LJ Archive

Driving Me Nuts

Device Classes

Greg Kroah-Hartman

Issue #112, August 2003

More necessary instructions for making your new device driver play nice in the 2.6 kernel.

In the last Driving Me Nuts column [see LJ, June 2003], we introduced the kernel driver model framework with an explanation of how the generic bus and driver and device code works. The i2c core was used as an example to show how these different subsystems work. This month, we cover how the driver class code works, again using the i2c code to provide some working examples.

As discussed in the last column, device classes do not meet the general object-oriented definition of a class; rather they are something that provides a single type of function to the user. For example, kernel classes are used for tty devices, block devices, network devices, SCSI hosts and, in the near future, filesystems.

In the 2.5.69 kernel, the driver class support was rewritten radically. In previous kernel versions, class support was tied tightly to the driver and device support. A class would be bound to the device at the same time it was registered to a driver. This did work for a number of devices and classes, but some real-world devices did not fit very well into this model. Now, class support is tied only loosely to devices and drivers; in fact, a device or driver is not even needed to use the class code now, as the tty class code shows. The class code is now split into three different types of structures: classes, class devices and class interfaces.


Classes in the kernel are defined with a simple struct class structure. Yes, class is not a reserved word in C. (Everyone who wants to build a kernel with a C++ compiler, go flame the author of the new class code.) To create a class structure, only the name variable in the struct class structure needs to be defined for it to be a valid class. This can be done with the following code:

static struct class i2c_adapter_class = {
	.name = "i2c_adapter"

After the class structure is defined, it can be registered with the driver core by calling the class_register function:

if (class_register(&i2c_adapter_class) != 0)
    printk(KERN_ERR "i2c adapter class failed "
                    "to register properly\n");

After the class_register function returns without reporting an error, the /sys/class/i2c_adapter directory has been created successfully. Later, when the class needs to be unloaded, the class_unregister function should be called:


Class Devices

Classes are used to manage a set of different class devices. A class device is defined in the kernel with the struct class_device structure. This structure contains of a lot of variables the driver core uses, and it can be ignored by the driver writer. Only the following variables should be set:

  • class: should point to the struct class that is going to manage the class device.

  • dev: should be set to the address of the struct device associated with the class device, if any. A single struct device can be pointed to by multiple class device structures. This is the main difference between the previous kernel class support and the current implementation. This variable does not have to be set for the kernel to work properly. If it is set, a device symbolic link is created in the sysfs entry for the class device that points to the struct device. See below for an example.

  • class_id: an array of characters used to describe the class device. It must be unique among all class device structures assigned to a single class structure.

  • class_data: used to store a pointer to any private data the class driver wants to associate with the class device. This variable should not be accessed directly, but the class_set_devdata and class_get_devdata functions should be used to set and retrieve the value of this variable.

To register a properly set up struct class_device structure, the class_device_register function should be called. An example of how to initialize a struct class_device and register it with the driver core can be seen in the following code from the drivers/i2c/i2c-core.c file:

/* Add this adapter to the i2c_adapter class */
memset(&adap->class_dev, 0x00,
       sizeof(struct class_device));
adap->class_dev.dev = &adap->dev;
adap->class_dev.class = &i2c_adapter_class;
        adap->dev.bus_id, BUS_ID_SIZE);

First, the struct class_device variable (embedded in the struct i2c_adapter variable) is initialized to zero. All driver model structures need to have all variables set to zero before they are registered, in order for the driver core to use them properly.

Then the dev variable is set to point to the i2c_adapter's struct device variable; in this case, the same structure, struct i2c_adapter, contains both a struct device and a struct class_device. The class variable is set to the address of the i2c_adapter_class variable, and then the class_id variable is set to the same value as the device's bus_id. Because the i2c_adapter device's bus_id is unique, it also ensures that the i2c_adapter class_device's class_id is unique. Finally, the class device structure is registered with the kernel driver core by a call to the class_device_register function.

