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.
