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 Some vendors are open with their USB protocols and 25help with the creation of Linux drivers, while others do not publish 26them, and developers are forced to reverse-engineer.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 .suspend = skel_suspend, 61 .resume = skel_resume, 62 .pre_reset = skel_pre_reset, 63 .post_reset = skel_post_reset, 64 .id_table = skel_table, 65 .supports_autosuspend = 1, 66 }; 67 68 69The variable name is a string that describes the driver. It is used in 70informational messages printed to the system log. The probe and 71disconnect function pointers are called when a device that matches the 72information provided in the ``id_table`` variable is either seen or 73removed. 74 75The fops and minor variables are optional. Most USB drivers hook into 76another kernel subsystem, such as the SCSI, network or TTY subsystem. 77These types of drivers register themselves with the other kernel 78subsystem, and any user-space interactions are provided through that 79interface. But for drivers that do not have a matching kernel subsystem, 80such as MP3 players or scanners, a method of interacting with user space 81is needed. The USB subsystem provides a way to register a minor device 82number and a set of :c:type:`file_operations` function pointers that enable 83this user-space interaction. The skeleton driver needs this kind of 84interface, so it provides a minor starting number and a pointer to its 85:c:type:`file_operations` functions. 86 87The USB driver is then registered with a call to usb_register(), 88usually in the driver's init function, as shown here:: 89 90 static int __init usb_skel_init(void) 91 { 92 int result; 93 94 /* register this driver with the USB subsystem */ 95 result = usb_register(&skel_driver); 96 if (result < 0) { 97 pr_err("usb_register failed for the %s driver. Error number %d\n", 98 skel_driver.name, result); 99 return -1; 100 } 101 102 return 0; 103 } 104 module_init(usb_skel_init); 105 106 107When the driver is unloaded from the system, it needs to deregister 108itself with the USB subsystem. This is done with usb_deregister() 109function:: 110 111 static void __exit usb_skel_exit(void) 112 { 113 /* deregister this driver with the USB subsystem */ 114 usb_deregister(&skel_driver); 115 } 116 module_exit(usb_skel_exit); 117 118 119To enable the linux-hotplug system to load the driver automatically when 120the device is plugged in, you need to create a ``MODULE_DEVICE_TABLE``. 121The following code tells the hotplug scripts that this module supports a 122single device with a specific vendor and product ID:: 123 124 /* table of devices that work with this driver */ 125 static struct usb_device_id skel_table [] = { 126 { USB_DEVICE(USB_SKEL_VENDOR_ID, USB_SKEL_PRODUCT_ID) }, 127 { } /* Terminating entry */ 128 }; 129 MODULE_DEVICE_TABLE (usb, skel_table); 130 131 132There are other macros that can be used in describing a struct 133:c:type:`usb_device_id` for drivers that support a whole class of USB 134drivers. See :ref:`usb.h <usb_header>` for more information on this. 135 136Device operation 137================ 138 139When a device is plugged into the USB bus that matches the device ID 140pattern that your driver registered with the USB core, the probe 141function is called. The :c:type:`usb_device` structure, interface number and 142the interface ID are passed to the function:: 143 144 static int skel_probe(struct usb_interface *interface, 145 const struct usb_device_id *id) 146 147 148The driver now needs to verify that this device is actually one that it 149can accept. If so, it returns 0. If not, or if any error occurs during 150initialization, an errorcode (such as ``-ENOMEM`` or ``-ENODEV``) is 151returned from the probe function. 152 153In the skeleton driver, we determine what end points are marked as 154bulk-in and bulk-out. We create buffers to hold the data that will be 155sent and received from the device, and a USB urb to write data to the 156device is initialized. 157 158Conversely, when the device is removed from the USB bus, the disconnect 159function is called with the device pointer. The driver needs to clean 160any private data that has been allocated at this time and to shut down 161any pending urbs that are in the USB system. 162 163Now that the device is plugged into the system and the driver is bound 164to the device, any of the functions in the :c:type:`file_operations` structure 165that were passed to the USB subsystem will be called from a user program 166trying to talk to the device. The first function called will be open, as 167the program tries to open the device for I/O. We increment our private 168usage count and save a pointer to our internal structure in the file 169structure. This is done so that future calls to file operations will 170enable the driver to determine which device the user is addressing. All 171of this is done with the following code:: 172 173 /* increment our usage count for the device */ 174 kref_get(&dev->kref); 175 176 /* save our object in the file's private structure */ 177 file->private_data = dev; 178 179 180After the open function is called, the read and write functions are 181called to receive and send data to the device. In the ``skel_write`` 182function, we receive a pointer to some data that the user wants to send 183to the device and the size of the data. The function determines how much 184data it can send to the device based on the size of the write urb it has 185created (this size depends on the size of the bulk out end point that 186the device has). Then it copies the data from user space to kernel 187space, points the urb to the data and submits the