With the above code and two i2c adapters loaded on a test machine, the /sys/class/i2c_adapter tree might look like the following:

$ tree /sys/class/i2c-adapter/
|-- i2c-0
|   |-- device -> ../../../devices/pci0/00:07.3/i2c-0
|   `-- driver -> ../../../bus/i2c/drivers/i2c_adapter
`-- i2c-2
    |-- device -> ../../../devices/legacy/i2c-2
    `-- driver -> ../../../bus/i2c/drivers/i2c_adapter

As you can see by the above tree output, a device and driver symbolic link are created automatically by the driver core to point to the proper place within the sysfs tree that represents those values. If the dev pointer was not set to point to a struct device, those symbolic links would not have been created. If you look in the /sys/class/tty directory, the majority of those class device entries do not have a corresponding struct device, so those symbolic links are not present.

Class Interfaces

Class interfaces simply are a way for your code to be notified whenever a struct class_device is registered or unregistered from a specific class. A class interface is defined with the struct class_interface structure. This structure is simple and looks like:

struct class_interface {
    struct list_head  node;
    struct class      *class;
    int (*add)        (struct class_device *);
    void (*remove)    (struct class_device *);

The class variable needs to be set to the class about which we want to be notified. The add and remove variables should be set to a function that is called when any devices are added or removed, respectively, from that class. It is not necessary to set both the add and remove variables if you do not want to be notified about one of those events.

To register a class interface with the kernel, the class_interface_register function is called. Likewise, to unregister a class interface, the class_interface_unregister function is called. An example of code that uses class interfaces is the CPU frequency core; this code can be found at kernel/cpufreq.c in the kernel source tree.

Creating Files

As described above, the i2c-adapter class is useful for easily determining all of the different i2c adapters present in the system and their specific location in the driver tree. But i2c adapters are not directly addressable by a user. To talk to an i2c adapter, an i2c chip driver needs to be loaded, or the i2c-dev driver can be used. The i2c-dev driver provides a character driver interface to all i2c adapters present in the system. Because it is useful to determine exactly which i2c-dev devices are attached to which i2c adapters, a i2c-dev class was created:

static struct class i2c_dev_class = {
    .name	= "i2c-dev"

Then, when every i2c adapter is found by the i2c-dev driver, a new i2c class device is added to the driver core. This addition is done in the i2c_add_class_device function:

static void
i2c_add_class_device(char *name, int minor,
                     struct i2c_adapter *adap)
   struct i2c_dev *i2c_dev;
   int retval;

   i2c_dev = kmalloc(sizeof(*i2c_dev), GFP_KERNEL);
   if (!i2c_dev)
   memset(i2c_dev, 0x00, sizeof(*i2c_dev));

   if (adap->dev.parent == &legacy_bus)
       i2c_dev->class_dev.dev = &adap->dev;
       i2c_dev->class_dev.dev = adap->dev.parent;
   i2c_dev->class_dev.class = &i2c_dev_class;
            BUS_ID_SIZE, "%s", name);
   retval =
   if (retval)
       goto error;
   class_device_create_file (&i2c_dev->class_dev,
   i2c_dev->minor = minor;
   list_add(&i2c_dev->node, &i2c_dev_list);

This function looks almost like the i2c_adapter class registration code, with two exceptions. First, the class_dev.dev field is set to be either the adapter's parent device or the adapter's device. This is done because some i2c adapters do not have a real parent in the global kernel device tree, as they live on a bus that has not been converted to the kernel driver model (like ISA) or they do not really live on a bus at all (like some i2c embedded controllers). When an i2c adapter does not have a place in the kernel device tree, it is assigned to the legacy bus. The legacy bus, located at /sys/devices/legacy, is used for these kinds of devices.

