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196 lines
6.2 KiB
196 lines
6.2 KiB
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The Basic Device Structure
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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struct device {
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struct list_head g_list;
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struct list_head node;
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struct list_head bus_list;
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struct list_head driver_list;
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struct list_head intf_list;
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struct list_head children;
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struct device * parent;
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char name[DEVICE_NAME_SIZE];
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char bus_id[BUS_ID_SIZE];
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spinlock_t lock;
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atomic_t refcount;
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struct bus_type * bus;
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struct driver_dir_entry dir;
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u32 class_num;
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struct device_driver *driver;
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void *driver_data;
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void *platform_data;
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u32 current_state;
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unsigned char *saved_state;
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void (*release)(struct device * dev);
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};
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Fields
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~~~~~~
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g_list: Node in the global device list.
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node: Node in device's parent's children list.
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bus_list: Node in device's bus's devices list.
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driver_list: Node in device's driver's devices list.
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intf_list: List of intf_data. There is one structure allocated for
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each interface that the device supports.
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children: List of child devices.
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parent: *** FIXME ***
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name: ASCII description of device.
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Example: " 3Com Corporation 3c905 100BaseTX [Boomerang]"
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bus_id: ASCII representation of device's bus position. This
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field should be a name unique across all devices on the
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bus type the device belongs to.
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Example: PCI bus_ids are in the form of
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<bus number>:<slot number>.<function number>
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This name is unique across all PCI devices in the system.
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lock: Spinlock for the device.
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refcount: Reference count on the device.
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bus: Pointer to struct bus_type that device belongs to.
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dir: Device's sysfs directory.
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class_num: Class-enumerated value of the device.
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driver: Pointer to struct device_driver that controls the device.
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driver_data: Driver-specific data.
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platform_data: Platform data specific to the device.
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Example: for devices on custom boards, as typical of embedded
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and SOC based hardware, Linux often uses platform_data to point
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to board-specific structures describing devices and how they
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are wired. That can include what ports are available, chip
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variants, which GPIO pins act in what additional roles, and so
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on. This shrinks the "Board Support Packages" (BSPs) and
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minimizes board-specific #ifdefs in drivers.
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current_state: Current power state of the device.
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saved_state: Pointer to saved state of the device. This is usable by
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the device driver controlling the device.
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release: Callback to free the device after all references have
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gone away. This should be set by the allocator of the
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device (i.e. the bus driver that discovered the device).
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Programming Interface
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~~~~~~~~~~~~~~~~~~~~~
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The bus driver that discovers the device uses this to register the
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device with the core:
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int device_register(struct device * dev);
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The bus should initialize the following fields:
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- parent
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- name
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- bus_id
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- bus
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A device is removed from the core when its reference count goes to
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0. The reference count can be adjusted using:
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struct device * get_device(struct device * dev);
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void put_device(struct device * dev);
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get_device() will return a pointer to the struct device passed to it
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if the reference is not already 0 (if it's in the process of being
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removed already).
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A driver can access the lock in the device structure using:
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void lock_device(struct device * dev);
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void unlock_device(struct device * dev);
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Attributes
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~~~~~~~~~~
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struct device_attribute {
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struct attribute attr;
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ssize_t (*show)(struct device *dev, struct device_attribute *attr,
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char *buf);
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ssize_t (*store)(struct device *dev, struct device_attribute *attr,
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const char *buf, size_t count);
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};
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Attributes of devices can be exported via drivers using a simple
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procfs-like interface.
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Please see Documentation/filesystems/sysfs.txt for more information
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on how sysfs works.
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Attributes are declared using a macro called DEVICE_ATTR:
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#define DEVICE_ATTR(name,mode,show,store)
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Example:
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DEVICE_ATTR(power,0644,show_power,store_power);
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This declares a structure of type struct device_attribute named
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'dev_attr_power'. This can then be added and removed to the device's
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directory using:
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int device_create_file(struct device *device, struct device_attribute * entry);
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void device_remove_file(struct device * dev, struct device_attribute * attr);
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Example:
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device_create_file(dev,&dev_attr_power);
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device_remove_file(dev,&dev_attr_power);
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The file name will be 'power' with a mode of 0644 (-rw-r--r--).
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Word of warning: While the kernel allows device_create_file() and
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device_remove_file() to be called on a device at any time, userspace has
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strict expectations on when attributes get created. When a new device is
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registered in the kernel, a uevent is generated to notify userspace (like
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udev) that a new device is available. If attributes are added after the
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device is registered, then userspace won't get notified and userspace will
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not know about the new attributes.
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This is important for device driver that need to publish additional
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attributes for a device at driver probe time. If the device driver simply
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calls device_create_file() on the device structure passed to it, then
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userspace will never be notified of the new attributes. Instead, it should
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probably use class_create() and class->dev_attrs to set up a list of
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desired attributes in the modules_init function, and then in the .probe()
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hook, and then use device_create() to create a new device as a child
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of the probed device. The new device will generate a new uevent and
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properly advertise the new attributes to userspace.
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For example, if a driver wanted to add the following attributes:
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struct device_attribute mydriver_attribs[] = {
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__ATTR(port_count, 0444, port_count_show),
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__ATTR(serial_number, 0444, serial_number_show),
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NULL
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};
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Then in the module init function is would do:
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mydriver_class = class_create(THIS_MODULE, "my_attrs");
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mydriver_class.dev_attr = mydriver_attribs;
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And assuming 'dev' is the struct device passed into the probe hook, the driver
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probe function would do something like:
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create_device(&mydriver_class, dev, chrdev, &private_data, "my_name");
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