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