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+ACPI Device Tree - Representation of ACPI Namespace
+
+Copyright (C) 2013, Intel Corporation
+Author: Lv Zheng <lv.zheng@intel.com>
+
+
+Abstract:
+
+The Linux ACPI subsystem converts ACPI namespace objects into a Linux
+device tree under the /sys/devices/LNXSYSTEM:00 and updates it upon
+receiving ACPI hotplug notification events.  For each device object in this
+hierarchy there is a corresponding symbolic link in the
+/sys/bus/acpi/devices.
+This document illustrates the structure of the ACPI device tree.
+
+
+Credit:
+
+Thanks for the help from Zhang Rui <rui.zhang@intel.com> and Rafael J.
+Wysocki <rafael.j.wysocki@intel.com>.
+
+
+1. ACPI Definition Blocks
+
+   The ACPI firmware sets up RSDP (Root System Description Pointer) in the
+   system memory address space pointing to the XSDT (Extended System
+   Description Table).  The XSDT always points to the FADT (Fixed ACPI
+   Description Table) using its first entry, the data within the FADT
+   includes various fixed-length entries that describe fixed ACPI features
+   of the hardware.  The FADT contains a pointer to the DSDT
+   (Differentiated System Descripition Table).  The XSDT also contains
+   entries pointing to possibly multiple SSDTs (Secondary System
+   Description Table).
+
+   The DSDT and SSDT data is organized in data structures called definition
+   blocks that contain definitions of various objects, including ACPI
+   control methods, encoded in AML (ACPI Machine Language).  The data block
+   of the DSDT along with the contents of SSDTs represents a hierarchical
+   data structure called the ACPI namespace whose topology reflects the
+   structure of the underlying hardware platform.
+
+   The relationships between ACPI System Definition Tables described above
+   are illustrated in the following diagram.
+
+     +---------+    +-------+    +--------+    +------------------------+
+     |  RSDP   | +->| XSDT  | +->|  FADT  |    |  +-------------------+ |
+     +---------+ |  +-------+ |  +--------+  +-|->|       DSDT        | |
+     | Pointer | |  | Entry |-+  | ...... |  | |  +-------------------+ |
+     +---------+ |  +-------+    | X_DSDT |--+ |  | Definition Blocks | |
+     | Pointer |-+  | ..... |    | ...... |    |  +-------------------+ |
+     +---------+    +-------+    +--------+    |  +-------------------+ |
+                    | Entry |------------------|->|       SSDT        | |
+                    +- - - -+                  |  +-------------------| |
+                    | Entry | - - - - - - - -+ |  | Definition Blocks | |
+                    +- - - -+                | |  +-------------------+ |
+                                             | |  +- - - - - - - - - -+ |
+                                             +-|->|       SSDT        | |
+                                               |  +-------------------+ |
+                                               |  | Definition Blocks | |
+                                               |  +- - - - - - - - - -+ |
+                                               +------------------------+
+                                                           |
+                                              OSPM Loading |
+                                                          \|/
+                                                    +----------------+
+                                                    | ACPI Namespace |
+                                                    +----------------+
+
+                     Figure 1. ACPI Definition Blocks
+
+   NOTE: RSDP can also contain a pointer to the RSDT (Root System
+         Description Table).  Platforms provide RSDT to enable
+         compatibility with ACPI 1.0 operating systems.  The OS is expected
+         to use XSDT, if present.
+
+
+2. Example ACPI Namespace
+
+   All definition blocks are loaded into a single namespace.  The namespace
+   is a hierarchy of objects identified by names and paths.
+   The following naming conventions apply to object names in the ACPI
+   namespace:
+   1. All names are 32 bits long.
+   2. The first byte of a name must be one of 'A' - 'Z', '_'.
+   3. Each of the remaining bytes of a name must be one of 'A' - 'Z', '0'
+      - '9', '_'.
+   4. Names starting with '_' are reserved by the ACPI specification.
+   5. The '\' symbol represents the root of the namespace (i.e. names
+      prepended with '\' are relative to the namespace root).
+   6. The '^' symbol represents the parent of the current namespace node
+      (i.e. names prepended with '^' are relative to the parent of the
+      current namespace node).
+
+   The figure below shows an example ACPI namespace.
