The Linux IPMI Driver
  			  ---------------------
  			      Corey Minyard
  			  <minyard@mvista.com>
  			    <minyard@acm.org>
  
  The Intelligent Platform Management Interface, or IPMI, is a
  standard for controlling intelligent devices that monitor a system.
  It provides for dynamic discovery of sensors in the system and the
  ability to monitor the sensors and be informed when the sensor's
  values change or go outside certain boundaries.  It also has a
  standardized database for field-replaceable units (FRUs) and a watchdog
  timer.
  
  To use this, you need an interface to an IPMI controller in your
  system (called a Baseboard Management Controller, or BMC) and
  management software that can use the IPMI system.
  
  This document describes how to use the IPMI driver for Linux.  If you
  are not familiar with IPMI itself, see the web site at
  http://www.intel.com/design/servers/ipmi/index.htm.  IPMI is a big
  subject and I can't cover it all here!
  
  Configuration
  -------------
  
  The Linux IPMI driver is modular, which means you have to pick several
  things to have it work right depending on your hardware.  Most of
  these are available in the 'Character Devices' menu then the IPMI
  menu.
  
  No matter what, you must pick 'IPMI top-level message handler' to use
  IPMI.  What you do beyond that depends on your needs and hardware.
  
  The message handler does not provide any user-level interfaces.
  Kernel code (like the watchdog) can still use it.  If you need access
  from userland, you need to select 'Device interface for IPMI' if you
  want access through a device driver.
  
  The driver interface depends on your hardware.  If your system
  properly provides the SMBIOS info for IPMI, the driver will detect it
  and just work.  If you have a board with a standard interface (These
  will generally be either "KCS", "SMIC", or "BT", consult your hardware
  manual), choose the 'IPMI SI handler' option.
  
  You should generally enable ACPI on your system, as systems with IPMI
  can have ACPI tables describing them.
  
  If you have a standard interface and the board manufacturer has done
  their job correctly, the IPMI controller should be automatically
  detected (via ACPI or SMBIOS tables) and should just work.  Sadly,
  many boards do not have this information.  The driver attempts
  standard defaults, but they may not work.  If you fall into this
  situation, you need to read the section below named 'The SI Driver'.
  
  IPMI defines a standard watchdog timer.  You can enable this with the
  'IPMI Watchdog Timer' config option.  If you compile the driver into
  the kernel, then via a kernel command-line option you can have the
  watchdog timer start as soon as it initializes.  It also have a lot
  of other options, see the 'Watchdog' section below for more details.
  Note that you can also have the watchdog continue to run if it is
  closed (by default it is disabled on close).  Go into the 'Watchdog
  Cards' menu, enable 'Watchdog Timer Support', and enable the option
  'Disable watchdog shutdown on close'.
  
  IPMI systems can often be powered off using IPMI commands.  Select
  'IPMI Poweroff' to do this.  The driver will auto-detect if the system
  can be powered off by IPMI.  It is safe to enable this even if your
  system doesn't support this option.  This works on ATCA systems, the
  Radisys CPI1 card, and any IPMI system that supports standard chassis
  management commands.
  
  If you want the driver to put an event into the event log on a panic,
  enable the 'Generate a panic event to all BMCs on a panic' option.  If
  you want the whole panic string put into the event log using OEM
  events, enable the 'Generate OEM events containing the panic string'
  option.
  
  Basic Design
  ------------
  
  The Linux IPMI driver is designed to be very modular and flexible, you
  only need to take the pieces you need and you can use it in many
  different ways.  Because of that, it's broken into many chunks of
  code.  These chunks (by module name) are:
  
  ipmi_msghandler - This is the central piece of software for the IPMI
  system.  It handles all messages, message timing, and responses.  The
  IPMI users tie into this, and the IPMI physical interfaces (called
  System Management Interfaces, or SMIs) also tie in here.  This
  provides the kernelland interface for IPMI, but does not provide an
  interface for use by application processes.
  
  ipmi_devintf - This provides a userland IOCTL interface for the IPMI
  driver, each open file for this device ties in to the message handler
  as an IPMI user.
  
  ipmi_si - A driver for various system interfaces.  This supports KCS,
  SMIC, and BT interfaces.
  
  ipmi_watchdog - IPMI requires systems to have a very capable watchdog
  timer.  This driver implements the standard Linux watchdog timer
  interface on top of the IPMI message handler.
  
  ipmi_poweroff - Some systems support the ability to be turned off via
  IPMI commands.
  
