829f4c362f
No need to be be, just be should be sufficient. Signed-off-by: Laurent Navet <laurent.navet@gmail.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
106 lines
3.2 KiB
Text
106 lines
3.2 KiB
Text
|
|
The Basic Device Structure
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
See the kerneldoc for the struct 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 by a device driver through sysfs.
|
|
|
|
Please see Documentation/filesystems/sysfs.txt for more information
|
|
on how sysfs works.
|
|
|
|
As explained in Documentation/kobject.txt, device attributes must be
|
|
created before the KOBJ_ADD uevent is generated. The only way to realize
|
|
that is by defining an attribute group.
|
|
|
|
Attributes are declared using a macro called DEVICE_ATTR:
|
|
|
|
#define DEVICE_ATTR(name,mode,show,store)
|
|
|
|
Example:
|
|
|
|
static DEVICE_ATTR(type, 0444, show_type, NULL);
|
|
static DEVICE_ATTR(power, 0644, show_power, store_power);
|
|
|
|
This declares two structures of type struct device_attribute with respective
|
|
names 'dev_attr_type' and 'dev_attr_power'. These two attributes can be
|
|
organized as follows into a group:
|
|
|
|
static struct attribute *dev_attrs[] = {
|
|
&dev_attr_type.attr,
|
|
&dev_attr_power.attr,
|
|
NULL,
|
|
};
|
|
|
|
static struct attribute_group dev_attr_group = {
|
|
.attrs = dev_attrs,
|
|
};
|
|
|
|
static const struct attribute_group *dev_attr_groups[] = {
|
|
&dev_attr_group,
|
|
NULL,
|
|
};
|
|
|
|
This array of groups can then be associated with a device by setting the
|
|
group pointer in struct device before device_register() is invoked:
|
|
|
|
dev->groups = dev_attr_groups;
|
|
device_register(dev);
|
|
|
|
The device_register() function will use the 'groups' pointer to create the
|
|
device attributes and the device_unregister() function will use this pointer
|
|
to remove the device attributes.
|
|
|
|
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.
|