Writing a Simple USB Driver

Armed with our new-found information, we set off to whip up a quick kernel driver. It should be a USB driver, but what kind of interface to user space should we use? A block device does not make sense, as this device does not need to store filesystem data, but a character device would work. If we use a character device driver, however, a major and minor number needs to be reserved for it. And how many minor numbers would we need for this driver? What if someone wanted to plug 100 different USB lamp devices in to this system? To anticipate this, we would need to reserve at least 100 minor numbers, which would be a total waste if all anyone ever used was one device at a time. If we make a character driver, we also would need to invent some way to tell the driver to turn on and off the different colors individually. Traditionally, that could be done using different ioctl commands on the character driver, but we know much better than ever to create a new ioctl command in the kernel.

As all USB devices show up in their own directory in the sysfs tree, so why not use sysfs and create three files in the USB device directory, blue, red and green? This would allow any user-space program, be it a C program or a shell script, to change the colors on our LED device. This also would keep us from having to write a character driver and beg for a chunk of minor numbers for our device.

To start out our USB driver, we need to provide the USB subsystem with five things:

The driver retrieves this information with the following bit of code:

static struct usb_driver led_driver = {
	.owner =	THIS_MODULE,
	.name =		"usbled",
	.probe =	led_probe,
	.disconnect =	led_disconnect,
	.id_table =	id_table,
};

The id_table variable is defined as:

static struct usb_device_id id_table [] = {
	{ USB_DEVICE(VENDOR_ID, PRODUCT_ID) },
	{ },
};
MODULE_DEVICE_TABLE (usb, id_table);

The led_probe() and led_disconnect() functions are described later.

When the driver module is loaded, this led_driver structure must be registered with the USB core. This is accomplished with a single call to the usb_register() function:

retval = usb_register(&led_driver);
if (retval)
        err("usb_register failed. "
            "Error number %d", retval);

Likewise, when the driver is unloaded from the system, it must unregister itself from the USB core:

usb_deregister(&led_driver);

The led_probe() function is called when the USB core has found our USB lamp device. All it needs to do is initialize the device and create the three sysfs files, in the proper location. This is done with the following code:

/* Initialize our local device structure */
dev = kmalloc(sizeof(struct usb_led), GFP_KERNEL);
memset (dev, 0x00, sizeof (*dev));

dev->udev = usb_get_dev(udev);
usb_set_intfdata (interface, dev);

/* Create our three sysfs files in the USB
* device directory */
device_create_file(&interface->dev, &dev_attr_blue);
device_create_file(&interface->dev, &dev_attr_red);
device_create_file(&interface->dev, &dev_attr_green);

dev_info(&interface->dev,
    "USB LED device now attached\n");
return 0;

The led_disconnect() function is equally as simple, as we need only to free our allocated memory and remove the sysfs files:

dev = usb_get_intfdata (interface);
usb_set_intfdata (interface, NULL);

device_remove_file(&interface->dev, &dev_attr_blue);
device_remove_file(&interface->dev, &dev_attr_red);
device_remove_file(&interface->dev, &dev_attr_green);

usb_put_dev(dev->udev);
kfree(dev);

dev_info(&interface->dev,
         "USB LED now disconnected\n");

When the sysfs files are read from, we want to show the current value of that LED; when it is written to, we want to set that specific LED. To do this, the following macro creates two functions for each color LED and declares a sysfs device attribute file:

#define show_set(value)	                           \
static ssize_t                                     \
show_##value(struct device *dev, char *buf)        \
{                                                  \
   struct usb_interface *intf =                    \
      to_usb_interface(dev);                       \
   struct usb_led *led = usb_get_intfdata(intf);   \
                                                   \
   return sprintf(buf, "%d\n", led->value);        \
}                                                  \
                                                   \
static ssize_t                                     \
set_##value(struct device *dev, const char *buf,   \
            size_t count)                          \
{                                                  \
   struct usb_interface *intf =                    \
      to_usb_interface(dev);                       \
   struct usb_led *led = usb_get_intfdata(intf);   \
   int temp = simple_strtoul(buf, NULL, 10);       \
                                                   \
   led->value = temp;                              \
   change_color(led);                              \
   return count;                                   \
}                                                  \

static DEVICE_ATTR(value, S_IWUGO | S_IRUGO,
                   show_##value, set_##value);
show_set(blue);
show_set(red);
show_set(green);

This creates six functions, show_blue(), set_blue(), show_red(), set_red(), show_green() and set_green(); and three attribute structures, dev_attr_blue, dev_attr_red and dev_attr_green. Due to the simple nature of the sysfs file callbacks and the fact that we need to do the same thing for every different value (blue, red and green), a macro was used to reduce typing. This is a common occurrence for sysfs file functions; an example of this in the kernel source tree is the I2C chip drivers in drivers/i2c/chips.

So, to enable the red LED, a user writes a 1 to the red file in sysfs, which calls the set_red() function in the driver, which calls the change_color() function. The change_color() function looks like:

#define BLUE	0x04
#define RED	0x02
#define GREEN	0x01
   buffer = kmalloc(8, GFP_KERNEL);

   color = 0x07;
   if (led->blue)
      color &= ~(BLUE);
   if (led->red)
      color &= ~(RED);
   if (led->green)
      color &= ~(GREEN);
   retval =
      usb_control_msg(led->udev,
                      usb_sndctrlpipe(led->udev, 0),
                      0x12,
                      0xc8,
                      (0x02 * 0x100) + 0x0a,
                      (0x00 * 0x100) + color,
                      buffer,
                      8,
                      2 * HZ);
   kfree(buffer);