urb to the USB 188subsystem. This can be seen in the following code:: 189 190 /* we can only write as much as 1 urb will hold */ 191 size_t writesize = min_t(size_t, count, MAX_TRANSFER); 192 193 /* copy the data from user space into our urb */ 194 copy_from_user(buf, user_buffer, writesize); 195 196 /* set up our urb */ 197 usb_fill_bulk_urb(urb, 198 dev->udev, 199 usb_sndbulkpipe(dev->udev, dev->bulk_out_endpointAddr), 200 buf, 201 writesize, 202 skel_write_bulk_callback, 203 dev); 204 205 /* send the data out the bulk port */ 206 retval = usb_submit_urb(urb, GFP_KERNEL); 207 if (retval) { 208 dev_err(&dev->interface->dev, 209 "%s - failed submitting write urb, error %d\n", 210 __func__, retval); 211 } 212 213 214When the write urb is filled up with the proper information using the 215:c:func:`usb_fill_bulk_urb` function, we point the urb's completion callback 216to call our own ``skel_write_bulk_callback`` function. This function is 217called when the urb is finished by the USB subsystem. The callback 218function is called in interrupt context, so caution must be taken not to 219do very much processing at that time. Our implementation of 220``skel_write_bulk_callback`` merely reports if the urb was completed 221successfully or not and then returns. 222 223The read function works a bit differently from the write function in 224that we do not use an urb to transfer data from the device to the 225driver. Instead we call the :c:func:`usb_bulk_msg` function, which can be used 226to send or receive data from a device without having to create urbs and 227handle urb completion callback functions. We call the :c:func:`usb_bulk_msg` 228function, giving it a buffer into which to place any data received from 229the device and a timeout value. If the timeout period expires without 230receiving any data from the device, the function will fail and return an 231error message. This can be shown with the following code:: 232 233 /* do an immediate bulk read to get data from the device */ 234 retval = usb_bulk_msg (skel->dev, 235 usb_rcvbulkpipe (skel->dev, 236 skel->bulk_in_endpointAddr), 237 skel->bulk_in_buffer, 238 skel->bulk_in_size, 239 &count, 5000); 240 /* if the read was successful, copy the data to user space */ 241 if (!retval) { 242 if (copy_to_user (buffer, skel->bulk_in_buffer, count)) 243 retval = -EFAULT; 244 else 245 retval = count; 246 } 247 248 249The :c:func:`usb_bulk_msg` function can be very useful for doing single reads 250or writes to a device; however, if you need to read or write constantly to 251a device, it is recommended to set up your own urbs and submit them to 252the USB subsystem. 253 254When the user program releases the file handle that it has been using to 255talk to the device, the release function in the driver is called. In 256this function we decrement our private usage count and wait for possible 257pending writes:: 258 259 /* decrement our usage count for the device */ 260 --skel->open_count; 261 262 263One of the more difficult problems that USB drivers must be able to 264handle smoothly is the fact that the USB device may be removed from the 265system at any point in time, even if a program is currently talking to 266it. It needs to be able to shut down any current reads and writes and 267notify the user-space programs that the device is no longer there. The 268following code (function ``skel_delete``) is an example of how to do 269this:: 270 271 static inline void skel_delete (struct usb_skel *dev) 272 { 273 kfree (dev->bulk_in_buffer); 274 if (dev->bulk_out_buffer != NULL) 275 usb_free_coherent (dev->udev, dev->bulk_out_size, 276 dev->bulk_out_buffer, 277 dev->write_urb->transfer_dma); 278 usb_free_urb (dev->write_urb); 279 kfree (dev); 280 } 281 282 283If a program currently has an open handle to the device, we reset the 284flag ``device_present``. For every read, write, release and other 285functions that expect a device to be present, the driver first checks 286this flag to see if the device is still present. If not, it releases 287that the device has disappeared, and a ``-ENODEV`` error is returned to the 288user-space program. When the release function is eventually called, it 289determines if there is no device and if not, it does the cleanup that 290the ``skel_disconnect`` function normally does if there are no open files 291on the device (see Listing 5). 292 293Isochronous Data 294================ 295 296This usb-skeleton driver does not have any examples of interrupt or 297isochronous data being sent to or from the device. Interrupt data is 298sent almost exactly as bulk data is, with a few minor exceptions. 299Isochronous data works differently with continuous streams of data being 300sent to or from the device. The audio and video camera drivers are very 301good examples of drivers that handle isochronous data and will be useful 302if you also need to do this. 303 304Conclusion 305========== 306 307Writing Linux USB device drivers is not a difficult task as the 308usb-skeleton driver shows. This driver, combined with the other current 309USB drivers, should provide enough examples to help a beginning author 310create a working driver in a minimal amount of time. The linux-usb-devel 311mailing list archives also contain a lot of helpful information. 312 313Resources 314========= 315 316The Linux USB Project: 317http://www.linux-usb.org/ 318 319Linux Hotplug Project: 320http://linux-hotplug.sourceforge.net/ 321 322linux-usb Mailing List Archives: 323https://lore.kernel.org/linux-usb/ 324 325Programming Guide for Linux USB Device Drivers: 326https://lmu.web.psi.ch/docu/manuals/software_manuals/linux_sl/usb_linux_programming_guide.pdf 327 328USB Home Page: https://www.usb.org 329