[net-next-2.6.git] / Documentation / driver-model / device.txt


The Basic Device Structure
~~~~~~~~~~~~~~~~~~~~~~~~~~

struct device {
        struct list_head g_list;
        struct list_head node;
        struct list_head bus_list;
        struct list_head driver_list;
        struct list_head intf_list;
        struct list_head children;
        struct device   * parent;

        char    name[DEVICE_NAME_SIZE];
        char    bus_id[BUS_ID_SIZE];

        spinlock_t      lock;
        atomic_t        refcount;

        struct bus_type * bus;
        struct driver_dir_entry dir;

	u32		class_num;

        struct device_driver *driver;
        void            *driver_data;
        void            *platform_data;

        u32             current_state;
        unsigned char *saved_state;

        void    (*release)(struct device * dev);
};

Fields 
~~~~~~
g_list:	Node in the global device list.

node:	Node in device's parent's children list.

bus_list: Node in device's bus's devices list.

driver_list:   Node in device's driver's devices list.

intf_list:     List of intf_data. There is one structure allocated for
	       each interface that the device supports.

children:      List of child devices.

parent:        *** FIXME ***

name:	       ASCII description of device. 
	       Example: " 3Com Corporation 3c905 100BaseTX [Boomerang]"

bus_id:	       ASCII representation of device's bus position. This 
	       field should be a name unique across all devices on the
	       bus type the device belongs to. 

	       Example: PCI bus_ids are in the form of
	       <bus number>:<slot number>.<function number> 
	       This name is unique across all PCI devices in the system.

lock:	       Spinlock for the device. 

refcount:      Reference count on the device.

bus:	       Pointer to struct bus_type that device belongs to.

dir:	       Device's sysfs directory.

class_num:     Class-enumerated value of the device.

driver:	       Pointer to struct device_driver that controls the device.

driver_data:   Driver-specific data.

platform_data: Platform data specific to the device.

	       Example:  for devices on custom boards, as typical of embedded
	       and SOC based hardware, Linux often uses platform_data to point
	       to board-specific structures describing devices and how they
	       are wired.  That can include what ports are available, chip
	       variants, which GPIO pins act in what additional roles, and so
	       on.  This shrinks the "Board Support Packages" (BSPs) and
	       minimizes board-specific #ifdefs in drivers.

current_state: Current power state of the device.

saved_state:   Pointer to saved state of the device. This is usable by
	       the device driver controlling the device.

release:       Callback to free the device after all references have 
	       gone away. This should be set by the allocator of the 
	       device (i.e. the bus driver that discovered the device).


Programming Interface
~~~~~~~~~~~~~~~~~~~~~
The bus driver that discovers the device uses this to register the
device with the core:

int device_register(struct device * dev);

The bus should initialize the following fields:

    - parent
    - name
    - bus_id
    - bus

A device is removed from the core when its reference count goes to
0. The reference count can be adjusted using:

struct device * get_device(struct device * dev);
void put_device(struct device * dev);

get_device() will return a pointer to the struct device passed to it
if the reference is not already 0 (if it's in the process of being
removed already).

A driver can access the lock in the device structure using: 

void lock_device(struct device * dev);
void unlock_device(struct device * dev);


Attributes
~~~~~~~~~~
struct device_attribute {
	struct attribute	attr;
	ssize_t (*show)(struct device *dev, struct device_attribute *attr,
			char *buf);
	ssize_t (*store)(struct device *dev, struct device_attribute *attr,
			 const char *buf, size_t count);
};

Attributes of devices can be exported via drivers using a simple
procfs-like interface. 

Please see Documentation/filesystems/sysfs.txt for more information
on how sysfs works.

Attributes are declared using a macro called DEVICE_ATTR:

#define DEVICE_ATTR(name,mode,show,store)

Example:

DEVICE_ATTR(power,0644,show_power,store_power);

This declares a structure of type struct device_attribute named
'dev_attr_power'. This can then be added and removed to the device's
directory using:

int device_create_file(struct device *device, struct device_attribute * entry);
void device_remove_file(struct device * dev, struct device_attribute * attr);

Example:

device_create_file(dev,&dev_attr_power);
device_remove_file(dev,&dev_attr_power);

The file name will be 'power' with a mode of 0644 (-rw-r--r--).