The second thing that is different with this class device is the line:

class_device_create_file (&i2c_dev->class_dev, &class_device_attr_dev);

The class_device_create_file function is used to create a file in the class device's directory. The filename and attributes are defined with the CLASS_DEVICE_ATTR macro as:

static ssize_t
show_dev(struct class_device *class_dev, char *buf)
   struct i2c_dev *i2c_dev = to_i2c_dev(class_dev);
   return sprintf(buf, "%04x\n",
                  MKDEV(I2C_MAJOR, i2c_dev->minor));

The CLASS_DEVICE_ATTR macro is itself defined as:

#define CLASS_DEVICE_ATTR(_name,_mode,_show,_store) \
struct class_device_attribute                       \
class_device_attr_##_name = { 	                    \
    .attr  = {.name = __stringify(_name),           \
              .mode = _mode },                      \
    .show  = _show,                                 \
    .store = _store,                                \

The arguments within the CLASS_DEVICE_ATTR macro are:

  • _name: both the name of the file to be created in sysfs and part of the variable name that describes this whole attribute.

  • _mode: the file access mode with which the file is created. Use the standard access macros to specify the proper value.

  • _show: points to a function that is called when the file is read from. This function must have the following return value and parameters. This variable does not have to be set if the file is not to be read from.

    show (struct class_device *class_dev, char *buf);

  • _store: points to a function that is called when the file is written to. This function must have the following return value and paramaters. This variable does not have to be set if the file is not to be written to.

    store (struct device *dev, const char *buf,
           size_t count);

Almost all driver model structures have an ATTR() macro that declares a file within the sysfs tree.

In this example, a file named dev is created when the class_device_create_file function is called. This file is created to be read-only by any user. If the file is read from, the show_dev function is called by the driver core. The show_dev function fills in the buffer passed to it with the information it wants to give the user. In this case, the major and minor number for this specific device are passed to the user. All class devices using a major and minor number should have a dev file within their sysfs class device directory.

The class_device_remove_file function can be used to remove any files created by the class_device_create_file function. But it is not necessary to remove manually any file created if the device is about to be removed. When devices are removed from sysfs, all files created in their directories are removed automatically by the sysfs core. So, when the i2c-dev class device is removed from the system, all that is needed is the following:

static void
i2c_remove_class_device(int minor)
    struct i2c_dev *i2c_dev = NULL;
    struct list_head *tmp;
    int found = 0;

    list_for_each (tmp, &i2c_dev_list) {
        i2c_dev = list_entry(tmp, struct i2c_dev,
        if (i2c_dev->minor == minor) {
            found = 1;
    if (found) {
    } else {

What It All Looks Like

With the i2c-dev driver and two i2c adapter drivers (the i2c-piix4 and i2c-isa drivers) loaded, the /sys/class/i2c-dev directory might look like the following:

$ tree /sys/class/i2c-dev/
|-- i2c-0
|   |-- dev
|   |-- device -> ../../../devices/pci0/00:07.3
|   `-- driver -> ../../../bus/pci/drivers/piix4-smbus
`-- i2c-2
    |-- dev
    |-- device -> ../../../devices/legacy/i2c-2
    `-- driver -> ../../../bus/i2c/drivers/i2c_adapter

The dev file in the /sys/class/i2c-dev/i2c-2/ directory would contain the following string:

$ cat /sys/class/i2c-dev/i2c-2/dev

which corresponds to major number 86 and minor number 2, the character major and minor numbers for this specific device.

