, modelled after the 31pci-skeleton.c file in the kernel source tree upon which many PCI 32network drivers have been based. This USB skeleton can be found at 33drivers/usb/usb-skeleton.c in the kernel source tree. In this article I 34will walk through the basics of the skeleton driver, explaining the 35different pieces and what needs to be done to customize it to your 36specific device. 37 38Linux USB Basics 39================ 40 41If you are going to write a Linux USB driver, please become familiar 42with the USB protocol specification. It can be found, along with many 43other useful documents, at the USB home page (see Resources). An 44excellent introduction to the Linux USB subsystem can be found at the 45USB Working Devices List (see Resources). It explains how the Linux USB 46subsystem is structured and introduces the reader to the concept of USB 47urbs (USB Request Blocks), which are essential to USB drivers. 48 49The first thing a Linux USB driver needs to do is register itself with 50the Linux USB subsystem, giving it some information about which devices 51the driver supports and which functions to call when a device supported 52by the driver is inserted or removed from the system. All of this 53information is passed to the USB subsystem in the :c:type:`usb_driver` 54structure. The skeleton driver declares a :c:type:`usb_driver` as:: 55 56 static struct usb_driver skel_driver = { 57 .name = "skeleton", 58 .probe = skel_probe, 59 .disconnect = skel_disconnect, 60 .fops = &skel_fops, 61 .minor = USB_SKEL_MINOR_BASE, 62 .id_table = skel_table, 63 }; 64 65 66The variable name is a string that describes the driver. It is used in 67informational messages printed to the system log. The probe and 68disconnect function pointers are called when a device that matches the 69information provided in the ``id_table`` variable is either seen or 70removed. 71 72The fops and minor variables are optional. Most USB drivers hook into 73another kernel subsystem, such as the SCSI, network or TTY subsystem. 74These types of drivers register themselves with the other kernel 75subsystem, and any user-space interactions are provided through that 76interface. But for drivers that do not have a matching kernel subsystem, 77such as MP3 players or scanners, a method of interacting with user space 78is needed. The USB subsystem provides a way to register a minor device 79number and a set of :c:type:`file_operations` function pointers that enable 80this user-space interaction. The skeleton driver needs this kind of 81interface, so it provides a minor starting number and a pointer to its 82:c:type:`file_operations` functions. 83 84The USB driver is then registered with a call to :c:func:`usb_register`, 85usually in the driver's init function, as shown here:: 86 87 static int __init usb_skel_init(void) 88 { 89 int result; 90 91 /* register this driver with the USB subsystem */ 92 result = usb_register(&skel_driver); 93 if (result < 0) { 94 err("usb_register failed for the "__FILE__ "driver." 95 "Error number %d", result); 96 return -1; 97 } 98 99 return 0; 100 } 101 module_init(usb_skel_init); 102 103 104When the driver is unloaded from the system, it needs to deregister 105itself with the USB subsystem. This is done with the :c:func:`usb_deregister` 106function:: 107 108 static void __exit usb_skel_exit(void) 109 { 110 /* deregister this driver with the USB subsystem */ 111 usb_deregister(&skel_driver); 112 } 113 module_exit(usb_skel_exit); 114 115 116To enable the linux-hotplug system to load the driver automatically when 117the device is plugged in, you need to create a ``MODULE_DEVICE_TABLE``. 118The following code tells the hotplug scripts that this module supports a 119single device with a specific vendor and product ID:: 120 121 /* table of devices that work with this driver */ 122 static struct usb_device_id skel_table [] = { 123 { USB_DEVICE(USB_SKEL_VENDOR_ID, USB_SKEL_PRODUCT_ID) }, 124 { } /* Terminating entry */ 125 }; 126 MODULE_DEVICE_TABLE (usb, skel_table); 127 128 129There are other macros that can be used in describing a struct 130:c:type:`usb_device_id` for drivers that support a whole class of USB 131drivers. See :ref:`usb.h <usb_header>` for more information on this. 132 133Device operation 134================ 135 136When a device is plugged into the USB bus that matches the device ID 137pattern that your driver registered with the USB core, the probe 138function is called. The :c:type:`usb_device` structure, interface number and 139the interface ID are passed to the function:: 140 141 static int skel_probe(struct usb_interface *interface, 142 const struct usb_device_id *id) 143 144 145The driver now needs to verify that this device is actually one that it 146can accept. If so, it returns 0. If not, or if any error occurs during 147initialization, an errorcode (such as ``-ENOMEM`` or ``-ENODEV``) is 148returned from the probe function. 149 150In the skeleton driver, we determine what end points are marked as 151bulk-in and bulk-out. We create buffers to hold the data that will be 152sent and received from the device, and a USB urb to write data to the 153device is initialized. 154 155Conversely, when the device is removed from the USB bus, the disconnect 156function is called with the device pointer. The driver needs to clean 157any private data that has been allocated at this time and to shut down 158any pending urbs that are in the USB system. 