1.. SPDX-License-Identifier: GPL-2.0
2.. include:: <isonum.txt>
3
4===================================================
5ACPI Device Tree - Representation of ACPI Namespace
6===================================================
7
8:Copyright: |copy| 2013, Intel Corporation
9
10:Author: Lv Zheng <lv.zheng@intel.com>
11
12:Credit:   Thanks for the help from Zhang Rui <rui.zhang@intel.com> and
13           Rafael J.Wysocki <rafael.j.wysocki@intel.com>.
14
15Abstract
16========
17The Linux ACPI subsystem converts ACPI namespace objects into a Linux
18device tree under the /sys/devices/LNXSYSTEM:00 and updates it upon
19receiving ACPI hotplug notification events.  For each device object
20in this hierarchy there is a corresponding symbolic link in the
21/sys/bus/acpi/devices.
22
23This document illustrates the structure of the ACPI device tree.
24
25ACPI Definition Blocks
26======================
27
28The ACPI firmware sets up RSDP (Root System Description Pointer) in the
29system memory address space pointing to the XSDT (Extended System
30Description Table).  The XSDT always points to the FADT (Fixed ACPI
31Description Table) using its first entry, the data within the FADT
32includes various fixed-length entries that describe fixed ACPI features
33of the hardware.  The FADT contains a pointer to the DSDT
34(Differentiated System Descripition Table).  The XSDT also contains
35entries pointing to possibly multiple SSDTs (Secondary System
36Description Table).
37
38The DSDT and SSDT data is organized in data structures called definition
39blocks that contain definitions of various objects, including ACPI
40control methods, encoded in AML (ACPI Machine Language).  The data block
41of the DSDT along with the contents of SSDTs represents a hierarchical
42data structure called the ACPI namespace whose topology reflects the
43structure of the underlying hardware platform.
44
45The relationships between ACPI System Definition Tables described above
46are illustrated in the following diagram::
47
48   +---------+    +-------+    +--------+    +------------------------+
49   |  RSDP   | +->| XSDT  | +->|  FADT  |    |  +-------------------+ |
50   +---------+ |  +-------+ |  +--------+  +-|->|       DSDT        | |
51   | Pointer | |  | Entry |-+  | ...... |  | |  +-------------------+ |
52   +---------+ |  +-------+    | X_DSDT |--+ |  | Definition Blocks | |
53   | Pointer |-+  | ..... |    | ...... |    |  +-------------------+ |
54   +---------+    +-------+    +--------+    |  +-------------------+ |
55                  | Entry |------------------|->|       SSDT        | |
56                  +- - - -+                  |  +-------------------| |
57                  | Entry | - - - - - - - -+ |  | Definition Blocks | |
58                  +- - - -+                | |  +-------------------+ |
59                                           | |  +- - - - - - - - - -+ |
60                                           +-|->|       SSDT        | |
61                                             |  +-------------------+ |
62                                             |  | Definition Blocks | |
63                                             |  +- - - - - - - - - -+ |
64                                             +------------------------+
65                                                          |
66                                             OSPM Loading |
67                                                         \|/
68                                                   +----------------+
69                                                   | ACPI Namespace |
70                                                   +----------------+
71
72                  Figure 1. ACPI Definition Blocks
73
74.. note:: RSDP can also contain a pointer to the RSDT (Root System
75   Description Table).  Platforms provide RSDT to enable
76   compatibility with ACPI 1.0 operating systems.  The OS is expected
77   to use XSDT, if present.
78
79
80Example ACPI Namespace
81======================
82
83All definition blocks are loaded into a single namespace.  The namespace
84is a hierarchy of objects identified by names and paths.
85The following naming conventions apply to object names in the ACPI
86namespace:
87
88   1. All names are 32 bits long.
89   2. The first byte of a name must be one of 'A' - 'Z', '_'.
90   3. Each of the remaining bytes of a name must be one of 'A' - 'Z', '0'
91      - '9', '_'.
92   4. Names starting with '_' are reserved by the ACPI specification.
93   5. The '\' symbol represents the root of the namespace (i.e. names
94      prepended with '\' are relative to the namespace root).
95   6. The '^' symbol represents the parent of the current namespace node
96      (i.e. names prepended with '^' are relative to the parent of the
97      current namespace node).