+
+   +------+
+   | \    |                     Root
+   +------+
+     |
+     | +------+
+     +-| _PR  |                 Scope(_PR): the processor namespace
+     | +------+
+     |   |
+     |   | +------+
+     |   +-| CPU0 |             Processor(CPU0): the first processor
+     |     +------+
+     |
+     | +------+
+     +-| _SB  |                 Scope(_SB): the system bus namespace
+     | +------+
+     |   |
+     |   | +------+
+     |   +-| LID0 |             Device(LID0); the lid device
+     |   | +------+
+     |   |   |
+     |   |   | +------+
+     |   |   +-| _HID |         Name(_HID, "PNP0C0D"): the hardware ID
+     |   |   | +------+
+     |   |   |
+     |   |   | +------+
+     |   |   +-| _STA |         Method(_STA): the status control method
+     |   |     +------+
+     |   |
+     |   | +------+
+     |   +-| PCI0 |             Device(PCI0); the PCI root bridge
+     |     +------+
+     |       |
+     |       | +------+
+     |       +-| _HID |         Name(_HID, "PNP0A08"): the hardware ID
+     |       | +------+
+     |       |
+     |       | +------+
+     |       +-| _CID |         Name(_CID, "PNP0A03"): the compatible ID
+     |       | +------+
+     |       |
+     |       | +------+
+     |       +-| RP03 |         Scope(RP03): the PCI0 power scope
+     |       | +------+
+     |       |   |
+     |       |   | +------+
+     |       |   +-| PXP3 |     PowerResource(PXP3): the PCI0 power resource
+     |       |     +------+
+     |       |
+     |       | +------+
+     |       +-| GFX0 |         Device(GFX0): the graphics adapter
+     |         +------+
+     |           |
+     |           | +------+
+     |           +-| _ADR |     Name(_ADR, 0x00020000): the PCI bus address
+     |           | +------+
+     |           |
+     |           | +------+
+     |           +-| DD01 |     Device(DD01): the LCD output device
+     |             +------+
+     |               |
+     |               | +------+
+     |               +-| _BCL | Method(_BCL): the backlight control method
+     |                 +------+
+     |
+     | +------+
+     +-| _TZ  |                 Scope(_TZ): the thermal zone namespace
+     | +------+
+     |   |
+     |   | +------+
+     |   +-| FN00 |             PowerResource(FN00): the FAN0 power resource
+     |   | +------+
+     |   |
+     |   | +------+
+     |   +-| FAN0 |             Device(FAN0): the FAN0 cooling device
+     |   | +------+
+     |   |   |
+     |   |   | +------+
+     |   |   +-| _HID |         Name(_HID, "PNP0A0B"): the hardware ID
+     |   |     +------+
+     |   |
+     |   | +------+
+     |   +-| TZ00 |             ThermalZone(TZ00); the FAN thermal zone
+     |     +------+
+     |
+     | +------+
+     +-| _GPE |                 Scope(_GPE): the GPE namespace
+       +------+
+
+                     Figure 2. Example ACPI Namespace
+
+
+3. Linux ACPI Device Objects
+
+   The Linux kernel's core ACPI subsystem creates struct acpi_device
+   objects for ACPI namespace objects representing devices, power resources
+   processors, thermal zones.  Those objects are exported to user space via
+   sysfs as directories in the subtree under /sys/devices/LNXSYSTM:00.  The
+   format of their names is <bus_id:instance>, where 'bus_id' refers to the
+   ACPI namespace representation of the given object and 'instance' is used
+   for distinguishing different object of the same 'bus_id' (it is
+   two-digit decimal representation of an unsigned integer).
+
+   The value of 'bus_id' depends on the type of the object whose name it is
+   part of as listed in the table below.
