Writing a Real Driver—In User Space

Last time we discussed how to create a simple USB driver that controlled a USB LED device manufactured by Delcom Engineering [LJ, April 2004]. I would like to thank all of the readers who have given me feedback on the column. It even enabled one reader to write a driver now in the main kernel tree. I also would like to thank everyone who has given me ideas about what kinds of devices to write about in future columns, but please remember, let's try to cover simple devices. We don't have the room here to go over how to reverse engineer a streaming video camera that has about 12 different modes of operation.

At the end of the last column, I said it is simple to talk to a USB device from user space, without needing a custom kernel driver. Way back during the development of the original Linux USB code, a few developers recognized that it would be wise to allow user programs to get at the raw USB data in all devices in addition to controlling the device. To that end, the usbfs filesystem was created. It originally was called usbdevfs, but many people confused it with the devfs filesystem and all of the nasty baggage that filesystem brought on, so it was renamed usbfs.

Traditionally, usbfs is mounted in the /proc/bus/usb directory on your machine. In that main directory exists a file called devices and a directory for every different USB bus connected to the machine. Those bus directories are named with a number that corresponds to the number the kernel has given that particular USB bus. In each bus directory is a file for every different USB device connected to the bus. For example, a box that has six different USB buses and a few USB devices connected might look like this:

$ tree /proc/bus/usb/
/proc/bus/usb/
|-- 001
|   `-- 001
|-- 002
|   `-- 001
|-- 003
|   `-- 001
|-- 004
|   |-- 001
|   |-- 002
|   `-- 003
|-- 005
|   `-- 001
|-- 006
|   `-- 001
`-- devices

If you do not have any USB host controller drivers loaded, the main /proc/bus/usb/ directory should be empty.

The /proc/bus/usb/devices file contains a list of all USB devices attached at that moment in time. It also shows how the devices are connected to one another and a lot of other USB-specific information about each device. For details on how the data in this file should be interpreted, see the documentation in the kernel tree at Documentation/usb/proc_usb_info.txt. Programs such as usbview or usbtree use this file to show information about USB devices.

The files within the /proc/bus/usb/BBB/ directories, where BBB is the number of the USB bus, allow programs to talk directly to the different USB devices. The name of the files are the same number as the USB number assigned to the device: 001 for the first device, 002 for the second and so on. Do not rely on these numbers to be unique, as the USB subsystem reuses the numbers when devices are removed and added. If device 003 is removed and another, different device is added, it gets the 003 number.

If you read from the device file, the raw USB descriptor is returned—first the USB device descriptor and then the different configuration descriptors. For a detailed description of what format these descriptors are in and what all of the data means, see the USB specification, which is available for download at the USB Developers Web site (www.usb.org/developers).

The device file also supports a wide range of ioctl calls that allows programs to send and receive USB data from the device. These ioctls and the structures needed for the ioctls are described in the kernel file include/linux/usbdevice_fs.h.

Armed with these ioctls, the structures defined in this header file and a copy of the USB specification, we are set to write a user-space program to talk to our device. But do we really want to do this? Wouldn't it be great if someone wrote a library on top of this interface that would enable us to write sane programs? Luckily, a group of developers has created such a library, allowing programmers to ignore the ioctl mess that usbfs uses. This library is called libusb.

libusb is a library that works on a number of different operating systems: Linux, the various BSDs and Mac OS X. It allows programs to be written in a portable manner and yet still control USB devices on vastly different operating systems. Using this library lets us create a program to control the USB LED device. libusb can be downloaded from libusb.sf.net if it is not included in your Linux distribution.

The first thing any libusb program must do is initialize the library and have it scan all USB buses for all USB devices. This is done with the following three function calls:

usb_init(); usb_find_busses(); usb_find_devices();

After the call is complete, the program needs to find a USB device that matches the desired description. As all USB devices have unique vendor and product identification values, it usually is easiest to look for these values. As we remember from the kernel driver we created last time, the USB LED device has the following vendor and product values:

#define LED_VENDOR_ID	0x0fc5
#define LED_PRODUCT_ID  0x1223

With this information, the code to find this device using libusb is the following:

for (usb_bus = usb_busses; usb_bus;
     usb_bus = usb_bus->next) {
        for (dev = usb_bus->devices; dev;
             dev = dev->next) {
            if ((dev->descriptor.idVendor ==
                 LED_VENDOR_ID) &&
                (dev->descriptor.idProduct ==
                 LED_PRODUCT_ID))
                return dev;
	}
}
return NULL;

If the device is found in the system, a pointer to it is returned, otherwise NULL is returned. This pointer is of type struct usb_device. After this structure is found, the USB device must be opened and a handle must be created by libusb for the program to communicate with the device. This is done with the following simple code:

usb_handle = usb_open(usb_dev);
if (usb_handle == NULL) {
    fprintf(stderr,
            "Not able to claim the USB device\n");
    goto exit;
}

This usb_handle variable is of type struct usb_dev_handle, and it is what libusb uses to determine with which USB device it should communicate. This handle is all that is needed to set up our USB device to be ready to communicate with it.

When the program is finished with the USB device, a call to usb_close(usb_handle); is all that is necessary to clean up all of our structures and notify libusb that the device is no longer needed.