b6badddccc
Fix a rather obvious typo. Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
107 lines
3.5 KiB
Text
107 lines
3.5 KiB
Text
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The Basic Device Structure
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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See the kerneldoc for the struct 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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device_create(&mydriver_class, dev, chrdev, &private_data, "my_name");
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