Word of warning:  While the kernel allows device_create_file() and
device_remove_file() to be called on a device at any time, userspace has
strict expectations on when attributes get created.  When a new device is
registered in the kernel, a uevent is generated to notify userspace (like
udev) that a new device is available.  If attributes are added after the
device is registered, then userspace won't get notified and userspace will
not know about the new attributes.

This is important for device driver that need to publish additional
attributes for a device at driver probe time.  If the device driver simply
calls device_create_file() on the device structure passed to it, then
userspace will never be notified of the new attributes.  Instead, it should
probably use class_create() and class->dev_attrs to set up a list of
desired attributes in the modules_init function, and then in the .probe()
hook, and then use device_create() to create a new device as a child
of the probed device.  The new device will generate a new uevent and
properly advertise the new attributes to userspace.

For example, if a driver wanted to add the following attributes:
struct device_attribute mydriver_attribs[] = {
	__ATTR(port_count, 0444, port_count_show),
	__ATTR(serial_number, 0444, serial_number_show),
	NULL
};

Then in the module init function is would do:
	mydriver_class = class_create(THIS_MODULE, "my_attrs");
	mydriver_class.dev_attr = mydriver_attribs;

And assuming 'dev' is the struct device passed into the probe hook, the driver
probe function would do something like:
	create_device(&mydriver_class, dev, chrdev, &private_data, "my_name");
Commit	Line	Data
1da177e4 LT	1
	2	The Basic Device Structure
	3	~~~~~~~~~~~~~~~~~~~~~~~~~~
	4
	5	struct device {
	6	struct list_head g_list;
	7	struct list_head node;
	8	struct list_head bus_list;
	9	struct list_head driver_list;
	10	struct list_head intf_list;
	11	struct list_head children;
	12	struct device * parent;
	13
	14	char name[DEVICE_NAME_SIZE];
	15	char bus_id[BUS_ID_SIZE];
	16
	17	spinlock_t lock;
	18	atomic_t refcount;
	19
	20	struct bus_type * bus;
	21	struct driver_dir_entry dir;
	22
	23	u32 class_num;
	24
	25	struct device_driver *driver;
	26	void *driver_data;
	27	void *platform_data;
	28
	29	u32 current_state;
	30	unsigned char *saved_state;
	31
	32	void (release)(struct device dev);
	33	};
	34
	35	Fields
	36	~~~~~~
	37	g_list: Node in the global device list.
	38
	39	node: Node in device's parent's children list.
	40
	41	bus_list: Node in device's bus's devices list.
	42
	43	driver_list: Node in device's driver's devices list.
	44
	45	intf_list: List of intf_data. There is one structure allocated for
	46	each interface that the device supports.
	47
	48	children: List of child devices.
	49
	50	parent: * FIXME *
	51
	52	name: ASCII description of device.
	53	Example: " 3Com Corporation 3c905 100BaseTX [Boomerang]"
	54
	55	bus_id: ASCII representation of device's bus position. This
	56	field should be a name unique across all devices on the
	57	bus type the device belongs to.
	58
	59	Example: PCI bus_ids are in the form of
	60	<bus number>:<slot number>.<function number>
	61	This name is unique across all PCI devices in the system.
	62
	63	lock: Spinlock for the device.
	64
65	refcount: Reference count on the device.
66
67	bus: Pointer to struct bus_type that device belongs to.
68
69	dir: Device's sysfs directory.
70
71	class_num: Class-enumerated value of the device.
72
73	driver: Pointer to struct device_driver that controls the device.
74
75	driver_data: Driver-specific data.
76
77	platform_data: Platform data specific to the device.
78
4109aca0 DB	79	Example: for devices on custom boards, as typical of embedded
	80	and SOC based hardware, Linux often uses platform_data to point
	81	to board-specific structures describing devices and how they
	82	are wired. That can include what ports are available, chip
	83	variants, which GPIO pins act in what additional roles, and so
	84	on. This shrinks the "Board Support Packages" (BSPs) and
	85	minimizes board-specific #ifdefs in drivers.
	86
1da177e4 LT	87	current_state: Current power state of the device.
	88
	89	saved_state: Pointer to saved state of the device. This is usable by