98
99The figure below shows an example ACPI namespace::
100
101   +------+
102   | \    |                     Root
103   +------+
104     |
105     | +------+
106     +-| _PR  |                 Scope(_PR): the processor namespace
107     | +------+
108     |   |
109     |   | +------+
110     |   +-| CPU0 |             Processor(CPU0): the first processor
111     |     +------+
112     |
113     | +------+
114     +-| _SB  |                 Scope(_SB): the system bus namespace
115     | +------+
116     |   |
117     |   | +------+
118     |   +-| LID0 |             Device(LID0); the lid device
119     |   | +------+
120     |   |   |
121     |   |   | +------+
122     |   |   +-| _HID |         Name(_HID, "PNP0C0D"): the hardware ID
123     |   |   | +------+
124     |   |   |
125     |   |   | +------+
126     |   |   +-| _STA |         Method(_STA): the status control method
127     |   |     +------+
128     |   |
129     |   | +------+
130     |   +-| PCI0 |             Device(PCI0); the PCI root bridge
131     |     +------+
132     |       |
133     |       | +------+
134     |       +-| _HID |         Name(_HID, "PNP0A08"): the hardware ID
135     |       | +------+
136     |       |
137     |       | +------+
138     |       +-| _CID |         Name(_CID, "PNP0A03"): the compatible ID
139     |       | +------+
140     |       |
141     |       | +------+
142     |       +-| RP03 |         Scope(RP03): the PCI0 power scope
143     |       | +------+
144     |       |   |
145     |       |   | +------+
146     |       |   +-| PXP3 |     PowerResource(PXP3): the PCI0 power resource
147     |       |     +------+
148     |       |
149     |       | +------+
150     |       +-| GFX0 |         Device(GFX0): the graphics adapter
151     |         +------+
152     |           |
153     |           | +------+
154     |           +-| _ADR |     Name(_ADR, 0x00020000): the PCI bus address
155     |           | +------+
156     |           |
157     |           | +------+
158     |           +-| DD01 |     Device(DD01): the LCD output device
159     |             +------+
160     |               |
161     |               | +------+
162     |               +-| _BCL | Method(_BCL): the backlight control method
163     |                 +------+
164     |
165     | +------+
166     +-| _TZ  |                 Scope(_TZ): the thermal zone namespace
167     | +------+
168     |   |
169     |   | +------+
170     |   +-| FN00 |             PowerResource(FN00): the FAN0 power resource
171     |   | +------+
172     |   |
173     |   | +------+
174     |   +-| FAN0 |             Device(FAN0): the FAN0 cooling device
175     |   | +------+
176     |   |   |
177     |   |   | +------+
178     |   |   +-| _HID |         Name(_HID, "PNP0A0B"): the hardware ID
179     |   |     +------+
180     |   |
181     |   | +------+
182     |   +-| TZ00 |             ThermalZone(TZ00); the FAN thermal zone
183     |     +------+
184     |
185     | +------+
186     +-| _GPE |                 Scope(_GPE): the GPE namespace
187       +------+
188
189                     Figure 2. Example ACPI Namespace
190
191
192Linux ACPI Device Objects
193=========================
194
195The Linux kernel's core ACPI subsystem creates struct acpi_device
196objects for ACPI namespace objects representing devices, power resources
197processors, thermal zones.  Those objects are exported to user space via
198sysfs as directories in the subtree under /sys/devices/LNXSYSTM:00.  The
199format of their names is <bus_id:instance>, where 'bus_id' refers to the
200ACPI namespace representation of the given object and 'instance' is used
201for distinguishing different object of the same 'bus_id' (it is
202two-digit decimal representation of an unsigned integer).
