1ACPI Scan Handlers
2
3Copyright (C) 2012, Intel Corporation
4Author: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
5
6During system initialization and ACPI-based device hot-add, the ACPI namespace
7is scanned in search of device objects that generally represent various pieces
8of hardware.  This causes a struct acpi_device object to be created and
9registered with the driver core for every device object in the ACPI namespace
10and the hierarchy of those struct acpi_device objects reflects the namespace
11layout (i.e. parent device objects in the namespace are represented by parent
12struct acpi_device objects and analogously for their children).  Those struct
13acpi_device objects are referred to as "device nodes" in what follows, but they
14should not be confused with struct device_node objects used by the Device Trees
15parsing code (although their role is analogous to the role of those objects).
16
17During ACPI-based device hot-remove device nodes representing pieces of hardware
18being removed are unregistered and deleted.
19
20The core ACPI namespace scanning code in drivers/acpi/scan.c carries out basic
21initialization of device nodes, such as retrieving common configuration
22information from the device objects represented by them and populating them with
23appropriate data, but some of them require additional handling after they have
24been registered.  For example, if the given device node represents a PCI host
25bridge, its registration should cause the PCI bus under that bridge to be
26enumerated and PCI devices on that bus to be registered with the driver core.
27Similarly, if the device node represents a PCI interrupt link, it is necessary
28to configure that link so that the kernel can use it.
29
30Those additional configuration tasks usually depend on the type of the hardware
31component represented by the given device node which can be determined on the
32basis of the device node's hardware ID (HID).  They are performed by objects
33called ACPI scan handlers represented by the following structure:
34
35struct acpi_scan_handler {
36	const struct acpi_device_id *ids;
37	struct list_head list_node;
38	int (*attach)(struct acpi_device *dev, const struct acpi_device_id *id);
39	void (*detach)(struct acpi_device *dev);
40};
41
42where ids is the list of IDs of device nodes the given handler is supposed to
43take care of, list_node is the hook to the global list of ACPI scan handlers
44maintained by the ACPI core and the .attach() and .detach() callbacks are
45executed, respectively, after registration of new device nodes and before
46unregistration of device nodes the handler attached to previously.
47
48The namespace scanning function, acpi_bus_scan(), first registers all of the
49device nodes in the given namespace scope with the driver core.  Then, it tries
50to match a scan handler against each of them using the ids arrays of the
51available scan handlers.  If a matching scan handler is found, its .attach()
52callback is executed for the given device node.  If that callback returns 1,
53that means that the handler has claimed the device node and is now responsible
54for carrying out any additional configuration tasks related to it.  It also will
55be responsible for preparing the device node for unregistration in that case.
56The device node's handler field is then populated with the address of the scan
57handler that has claimed it.
58
59If the .attach() callback returns 0, it means that the device node is not
60interesting to the given scan handler and may be matched against the next scan
61handler in the list.  If it returns a (negative) error code, that means that
62the namespace scan should be terminated due to a serious error.  The error code
63returned should then reflect the type of the error.
64
65The namespace trimming function, acpi_bus_trim(), first executes .detach()
66callbacks from the scan handlers of all device nodes in the given namespace
67scope (if they have scan handlers).  Next, it unregisters all of the device
68nodes in that scope.
69
70ACPI scan handlers can be added to the list maintained by the ACPI core with the
71help of the acpi_scan_add_handler() function taking a pointer to the new scan
72handler as an argument.  The order in which scan handlers are added to the list
73is the order in which they are matched against device nodes during namespace
74scans.
75
76All scan handles must be added to the list before acpi_bus_scan() is run for the
77first time and they cannot be removed from it.
78