  These are all individually selectable via configuration options.
  
  Note that the KCS-only interface has been removed.  The af_ipmi driver
  is no longer supported and has been removed because it was impossible
  to do 32 bit emulation on 64-bit kernels with it.
  
  Much documentation for the interface is in the include files.  The
  IPMI include files are:
  
  net/af_ipmi.h - Contains the socket interface.
  
  linux/ipmi.h - Contains the user interface and IOCTL interface for IPMI.
  
  linux/ipmi_smi.h - Contains the interface for system management interfaces
  (things that interface to IPMI controllers) to use.
  
  linux/ipmi_msgdefs.h - General definitions for base IPMI messaging.
  
  
  Addressing
  ----------
  
  The IPMI addressing works much like IP addresses, you have an overlay
  to handle the different address types.  The overlay is:
  
    struct ipmi_addr
    {
  	int   addr_type;
  	short channel;
  	char  data[IPMI_MAX_ADDR_SIZE];
    };
  
  The addr_type determines what the address really is.  The driver
  currently understands two different types of addresses.
  
  "System Interface" addresses are defined as:
  
    struct ipmi_system_interface_addr
    {
  	int   addr_type;
  	short channel;
    };
  
  and the type is IPMI_SYSTEM_INTERFACE_ADDR_TYPE.  This is used for talking
  straight to the BMC on the current card.  The channel must be
  IPMI_BMC_CHANNEL.
  
  Messages that are destined to go out on the IPMB bus use the
  IPMI_IPMB_ADDR_TYPE address type.  The format is
  
    struct ipmi_ipmb_addr
    {
  	int           addr_type;
  	short         channel;
  	unsigned char slave_addr;
  	unsigned char lun;
    };
  
  The "channel" here is generally zero, but some devices support more
  than one channel, it corresponds to the channel as defined in the IPMI
  spec.
  
  
  Messages
  --------
  
  Messages are defined as:
  
  struct ipmi_msg
  {
  	unsigned char netfn;
  	unsigned char lun;
  	unsigned char cmd;
  	unsigned char *data;
  	int           data_len;
  };
  
  The driver takes care of adding/stripping the header information.  The
  data portion is just the data to be send (do NOT put addressing info
  here) or the response.  Note that the completion code of a response is
  the first item in "data", it is not stripped out because that is how
  all the messages are defined in the spec (and thus makes counting the
  offsets a little easier :-).
  
  When using the IOCTL interface from userland, you must provide a block
  of data for "data", fill it, and set data_len to the length of the
  block of data, even when receiving messages.  Otherwise the driver
  will have no place to put the message.
  
  Messages coming up from the message handler in kernelland will come in
  as:
  
    struct ipmi_recv_msg
    {
  	struct list_head link;
  
  	/* The type of message as defined in the "Receive Types"
             defines above. */
  	int         recv_type;
  
  	ipmi_user_t      *user;
  	struct ipmi_addr addr;
  	long             msgid;
  	struct ipmi_msg  msg;
  
  	/* Call this when done with the message.  It will presumably free
  	   the message and do any other necessary cleanup. */
  	void (*done)(struct ipmi_recv_msg *msg);
  
  	/* Place-holder for the data, don't make any assumptions about
  	   the size or existence of this, since it may change. */
  	unsigned char   msg_data[IPMI_MAX_MSG_LENGTH];
    };
  
  You should look at the receive type and handle the message
  appropriately.
  