Also, the /sys/bus/i2c/ directory with a few i2c client drivers loaded looks like:

$ tree /sys/bus/i2c/
|-- devices
|   |-- 0-0050 -> ../../../devices/pci0/00:07.3/i2c-0/0-0050
|   |-- 0-0051 -> ../../../devices/pci0/00:07.3/i2c-0/0-0051
|   |-- 0-0052 -> ../../../devices/pci0/00:07.3/i2c-0/0-0052
|   |-- 0-0053 -> ../../../devices/pci0/00:07.3/i2c-0/0-0053
|   `-- 2-0290 -> ../../../devices/legacy/i2c-2/2-0290
`-- drivers
    |-- dev driver
    |-- eeprom
    |   |-- 0-0050 -> ../../../../devices/pci0/00:07.3/i2c-0/0-0050
    |   |-- 0-0051 -> ../../../../devices/pci0/00:07.3/i2c-0/0-0051
    |   |-- 0-0052 -> ../../../../devices/pci0/00:07.3/i2c-0/0-0052
    |   `-- 0-0053 -> ../../../../devices/pci0/00:07.3/i2c-0/0-0053
    |-- i2c_adapter
    `-- w83781d
        `-- 2-0290 -> ../../../../devices/legacy/i2c-2/2-0290

And, the actual /sys/devices/ directories for the i2c adapters look like:

$ tree /sys/devices/pci0/00:07.3
|-- class
|-- device
|-- i2c-0
|   |-- 0-0050
|   |   |-- eeprom_00
|   |   |-- name
|   |   `-- power
|   |-- 0-0051
|   |   |-- eeprom_00
|   |   |-- name
|   |   `-- power
|   |-- 0-0052
|   |   |-- eeprom_00
|   |   |-- name
|   |   `-- power
|   |-- 0-0053
|   |   |-- eeprom_00
|   |   |-- name
|   |   `-- power
|   |-- name
|   `-- power
|-- irq
|-- name
|-- power
|-- resource
|-- subsystem_device
|-- subsystem_vendor
`-- vendor


$ tree /sys/devices/legacy/i2c-2/
|-- 2-0290
|   |-- alarms
|   |-- beep_enable
|   |-- beep_mask
|   |-- fan_div1
|   |-- fan_div2
|   |-- fan_div3
|   |-- fan_input1
|   |-- fan_input2
|   |-- fan_input3
|   |-- fan_min1
|   |-- fan_min2
|   |-- fan_min3
|   |-- in_input0
|   |-- in_input1
|   |-- in_input2
|   |-- in_input3
|   |-- in_input4
|   |-- in_input5
|   |-- in_input6
|   |-- in_input7
|   |-- in_input8
|   |-- in_max0
|   |-- in_max1
|   |-- in_max2
|   |-- in_max3
|   |-- in_max4
|   |-- in_max5
|   |-- in_max6
|   |-- in_max7
|   |-- in_max8
|   |-- in_min0
|   |-- in_min1
|   |-- in_min2
|   |-- in_min3
|   |-- in_min4
|   |-- in_min5
|   |-- in_min6
|   |-- in_min7
|   |-- in_min8
|   |-- name
|   |-- power
|   |-- pwm1
|   |-- pwm2
|   |-- pwm_enable2
|   |-- sensor1
|   |-- sensor2
|   |-- sensor3
|   |-- temp_input1
|   |-- temp_input2
|   |-- temp_input3
|   |-- temp_max1
|   |-- temp_max2
|   |-- temp_max3
|   |-- temp_min1
|   |-- temp_min2
|   |-- temp_min3
|   |-- vid
|   `-- vrm
|-- name
`-- power

I think the best description of the kernel driver model's use of interconnected structure pointers and representation to the user was issued by Jonathan Corbet: “web woven by a spider on drugs” (lwn.net/Articles/31185/). Hopefully, these two articles have helped you unravel the loony web, showing the true interconnectedness of all devices within the kernel.


I would like to thank Pat Mochel for creating such a powerful and complete framework in which all kernel drivers and devices easily can be shown to the user. Also, a big thanks to all of the kernel driver subsystem maintainers who have gladly converted their subsystems over to this model; without their help, the driver core code would have been little more than a nice academic exercise.

Greg Kroah-Hartman is currently the Linux USB and PCI Hot Plug kernel maintainer. He works for IBM, doing various Linux kernel-related things and can be reached at greg@kroah.com.

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