+
+                +---+-----------------+-------+----------+
+                |   | Object/Feature  | Table | bus_id   |
+                +---+-----------------+-------+----------+
+                | N | Root            | xSDT  | LNXSYSTM |
+                +---+-----------------+-------+----------+
+                | N | Device          | xSDT  | _HID     |
+                +---+-----------------+-------+----------+
+                | N | Processor       | xSDT  | LNXCPU   |
+                +---+-----------------+-------+----------+
+                | N | ThermalZone     | xSDT  | LNXTHERM |
+                +---+-----------------+-------+----------+
+                | N | PowerResource   | xSDT  | LNXPOWER |
+                +---+-----------------+-------+----------+
+                | N | Other Devices   | xSDT  | device   |
+                +---+-----------------+-------+----------+
+                | F | PWR_BUTTON      | FADT  | LNXPWRBN |
+                +---+-----------------+-------+----------+
+                | F | SLP_BUTTON      | FADT  | LNXSLPBN |
+                +---+-----------------+-------+----------+
+                | M | Video Extension | xSDT  | LNXVIDEO |
+                +---+-----------------+-------+----------+
+                | M | ATA Controller  | xSDT  | LNXIOBAY |
+                +---+-----------------+-------+----------+
+                | M | Docking Station | xSDT  | LNXDOCK  |
+                +---+-----------------+-------+----------+
+
+                 Table 1. ACPI Namespace Objects Mapping
+
+   The following rules apply when creating struct acpi_device objects on
+   the basis of the contents of ACPI System Description Tables (as
+   indicated by the letter in the first column and the notation in the
+   second column of the table above):
+   N:
+      The object's source is an ACPI namespace node (as indicated by the
+      named object's type in the second column).  In that case the object's
+      directory in sysfs will contain the 'path' attribute whose value is
+      the full path to the node from the namespace root.
+      struct acpi_device objects are created for the ACPI namespace nodes
+      whose _STA control methods return PRESENT or FUNCTIONING.  The power
+      resource nodes or nodes without _STA are assumed to be both PRESENT
+      and FUNCTIONING.
+   F:
+      The struct acpi_device object is created for a fixed hardware
+      feature (as indicated by the fixed feature flag's name in the second
+      column), so its sysfs directory will not contain the 'path'
+      attribute.
+   M:
+      The struct acpi_device object is created for an ACPI namespace node
+      with specific control methods (as indicated by the ACPI defined
+      device's type in the second column).  The 'path' attribute containing
+      its namespace path will be present in its sysfs directory.  For
+      example, if the _BCL method is present for an ACPI namespace node, a
+      struct acpi_device object with LNXVIDEO 'bus_id' will be created for
+      it.
+
+   The third column of the above table indicates which ACPI System
+   Description Tables contain information used for the creation of the
+   struct acpi_device objects represented by the given row (xSDT means DSDT
+   or SSDT).
+
+   The forth column of the above table indicates the 'bus_id' generation
+   rule of the struct acpi_device object:
+   _HID:
+      _HID in the last column of the table means that the object's bus_id
+      is derived from the _HID/_CID identification objects present under
+      the corresponding ACPI namespace node. The object's sysfs directory
+      will then contain the 'hid' and 'modalias' attributes that can be
+      used to retrieve the _HID and _CIDs of that object.
+   LNXxxxxx:
+      The 'modalias' attribute is also present for struct acpi_device
+      objects having bus_id of the "LNXxxxxx" form (pseudo devices), in
+      which cases it contains the bus_id string itself.
+   device:
+      'device' in the last column of the table indicates that the object's
+      bus_id cannot be determined from _HID/_CID of the corresponding
+      ACPI namespace node, although that object represents a device (for
+      example, it may be a PCI device with _ADR defined and without _HID
+      or _CID).  In that case the string 'device' will be used as the
+      object's bus_id.
+
+
+4. Linux ACPI Physical Device Glue
+
+   ACPI device (i.e. struct acpi_device) objects may be linked to other
+   objects in the Linux' device hierarchy that represent "physical" devices
+   (for example, devices on the PCI bus).  If that happens, it means that
+   the ACPI device object is a "companion" of a device otherwise
+   represented in a different way and is used (1) to provide configuration
+   information on that device which cannot be obtained by other means and
+   (2) to do specific things to the device with the help of its ACPI
+   control methods.  One ACPI device object may be linked this way to
+   multiple "physical" devices.
+
+   If an ACPI device object is linked to a "physical" device, its sysfs
+   directory contains the "physical_node" symbolic link to the sysfs
+   directory of the target device object.  In turn, the target device's
+   sysfs directory will then contain the "firmware_node" symbolic link to
+   the sysfs directory of the companion ACPI device object.
+   The linking mechanism relies on device identification provided by the
+   ACPI namespace.  For example, if there's an ACPI namespace object
+   representing a PCI device (i.e. a device object under an ACPI namespace
+   object representing a PCI bridge) whose _ADR returns 0x00020000 and the
+   bus number of the parent PCI bridge is 0, the sysfs directory
+   representing the struct acpi_device object created for that ACPI
+   namespace object will contain the 'physical_node' symbolic link to the
+   /sys/devices/pci0000:00/0000:00:02:0/ sysfs directory of the
+   corresponding PCI device.