159 160Now that the device is plugged into the system and the driver is bound 161to the device, any of the functions in the :c:type:`file_operations` structure 162that were passed to the USB subsystem will be called from a user program 163trying to talk to the device. The first function called will be open, as 164the program tries to open the device for I/O. We increment our private 165usage count and save a pointer to our internal structure in the file 166structure. This is done so that future calls to file operations will 167enable the driver to determine which device the user is addressing. All 168of this is done with the following code:: 169 170 /* increment our usage count for the module */ 171 ++skel->open_count; 172 173 /* save our object in the file's private structure */ 174 file->private_data = dev; 175 176 177After the open function is called, the read and write functions are 178called to receive and send data to the device. In the ``skel_write`` 179function, we receive a pointer to some data that the user wants to send 180to the device and the size of the data. The function determines how much 181data it can send to the device based on the size of the write urb it has 182created (this size depends on the size of the bulk out end point that 183the device has). Then it copies the data from user space to kernel 184space, points the urb to the data and submits the urb to the USB 185subsystem. This can be seen in the following code:: 186 187 /* we can only write as much as 1 urb will hold */ 188 bytes_written = (count > skel->bulk_out_size) ? skel->bulk_out_size : count; 189 190 /* copy the data from user space into our urb */ 191 copy_from_user(skel->write_urb->transfer_buffer, buffer, bytes_written); 192 193 /* set up our urb */ 194 usb_fill_bulk_urb(skel->write_urb, 195 skel->dev, 196 usb_sndbulkpipe(skel->dev, skel->bulk_out_endpointAddr), 197 skel->write_urb->transfer_buffer, 198 bytes_written, 199 skel_write_bulk_callback, 200 skel); 201 202 /* send the data out the bulk port */ 203 result = usb_submit_urb(skel->write_urb); 204 if (result) { 205 err("Failed submitting write urb, error %d", result); 206 } 207 208 209When the write urb is filled up with the proper information using the 210:c:func:`usb_fill_bulk_urb` function, we point the urb's completion callback 211to call our own ``skel_write_bulk_callback`` function. This function is 212called when the urb is finished by the USB subsystem. The callback 213function is called in interrupt context, so caution must be taken not to 214do very much processing at that time. Our implementation of 215``skel_write_bulk_callback`` merely reports if the urb was completed 216successfully or not and then returns. 217 218The read function works a bit differently from the write function in 219that we do not use an urb to transfer data from the device to the 220driver. Instead we call the :c:func:`usb_bulk_msg` function, which can be used 221to send or receive data from a device without having to create urbs and 222handle urb completion callback functions. We call the :c:func:`usb_bulk_msg` 223function, giving it a buffer into which to place any data received from 224the device and a timeout value. If the timeout period expires without 225receiving any data from the device, the function will fail and return an 226error message. This can be shown with the following code:: 227 228 /* do an immediate bulk read to get data from the device */ 229 retval = usb_bulk_msg (skel->dev, 230 usb_rcvbulkpipe (skel->dev, 231 skel->bulk_in_endpointAddr), 232 skel->bulk_in_buffer, 233 skel->bulk_in_size, 234 &count, HZ*10); 235 /* if the read was successful, copy the data to user space */ 236 if (!retval) { 237 if (copy_to_user (buffer, skel->bulk_in_buffer, count)) 238 retval = -EFAULT; 239 else 240 retval = count; 241 } 242 243 244The :c:func:`usb_bulk_msg` function can be very useful for doing single reads 245or writes to a device; however, if you need to read or write constantly to 246a device, it is recommended to set up your own urbs and submit them to 247the USB subsystem. 248 249When the user program releases the file handle that it has been using to 250talk to the device, the release function in the driver is called. In 251this function we decrement our private usage count and wait for possible 252pending writes:: 253 254 /* decrement our usage count for the device */ 255 --skel->open_count; 256 257 258One of the more difficult problems that USB drivers must be able to 259handle smoothly is the fact that the USB device may be removed from the 260system at any point in time, even if a program is currently talking to 261it. It needs to be able to shut down any current reads and writes and 262notify the user-space programs that the device is no longer there. The 263following code (function ``skel_delete``) is an example of how to do 264this:: 265 266 static inline void skel_delete (struct usb_skel *dev) 267 { 268 kfree (dev->bulk_in_buffer); 269 if (dev->bulk_out_buffer != NULL) 270 usb_free_coherent (dev->udev, dev->bulk_out_size, 271 dev->bulk_out_buffer, 272 dev->write_urb->transfer_dma); 273 usb_free_urb (dev->write_urb); 274 kfree (dev); 275 } 276 277 278If a program currently has an open handle to the device, we reset the 279flag ``device_present``. For every read, write, release and other 280functions that expect a device to be present, the driver first checks 281this flag to see if the device is still present. If not, it releases 282that the device has disappeared, and a ``-ENODEV`` error is returned to the 283user-space program. When the release function is eventually called, it 284determines if there is no device and if not, it does the cleanup that 285the ``skel_disconnect`` function normally does if there are no open files 286on the device (see Listing 5). 287 288Isochronous Data 289================ 290 291This usb-skeleton driver does not have any examples of interrupt or 292isochronous data being sent to or from the device. Interrupt data is 293sent almost exactly as bulk data is, with a few minor exceptions. 294Isochronous data works differently with continuous streams of data being 295sent to or from the device. The audio and video camera drivers are very 296good examples of drivers that handle isochronous data and will be useful 297if you also need to do this. 298 299Conclusion 300========== 301 302Writing Linux USB device drivers is not a difficult task as the 303usb-skeleton driver shows. This driver, combined with the other current 304USB drivers, should provide enough examples to help a beginning author 305create a working driver in a minimal amount of time. The linux-usb-devel 306mailing list archives also contain a lot of helpful information. 307 308Resources 309========= 310 311The Linux USB Project: 312http://www.linux-usb.org/ 313 314Linux Hotplug Project: 315http://linux-hotplug.sourceforge.net/ 316 317linux-usb Mailing List Archives: 318https://lore.kernel.org/linux-usb/ 319 320Programming Guide for Linux USB Device Drivers: 321https://lmu.web.psi.ch/docu/manuals/software_manuals/linux_sl/usb_linux_programming_guide.pdf 322 323USB Home Page: https://www.usb.org 324