  
  The Upper Layer Interface (Message Handler)
  -------------------------------------------
  
  The upper layer of the interface provides the users with a consistent
  view of the IPMI interfaces.  It allows multiple SMI interfaces to be
  addressed (because some boards actually have multiple BMCs on them)
  and the user should not have to care what type of SMI is below them.
  
  
  Creating the User
  
  To user the message handler, you must first create a user using
  ipmi_create_user.  The interface number specifies which SMI you want
  to connect to, and you must supply callback functions to be called
  when data comes in.  The callback function can run at interrupt level,
  so be careful using the callbacks.  This also allows to you pass in a
  piece of data, the handler_data, that will be passed back to you on
  all calls.
  
  Once you are done, call ipmi_destroy_user() to get rid of the user.
  
  From userland, opening the device automatically creates a user, and
  closing the device automatically destroys the user.
  
  
  Messaging
  
  To send a message from kernel-land, the ipmi_request() call does
  pretty much all message handling.  Most of the parameter are
  self-explanatory.  However, it takes a "msgid" parameter.  This is NOT
  the sequence number of messages.  It is simply a long value that is
  passed back when the response for the message is returned.  You may
  use it for anything you like.
  
  Responses come back in the function pointed to by the ipmi_recv_hndl
  field of the "handler" that you passed in to ipmi_create_user().
  Remember again, these may be running at interrupt level.  Remember to
  look at the receive type, too.
  
  From userland, you fill out an ipmi_req_t structure and use the
  IPMICTL_SEND_COMMAND ioctl.  For incoming stuff, you can use select()
  or poll() to wait for messages to come in.  However, you cannot use
  read() to get them, you must call the IPMICTL_RECEIVE_MSG with the
  ipmi_recv_t structure to actually get the message.  Remember that you
  must supply a pointer to a block of data in the msg.data field, and
  you must fill in the msg.data_len field with the size of the data.
  This gives the receiver a place to actually put the message.
  
  If the message cannot fit into the data you provide, you will get an
  EMSGSIZE error and the driver will leave the data in the receive
  queue.  If you want to get it and have it truncate the message, us
  the IPMICTL_RECEIVE_MSG_TRUNC ioctl.
  
  When you send a command (which is defined by the lowest-order bit of
  the netfn per the IPMI spec) on the IPMB bus, the driver will
  automatically assign the sequence number to the command and save the
  command.  If the response is not receive in the IPMI-specified 5
  seconds, it will generate a response automatically saying the command
  timed out.  If an unsolicited response comes in (if it was after 5
  seconds, for instance), that response will be ignored.
  
  In kernelland, after you receive a message and are done with it, you
  MUST call ipmi_free_recv_msg() on it, or you will leak messages.  Note
  that you should NEVER mess with the "done" field of a message, that is
  required to properly clean up the message.
  
  Note that when sending, there is an ipmi_request_supply_msgs() call
  that lets you supply the smi and receive message.  This is useful for
  pieces of code that need to work even if the system is out of buffers
  (the watchdog timer uses this, for instance).  You supply your own
  buffer and own free routines.  This is not recommended for normal use,
  though, since it is tricky to manage your own buffers.
  
  
  Events and Incoming Commands
  
  The driver takes care of polling for IPMI events and receiving
  commands (commands are messages that are not responses, they are
  commands that other things on the IPMB bus have sent you).  To receive
  these, you must register for them, they will not automatically be sent
  to you.
  
  To receive events, you must call ipmi_set_gets_events() and set the
  "val" to non-zero.  Any events that have been received by the driver
  since startup will immediately be delivered to the first user that
  registers for events.  After that, if multiple users are registered
  for events, they will all receive all events that come in.
  
  For receiving commands, you have to individually register commands you
  want to receive.  Call ipmi_register_for_cmd() and supply the netfn
  and command name for each command you want to receive.  You also
  specify a bitmask of the channels you want to receive the command from
  (or use IPMI_CHAN_ALL for all channels if you don't care).  Only one
  user may be registered for each netfn/cmd/channel, but different users
  may register for different commands, or the same command if the
  channel bitmasks do not overlap.
  