203
204The value of 'bus_id' depends on the type of the object whose name it is
205part of as listed in the table below::
206
207                +---+-----------------+-------+----------+
208                |   | Object/Feature  | Table | bus_id   |
209                +---+-----------------+-------+----------+
210                | N | Root            | xSDT  | LNXSYSTM |
211                +---+-----------------+-------+----------+
212                | N | Device          | xSDT  | _HID     |
213                +---+-----------------+-------+----------+
214                | N | Processor       | xSDT  | LNXCPU   |
215                +---+-----------------+-------+----------+
216                | N | ThermalZone     | xSDT  | LNXTHERM |
217                +---+-----------------+-------+----------+
218                | N | PowerResource   | xSDT  | LNXPOWER |
219                +---+-----------------+-------+----------+
220                | N | Other Devices   | xSDT  | device   |
221                +---+-----------------+-------+----------+
222                | F | PWR_BUTTON      | FADT  | LNXPWRBN |
223                +---+-----------------+-------+----------+
224                | F | SLP_BUTTON      | FADT  | LNXSLPBN |
225                +---+-----------------+-------+----------+
226                | M | Video Extension | xSDT  | LNXVIDEO |
227                +---+-----------------+-------+----------+
228                | M | ATA Controller  | xSDT  | LNXIOBAY |
229                +---+-----------------+-------+----------+
230                | M | Docking Station | xSDT  | LNXDOCK  |
231                +---+-----------------+-------+----------+
232
233                 Table 1. ACPI Namespace Objects Mapping
234
235The following rules apply when creating struct acpi_device objects on
236the basis of the contents of ACPI System Description Tables (as
237indicated by the letter in the first column and the notation in the
238second column of the table above):
239
240   N:
241      The object's source is an ACPI namespace node (as indicated by the
242      named object's type in the second column).  In that case the object's
243      directory in sysfs will contain the 'path' attribute whose value is
244      the full path to the node from the namespace root.
245   F:
246      The struct acpi_device object is created for a fixed hardware
247      feature (as indicated by the fixed feature flag's name in the second
248      column), so its sysfs directory will not contain the 'path'
249      attribute.
250   M:
251      The struct acpi_device object is created for an ACPI namespace node
252      with specific control methods (as indicated by the ACPI defined
253      device's type in the second column).  The 'path' attribute containing
254      its namespace path will be present in its sysfs directory.  For
255      example, if the _BCL method is present for an ACPI namespace node, a
256      struct acpi_device object with LNXVIDEO 'bus_id' will be created for
257      it.
258
259The third column of the above table indicates which ACPI System
260Description Tables contain information used for the creation of the
261struct acpi_device objects represented by the given row (xSDT means DSDT
262or SSDT).
263
264The fourth column of the above table indicates the 'bus_id' generation
265rule of the struct acpi_device object:
266
267   _HID:
268      _HID in the last column of the table means that the object's bus_id
269      is derived from the _HID/_CID identification objects present under
270      the corresponding ACPI namespace node. The object's sysfs directory
271      will then contain the 'hid' and 'modalias' attributes that can be
272      used to retrieve the _HID and _CIDs of that object.
273   LNXxxxxx:
274      The 'modalias' attribute is also present for struct acpi_device
275      objects having bus_id of the "LNXxxxxx" form (pseudo devices), in
276      which cases it contains the bus_id string itself.
277   device:
278      'device' in the last column of the table indicates that the object's
279      bus_id cannot be determined from _HID/_CID of the corresponding
280      ACPI namespace node, although that object represents a device (for
281      example, it may be a PCI device with _ADR defined and without _HID
282      or _CID).  In that case the string 'device' will be used as the
283      object's bus_id.
284
285
286Linux ACPI Physical Device Glue
287===============================
288
289ACPI device (i.e. struct acpi_device) objects may be linked to other
290objects in the Linux' device hierarchy that represent "physical" devices
291(for example, devices on the PCI bus).  If that happens, it means that
292the ACPI device object is a "companion" of a device otherwise
293represented in a different way and is used (1) to provide configuration
294information on that device which cannot be obtained by other means and
295(2) to do specific things to the device with the help of its ACPI
296control methods.  One ACPI device object may be linked this way to
297multiple "physical" devices.
298
299If an ACPI device object is linked to a "physical" device, its sysfs
300directory contains the "physical_node" symbolic link to the sysfs
301directory of the target device object.  In turn, the target device's
302sysfs directory will then contain the "firmware_node" symbolic link to
303the sysfs directory of the companion ACPI device object.
304The linking mechanism relies on device identification provided by the
305ACPI namespace.  For example, if there's an ACPI namespace object
306representing a PCI device (i.e. a device object under an ACPI namespace
307object representing a PCI bridge) whose _ADR returns 0x00020000 and the
308bus number of the parent PCI bridge is 0, the sysfs directory
309representing the struct acpi_device object created for that ACPI
310namespace object will contain the 'physical_node' symbolic link to the
311/sys/devices/pci0000:00/0000:00:02:0/ sysfs directory of the
312corresponding PCI device.