	90	the device driver controlling the device.
	91
	92	release: Callback to free the device after all references have
	93	gone away. This should be set by the allocator of the
	94	device (i.e. the bus driver that discovered the device).
	95
	96
	97	Programming Interface
	98	~~~~~~~~~~~~~~~~~~~~~
	99	The bus driver that discovers the device uses this to register the
	100	device with the core:
	101
	102	int device_register(struct device * dev);
	103
	104	The bus should initialize the following fields:
	105
	106	- parent
	107	- name
	108	- bus_id
	109	- bus
	110
	111	A device is removed from the core when its reference count goes to
	112	0. The reference count can be adjusted using:
	113
	114	struct device * get_device(struct device * dev);
	115	void put_device(struct device * dev);
	116
	117	get_device() will return a pointer to the struct device passed to it
	118	if the reference is not already 0 (if it's in the process of being
	119	removed already).
	120
	121	A driver can access the lock in the device structure using:
	122
	123	void lock_device(struct device * dev);
	124	void unlock_device(struct device * dev);
	125
	126
	127	Attributes
	128	~~~~~~~~~~
	129	struct device_attribute {
245127db MM	130	struct attribute attr;
	131	ssize_t (show)(struct device dev, struct device_attribute *attr,
	132	char *buf);
	133	ssize_t (store)(struct device dev, struct device_attribute *attr,
	134	const char *buf, size_t count);
1da177e4 LT	135	};
	136
	137	Attributes of devices can be exported via drivers using a simple
	138	procfs-like interface.
	139
	140	Please see Documentation/filesystems/sysfs.txt for more information
	141	on how sysfs works.
	142
	143	Attributes are declared using a macro called DEVICE_ATTR:
	144
	145	#define DEVICE_ATTR(name,mode,show,store)
	146
	147	Example:
	148
	149	DEVICE_ATTR(power,0644,show_power,store_power);
	150
	151	This declares a structure of type struct device_attribute named
	152	'dev_attr_power'. This can then be added and removed to the device's
	153	directory using:
	154
	155	int device_create_file(struct device device, struct device_attribute entry);
	156	void device_remove_file(struct device * dev, struct device_attribute * attr);
	157
	158	Example:
	159
	160	device_create_file(dev,&dev_attr_power);
	161	device_remove_file(dev,&dev_attr_power);
	162
	163	The file name will be 'power' with a mode of 0644 (-rw-r--r--).
	164
b22813b3 GL	165	Word of warning: While the kernel allows device_create_file() and
	166	device_remove_file() to be called on a device at any time, userspace has
	167	strict expectations on when attributes get created. When a new device is
	168	registered in the kernel, a uevent is generated to notify userspace (like
	169	udev) that a new device is available. If attributes are added after the
	170	device is registered, then userspace won't get notified and userspace will
	171	not know about the new attributes.
	172
	173	This is important for device driver that need to publish additional
	174	attributes for a device at driver probe time. If the device driver simply
	175	calls device_create_file() on the device structure passed to it, then
	176	userspace will never be notified of the new attributes. Instead, it should
	177	probably use class_create() and class->dev_attrs to set up a list of
	178	desired attributes in the modules_init function, and then in the .probe()
	179	hook, and then use device_create() to create a new device as a child
	180	of the probed device. The new device will generate a new uevent and
	181	properly advertise the new attributes to userspace.
	182
	183	For example, if a driver wanted to add the following attributes:
	184	struct device_attribute mydriver_attribs[] = {
	185	__ATTR(port_count, 0444, port_count_show),
	186	__ATTR(serial_number, 0444, serial_number_show),
	187	NULL
	188	};
	189
	190	Then in the module init function is would do:
	191	mydriver_class = class_create(THIS_MODULE, "my_attrs");
	192	mydriver_class.dev_attr = mydriver_attribs;
	193
	194	And assuming 'dev' is the struct device passed into the probe hook, the driver
	195	probe function would do something like:
	196	create_device(&mydriver_class, dev, chrdev, &private_data, "my_name");