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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 18:49:45 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-07 18:49:45 +0000
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Adding upstream version 6.1.76.upstream/6.1.76upstream
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+==========================
+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.rst for more information
+on how sysfs works.
+
+As explained in Documentation/core-api/kobject.rst, 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, type_show, NULL);
+ static DEVICE_ATTR(power, 0644, power_show, power_store);
+
+Helper macros are available for common values of mode, so the above examples
+can be simplified to:::
+
+ static DEVICE_ATTR_RO(type);
+ static DEVICE_ATTR_RW(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_group = {
+ .attrs = dev_attrs,
+ };
+
+ static const struct attribute_group *dev_groups[] = {
+ &dev_group,
+ NULL,
+ };
+
+A helper macro is available for the common case of a single group, so the
+above two structures can be declared using:::
+
+ ATTRIBUTE_GROUPS(dev);
+
+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_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.