  From userland, equivalent IOCTLs are provided to do these functions.
  
  
  The Lower Layer (SMI) Interface
  -------------------------------
  
  As mentioned before, multiple SMI interfaces may be registered to the
  message handler, each of these is assigned an interface number when
  they register with the message handler.  They are generally assigned
  in the order they register, although if an SMI unregisters and then
  another one registers, all bets are off.
  
  The ipmi_smi.h defines the interface for management interfaces, see
  that for more details.
  
  
  The SI Driver
  -------------
  
  The SI driver allows up to 4 KCS or SMIC interfaces to be configured
  in the system.  By default, scan the ACPI tables for interfaces, and
  if it doesn't find any the driver will attempt to register one KCS
  interface at the spec-specified I/O port 0xca2 without interrupts.
  You can change this at module load time (for a module) with:
  
    modprobe ipmi_si.o type=<type1>,<type2>....
         ports=<port1>,<port2>... addrs=<addr1>,<addr2>...
         irqs=<irq1>,<irq2>...
         regspacings=<sp1>,<sp2>,... regsizes=<size1>,<size2>,...
         regshifts=<shift1>,<shift2>,...
         slave_addrs=<addr1>,<addr2>,...
         force_kipmid=<enable1>,<enable2>,...
         kipmid_max_busy_us=<ustime1>,<ustime2>,...
         unload_when_empty=[0|1]
         trydefaults=[0|1] trydmi=[0|1] tryacpi=[0|1]
         tryplatform=[0|1] trypci=[0|1]
  
  Each of these except try... items is a list, the first item for the
  first interface, second item for the second interface, etc.
  
  The si_type may be either "kcs", "smic", or "bt".  If you leave it blank, it
  defaults to "kcs".
  
  If you specify addrs as non-zero for an interface, the driver will
  use the memory address given as the address of the device.  This
  overrides si_ports.
  
  If you specify ports as non-zero for an interface, the driver will
  use the I/O port given as the device address.
  
  If you specify irqs as non-zero for an interface, the driver will
  attempt to use the given interrupt for the device.
  
  trydefaults sets whether the standard IPMI interface at 0xca2 and
  any interfaces specified by ACPE are tried.  By default, the driver
  tries it, set this value to zero to turn this off.
  
  The other try... items disable discovery by their corresponding
  names.  These are all enabled by default, set them to zero to disable
  them.  The tryplatform disables openfirmware.
  
  The next three parameters have to do with register layout.  The
  registers used by the interfaces may not appear at successive
  locations and they may not be in 8-bit registers.  These parameters
  allow the layout of the data in the registers to be more precisely
  specified.
  
  The regspacings parameter give the number of bytes between successive
  register start addresses.  For instance, if the regspacing is set to 4
  and the start address is 0xca2, then the address for the second
  register would be 0xca6.  This defaults to 1.
  
  The regsizes parameter gives the size of a register, in bytes.  The
  data used by IPMI is 8-bits wide, but it may be inside a larger
  register.  This parameter allows the read and write type to specified.
  It may be 1, 2, 4, or 8.  The default is 1.
  
  Since the register size may be larger than 32 bits, the IPMI data may not
  be in the lower 8 bits.  The regshifts parameter give the amount to shift
  the data to get to the actual IPMI data.
  
  The slave_addrs specifies the IPMI address of the local BMC.  This is
  usually 0x20 and the driver defaults to that, but in case it's not, it
  can be specified when the driver starts up.
  
  The force_ipmid parameter forcefully enables (if set to 1) or disables
  (if set to 0) the kernel IPMI daemon.  Normally this is auto-detected
  by the driver, but systems with broken interrupts might need an enable,
  or users that don't want the daemon (don't need the performance, don't
  want the CPU hit) can disable it.
  
  If unload_when_empty is set to 1, the driver will be unloaded if it
  doesn't find any interfaces or all the interfaces fail to work.  The
  default is one.  Setting to 0 is useful with the hotmod, but is
  obviously only useful for modules.
  