+
+   The linking mechanism is generally bus-specific.  The core of its
+   implementation is located in the drivers/acpi/glue.c file, but there are
+   complementary parts depending on the bus types in question located
+   elsewhere.  For example, the PCI-specific part of it is located in
+   drivers/pci/pci-acpi.c.
+
+
+5. Example Linux ACPI Device Tree
+
+   The sysfs hierarchy of struct acpi_device objects corresponding to the
+   example ACPI namespace illustrated in Figure 2 with the addition of
+   fixed PWR_BUTTON/SLP_BUTTON devices is shown below.
+
+   +--------------+---+-----------------+
+   | LNXSYSTEM:00 | \ | acpi:LNXSYSTEM: |
+   +--------------+---+-----------------+
+     |
+     | +-------------+-----+----------------+
+     +-| LNXPWRBN:00 | N/A | acpi:LNXPWRBN: |
+     | +-------------+-----+----------------+
+     |
+     | +-------------+-----+----------------+
+     +-| LNXSLPBN:00 | N/A | acpi:LNXSLPBN: |
+     | +-------------+-----+----------------+
+     |
+     | +-----------+------------+--------------+
+     +-| LNXCPU:00 | \_PR_.CPU0 | acpi:LNXCPU: |
+     | +-----------+------------+--------------+
+     |
+     | +-------------+-------+----------------+
+     +-| LNXSYBUS:00 | \_SB_ | acpi:LNXSYBUS: |
+     | +-------------+-------+----------------+
+     |   |
+     |   | +- - - - - - - +- - - - - - +- - - - - - - -+
+     |   +-| * PNP0C0D:00 | \_SB_.LID0 | acpi:PNP0C0D: |
+     |   | +- - - - - - - +- - - - - - +- - - - - - - -+
+     |   |
+     |   | +------------+------------+-----------------------+
+     |   +-| PNP0A08:00 | \_SB_.PCI0 | acpi:PNP0A08:PNP0A03: |
+     |     +------------+------------+-----------------------+
+     |       |
+     |       | +-----------+-----------------+-----+
+     |       +-| device:00 | \_SB_.PCI0.RP03 | N/A |
+     |       | +-----------+-----------------+-----+
+     |       |   |
+     |       |   | +-------------+----------------------+----------------+
+     |       |   +-| LNXPOWER:00 | \_SB_.PCI0.RP03.PXP3 | acpi:LNXPOWER: |
+     |       |     +-------------+----------------------+----------------+
+     |       |
+     |       | +-------------+-----------------+----------------+
+     |       +-| LNXVIDEO:00 | \_SB_.PCI0.GFX0 | acpi:LNXVIDEO: |
+     |         +-------------+-----------------+----------------+
+     |           |
+     |           | +-----------+-----------------+-----+
+     |           +-| device:01 | \_SB_.PCI0.DD01 | N/A |
+     |             +-----------+-----------------+-----+
+     |
+     | +-------------+-------+----------------+
+     +-| LNXSYBUS:01 | \_TZ_ | acpi:LNXSYBUS: |
+       +-------------+-------+----------------+
+         |
+         | +-------------+------------+----------------+
+         +-| LNXPOWER:0a | \_TZ_.FN00 | acpi:LNXPOWER: |
+         | +-------------+------------+----------------+
+         |
+         | +------------+------------+---------------+
+         +-| PNP0C0B:00 | \_TZ_.FAN0 | acpi:PNP0C0B: |
+         | +------------+------------+---------------+
+         |
+         | +-------------+------------+----------------+
+         +-| LNXTHERM:00 | \_TZ_.TZ00 | acpi:LNXTHERM: |
+           +-------------+------------+----------------+
+
+                  Figure 3. Example Linux ACPI Device Tree
+
+   NOTE: Each node is represented as "object/path/modalias", where:
+         1. 'object' is the name of the object's directory in sysfs.
+         2. 'path' is the ACPI namespace path of the corresponding
+            ACPI namespace object, as returned by the object's 'path'
+            sysfs attribute.
+         3. 'modalias' is the value of the object's 'modalias' sysfs
+            attribute (as described earlier in this document).
+   NOTE: N/A indicates the device object does not have the 'path' or the
+         'modalias' attribute.
+   NOTE: The PNP0C0D device listed above is highlighted (marked by "*")
+         to indicate it will be created only when its _STA methods return
+         PRESENT or FUNCTIONING.