313
314The linking mechanism is generally bus-specific.  The core of its
315implementation is located in the drivers/acpi/glue.c file, but there are
316complementary parts depending on the bus types in question located
317elsewhere.  For example, the PCI-specific part of it is located in
318drivers/pci/pci-acpi.c.
319
320
321Example Linux ACPI Device Tree
322=================================
323
324The sysfs hierarchy of struct acpi_device objects corresponding to the
325example ACPI namespace illustrated in Figure 2 with the addition of
326fixed PWR_BUTTON/SLP_BUTTON devices is shown below::
327
328   +--------------+---+-----------------+
329   | LNXSYSTEM:00 | \ | acpi:LNXSYSTEM: |
330   +--------------+---+-----------------+
331     |
332     | +-------------+-----+----------------+
333     +-| LNXPWRBN:00 | N/A | acpi:LNXPWRBN: |
334     | +-------------+-----+----------------+
335     |
336     | +-------------+-----+----------------+
337     +-| LNXSLPBN:00 | N/A | acpi:LNXSLPBN: |
338     | +-------------+-----+----------------+
339     |
340     | +-----------+------------+--------------+
341     +-| LNXCPU:00 | \_PR_.CPU0 | acpi:LNXCPU: |
342     | +-----------+------------+--------------+
343     |
344     | +-------------+-------+----------------+
345     +-| LNXSYBUS:00 | \_SB_ | acpi:LNXSYBUS: |
346     | +-------------+-------+----------------+
347     |   |
348     |   | +- - - - - - - +- - - - - - +- - - - - - - -+
349     |   +-| PNP0C0D:00 | \_SB_.LID0 | acpi:PNP0C0D: |
350     |   | +- - - - - - - +- - - - - - +- - - - - - - -+
351     |   |
352     |   | +------------+------------+-----------------------+
353     |   +-| PNP0A08:00 | \_SB_.PCI0 | acpi:PNP0A08:PNP0A03: |
354     |     +------------+------------+-----------------------+
355     |       |
356     |       | +-----------+-----------------+-----+
357     |       +-| device:00 | \_SB_.PCI0.RP03 | N/A |
358     |       | +-----------+-----------------+-----+
359     |       |   |
360     |       |   | +-------------+----------------------+----------------+
361     |       |   +-| LNXPOWER:00 | \_SB_.PCI0.RP03.PXP3 | acpi:LNXPOWER: |
362     |       |     +-------------+----------------------+----------------+
363     |       |
364     |       | +-------------+-----------------+----------------+
365     |       +-| LNXVIDEO:00 | \_SB_.PCI0.GFX0 | acpi:LNXVIDEO: |
366     |         +-------------+-----------------+----------------+
367     |           |
368     |           | +-----------+-----------------+-----+
369     |           +-| device:01 | \_SB_.PCI0.DD01 | N/A |
370     |             +-----------+-----------------+-----+
371     |
372     | +-------------+-------+----------------+
373     +-| LNXSYBUS:01 | \_TZ_ | acpi:LNXSYBUS: |
374       +-------------+-------+----------------+
375         |
376         | +-------------+------------+----------------+
377         +-| LNXPOWER:0a | \_TZ_.FN00 | acpi:LNXPOWER: |
378         | +-------------+------------+----------------+
379         |
380         | +------------+------------+---------------+
381         +-| PNP0C0B:00 | \_TZ_.FAN0 | acpi:PNP0C0B: |
382         | +------------+------------+---------------+
383         |
384         | +-------------+------------+----------------+
385         +-| LNXTHERM:00 | \_TZ_.TZ00 | acpi:LNXTHERM: |
386           +-------------+------------+----------------+
387
388                  Figure 3. Example Linux ACPI Device Tree
389
390.. note:: Each node is represented as "object/path/modalias", where:
391
392   1. 'object' is the name of the object's directory in sysfs.
393   2. 'path' is the ACPI namespace path of the corresponding
394      ACPI namespace object, as returned by the object's 'path'
395      sysfs attribute.
396   3. 'modalias' is the value of the object's 'modalias' sysfs
397      attribute (as described earlier in this document).
398
399.. note:: N/A indicates the device object does not have the 'path' or the
400   'modalias' attribute.
401