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diff --git a/Documentation/driver-api/usb/writing_usb_driver.rst b/Documentation/driver-api/usb/writing_usb_driver.rst new file mode 100644 index 0000000000..95c4f5d140 --- /dev/null +++ b/Documentation/driver-api/usb/writing_usb_driver.rst @@ -0,0 +1,328 @@ +.. _writing-usb-driver: + +========================== +Writing USB Device Drivers +========================== + +:Author: Greg Kroah-Hartman + +Introduction +============ + +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. + +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 Some vendors are open with their USB protocols and +help with the creation of Linux drivers, while others do not publish +them, and developers are forced to reverse-engineer.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. + +Because each different protocol causes a new driver to be created, I +have written a generic USB driver skeleton, modelled 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. + +Linux USB Basics +================ + +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 (USB Request Blocks), which are essential to USB drivers. + +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 :c:type:`usb_driver` +structure. The skeleton driver declares a :c:type:`usb_driver` as:: + + static struct usb_driver skel_driver = { + .name = "skeleton", + .probe = skel_probe, + .disconnect = skel_disconnect, + .suspend = skel_suspend, + .resume = skel_resume, + .pre_reset = skel_pre_reset, + .post_reset = skel_post_reset, + .id_table = skel_table, + .supports_autosuspend = 1, + }; + + +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. + +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 :c:type:`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 +:c:type:`file_operations` functions. + +The USB driver is then registered with a call to usb_register(), +usually in the driver's init function, as shown here:: + + static int __init usb_skel_init(void) + { + int result; + + /* register this driver with the USB subsystem */ + result = usb_register(&skel_driver); + if (result < 0) { + pr_err("usb_register failed for the %s driver. Error number %d\n", + skel_driver.name, result); + return -1; + } + + return 0; + } + module_init(usb_skel_init); + + +When the driver is unloaded from the system, it needs to deregister +itself with the USB subsystem. This is done with usb_deregister() +function:: + + static void __exit usb_skel_exit(void) + { + /* deregister this driver with the USB subsystem */ + usb_deregister(&skel_driver); + } + module_exit(usb_skel_exit); + + +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:: + + /* 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); + + +There are other macros that can be used in describing a struct +:c:type:`usb_device_id` for drivers that support a whole class of USB +drivers. See :ref:`usb.h <usb_header>` for more information on this. + +Device operation +================ + +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 :c:type:`usb_device` structure, interface number and +the interface ID are passed to the function:: + + static int skel_probe(struct usb_interface *interface, + const struct usb_device_id *id) + + +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 ``-ENOMEM`` or ``-ENODEV``) is +returned from the probe function. + +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. + +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. + +Now that the device is plugged into the system and the driver is bound +to the device, any of the functions in the :c:type:`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 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:: + + /* increment our usage count for the device */ + kref_get(&dev->kref); + + /* save our object in the file's private structure */ + file->private_data = dev; + + +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 seen in the following code:: + + /* we can only write as much as 1 urb will hold */ + size_t writesize = min_t(size_t, count, MAX_TRANSFER); + + /* copy the data from user space into our urb */ + copy_from_user(buf, user_buffer, writesize); + + /* set up our urb */ + usb_fill_bulk_urb(urb, + dev->udev, + usb_sndbulkpipe(dev->udev, dev->bulk_out_endpointAddr), + buf, + writesize, + skel_write_bulk_callback, + dev); + + /* send the data out the bulk port */ + retval = usb_submit_urb(urb, GFP_KERNEL); + if (retval) { + dev_err(&dev->interface->dev, + "%s - failed submitting write urb, error %d\n", + __func__, retval); + } + + +When the write urb is filled up with the proper information using the +:c:func:`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. + +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 :c:func:`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 :c:func:`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:: + + /* 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, 5000); + /* 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; + } + + +The :c:func:`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. + +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:: + + /* decrement our usage count for the device */ + --skel->open_count; + + +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 ``skel_delete``) is an example of how to do +this:: + + static inline void skel_delete (struct usb_skel *dev) + { + kfree (dev->bulk_in_buffer); + if (dev->bulk_out_buffer != NULL) + usb_free_coherent (dev->udev, dev->bulk_out_size, + dev->bulk_out_buffer, + dev->write_urb->transfer_dma); + usb_free_urb (dev->write_urb); + kfree (dev); + } + + +If a program currently has an open handle to the device, we reset the +flag ``device_present``. 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). + +Isochronous Data +================ + +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. + +Conclusion +========== + +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. + +Resources +========= + +The Linux USB Project: +http://www.linux-usb.org/ + +Linux Hotplug Project: +http://linux-hotplug.sourceforge.net/ + +linux-usb Mailing List Archives: +https://lore.kernel.org/linux-usb/ + +Programming Guide for Linux USB Device Drivers: +https://lmu.web.psi.ch/docu/manuals/software_manuals/linux_sl/usb_linux_programming_guide.pdf + +USB Home Page: https://www.usb.org |