329 lines
13 KiB
ReStructuredText
329 lines
13 KiB
ReStructuredText
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.. _writing-usb-driver:
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==========================
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Writing USB Device Drivers
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==========================
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:Author: Greg Kroah-Hartman
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Introduction
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============
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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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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
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needs to be created. Some vendors are open with their USB protocols and
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help with the creation of Linux drivers, while others do not publish
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them, and developers are forced to reverse-engineer. See Resources for
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some links to handy reverse-engineering tools.
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Because each different protocol causes a new driver to be created, I
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have written a generic USB driver skeleton, modelled after the
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pci-skeleton.c file in the kernel source tree upon which many PCI
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network drivers have been based. This USB skeleton can be found at
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drivers/usb/usb-skeleton.c in the kernel source tree. In this article I
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will walk through the basics of the skeleton driver, explaining the
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different pieces and what needs to be done to customize it to your
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specific device.
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Linux USB Basics
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================
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If you are going to write a Linux USB driver, please become familiar
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with the USB protocol specification. It can be found, along with many
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other useful documents, at the USB home page (see Resources). An
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excellent introduction to the Linux USB subsystem can be found at the
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USB Working Devices List (see Resources). It explains how the Linux USB
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subsystem is structured and introduces the reader to the concept of USB
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urbs (USB Request Blocks), which are essential to USB drivers.
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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 :c:type:`usb_driver`
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structure. The skeleton driver declares a :c:type:`usb_driver` as::
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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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.suspend = skel_suspend,
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.resume = skel_resume,
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.pre_reset = skel_pre_reset,
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.post_reset = skel_post_reset,
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.id_table = skel_table,
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.supports_autosuspend = 1,
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};
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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
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removed.
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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 :c:type:`file_operations` function pointers that enable
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this user-space interaction. The skeleton driver needs this kind of
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interface, so it provides a minor starting number and a pointer to its
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:c:type:`file_operations` functions.
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The USB driver is then registered with a call to usb_register(),
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usually in the driver's init function, as shown here::
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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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/* 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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pr_err("usb_register failed for the %s driver. Error number %d\n",
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skel_driver.name, 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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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 usb_deregister()
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function::
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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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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``.
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The 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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/* 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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There are other macros that can be used in describing a struct
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:c:type:`usb_device_id` for drivers that support a whole class of USB
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drivers. See :ref:`usb.h <usb_header>` for more information on this.
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Device operation
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================
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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
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function is called. The :c:type:`usb_device` structure, interface number and
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the interface ID are passed to the function::
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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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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. If not, or if any error occurs during
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initialization, an errorcode (such as ``-ENOMEM`` or ``-ENODEV``) is
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returned from the probe function.
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In the skeleton driver, we determine what end points are marked as
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bulk-in and bulk-out. We create buffers to hold the data that will be
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sent and received from the device, and a USB urb to write data to the
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device is initialized.
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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
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any private data that has been allocated at this time and to shut down
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any pending urbs that are in the USB system.
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Now that the device is plugged into the system and the driver is bound
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to the device, any of the functions in the :c:type:`file_operations` structure
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that were passed to the USB subsystem will be called from a user program
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trying to talk to the device. The first function called will be open, as
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the program tries to open the device for I/O. We increment our private
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usage 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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/* increment our usage count for the device */
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kref_get(&dev->kref);
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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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After the open function is called, the read and write functions are
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called to receive and send data to the device. In the ``skel_write``
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function, we receive a pointer to some data that the user wants to send
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to the device and the size of the data. The function determines how much
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data it can send to the device based on the size of the write urb it has
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created (this size depends on the size of the bulk out end point that
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the device has). Then it copies the data from user space to kernel
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space, points the urb to the data and submits the urb to the USB
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subsystem. This can be seen in the following code::
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/* we can only write as much as 1 urb will hold */
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size_t writesize = min_t(size_t, count, MAX_TRANSFER);
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/* copy the data from user space into our urb */
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copy_from_user(buf, user_buffer, writesize);
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/* set up our urb */
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usb_fill_bulk_urb(urb,
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dev->udev,
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usb_sndbulkpipe(dev->udev, dev->bulk_out_endpointAddr),
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buf,
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writesize,
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skel_write_bulk_callback,
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dev);
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/* send the data out the bulk port */
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retval = usb_submit_urb(urb, GFP_KERNEL);
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if (retval) {
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dev_err(&dev->interface->dev,
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"%s - failed submitting write urb, error %d\n",
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__func__, retval);
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}
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When the write urb is filled up with the proper information using the
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:c:func:`usb_fill_bulk_urb` function, we point the urb's completion callback
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to call our own ``skel_write_bulk_callback`` function. This function is
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called when the urb is finished by the USB subsystem. The callback
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function is called in interrupt context, so caution must be taken not to
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do very much processing at that time. Our implementation of
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``skel_write_bulk_callback`` merely reports if the urb was completed
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successfully or not and then returns.
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The read function works a bit differently from the write function in
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that we do not use an urb to transfer data from the device to the
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driver. Instead we call the :c:func:`usb_bulk_msg` function, which can be used
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to send or receive data from a device without having to create urbs and
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handle urb completion callback functions. We call the :c:func:`usb_bulk_msg`
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function, giving it a buffer into which to place any data received from
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the device and a timeout value. If the timeout period expires without
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receiving any data from the device, the function will fail and return an
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error message. This can be shown with the following code::
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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, 5000);
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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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The :c:func:`usb_bulk_msg` function can be very useful for doing single reads
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or writes to a device; however, if you need to read or write constantly to
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a device, it is recommended to set up your own urbs and submit them to
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the USB subsystem.
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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
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this function we decrement our private usage count and wait for possible
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pending writes::
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/* decrement our usage count for the device */
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--skel->open_count;
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One of the more difficult problems that USB drivers must be able to
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handle smoothly is the fact that the USB device may be removed from the
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system at any point in time, even if a program is currently talking to
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it. It needs to be able to shut down any current reads and writes and
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notify the user-space programs that the device is no longer there. The
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following code (function ``skel_delete``) is an example of how to do
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this::
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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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If a program currently has an open handle to the device, we reset the
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flag ``device_present``. For every read, write, release and other
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functions that expect a device to be present, the driver first checks
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this flag to see if the device is still present. If not, it releases
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that the device has disappeared, and a ``-ENODEV`` error is returned to the
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user-space program. When the release function is eventually called, it
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determines if there is no device and if not, it does the cleanup that
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the ``skel_disconnect`` function normally does if there are no open files
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on the device (see Listing 5).
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Isochronous Data
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================
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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
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sent almost exactly as bulk data is, with a few minor exceptions.
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Isochronous data works differently with continuous streams of data being
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sent to or from the device. The audio and video camera drivers are very
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good examples of drivers that handle isochronous data and will be useful
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if you also need to do this.
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Conclusion
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==========
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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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Resources
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=========
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The Linux USB Project:
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http://www.linux-usb.org/
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Linux Hotplug Project:
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http://linux-hotplug.sourceforge.net/
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linux-usb Mailing List Archives:
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https://lore.kernel.org/linux-usb/
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Programming Guide for Linux USB Device Drivers:
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https://lmu.web.psi.ch/docu/manuals/software_manuals/linux_sl/usb_linux_programming_guide.pdf
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USB Home Page: https://www.usb.org
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