  When compiled into the kernel, the parameters can be specified on the
  kernel command line as:
  
    ipmi_si.type=<type1>,<type2>...
         ipmi_si.ports=<port1>,<port2>... ipmi_si.addrs=<addr1>,<addr2>...
         ipmi_si.irqs=<irq1>,<irq2>... ipmi_si.trydefaults=[0|1]
         ipmi_si.regspacings=<sp1>,<sp2>,...
         ipmi_si.regsizes=<size1>,<size2>,...
         ipmi_si.regshifts=<shift1>,<shift2>,...
         ipmi_si.slave_addrs=<addr1>,<addr2>,...
         ipmi_si.force_kipmid=<enable1>,<enable2>,...
         ipmi_si.kipmid_max_busy_us=<ustime1>,<ustime2>,...
  
  It works the same as the module parameters of the same names.
  
  By default, the driver will attempt to detect any device specified by
  ACPI, and if none of those then a KCS device at the spec-specified
  0xca2.  If you want to turn this off, set the "trydefaults" option to
  false.
  
  If your IPMI interface does not support interrupts and is a KCS or
  SMIC interface, the IPMI driver will start a kernel thread for the
  interface to help speed things up.  This is a low-priority kernel
  thread that constantly polls the IPMI driver while an IPMI operation
  is in progress.  The force_kipmid module parameter will all the user to
  force this thread on or off.  If you force it off and don't have
  interrupts, the driver will run VERY slowly.  Don't blame me,
  these interfaces suck.
  
  Unfortunately, this thread can use a lot of CPU depending on the
  interface's performance.  This can waste a lot of CPU and cause
  various issues with detecting idle CPU and using extra power.  To
  avoid this, the kipmid_max_busy_us sets the maximum amount of time, in
  microseconds, that kipmid will spin before sleeping for a tick.  This
  value sets a balance between performance and CPU waste and needs to be
  tuned to your needs.  Maybe, someday, auto-tuning will be added, but
  that's not a simple thing and even the auto-tuning would need to be
  tuned to the user's desired performance.
  
  The driver supports a hot add and remove of interfaces.  This way,
  interfaces can be added or removed after the kernel is up and running.
  This is done using /sys/modules/ipmi_si/parameters/hotmod, which is a
  write-only parameter.  You write a string to this interface.  The string
  has the format:
     <op1>[:op2[:op3...]]
  The "op"s are:
     add|remove,kcs|bt|smic,mem|i/o,<address>[,<opt1>[,<opt2>[,...]]]
  You can specify more than one interface on the line.  The "opt"s are:
     rsp=<regspacing>
     rsi=<regsize>
     rsh=<regshift>
     irq=<irq>
     ipmb=<ipmb slave addr>
  and these have the same meanings as discussed above.  Note that you
  can also use this on the kernel command line for a more compact format
  for specifying an interface.  Note that when removing an interface,
  only the first three parameters (si type, address type, and address)
  are used for the comparison.  Any options are ignored for removing.
  
  
  Other Pieces
  ------------
  
  Get the detailed info related with the IPMI device
  --------------------------------------------------
  
  Some users need more detailed information about a device, like where
  the address came from or the raw base device for the IPMI interface.
  You can use the IPMI smi_watcher to catch the IPMI interfaces as they
  come or go, and to grab the information, you can use the function
  ipmi_get_smi_info(), which returns the following structure:
  
  struct ipmi_smi_info {
  	enum ipmi_addr_src addr_src;
  	struct device *dev;
  	union {
  		struct {
  			void *acpi_handle;
  		} acpi_info;
  	} addr_info;
  };
  
  Currently special info for only for SI_ACPI address sources is
  returned.  Others may be added as necessary.
  
  Note that the dev pointer is included in the above structure, and
  assuming ipmi_smi_get_info returns success, you must call put_device
  on the dev pointer.
  
  
  Watchdog
  --------
  
  A watchdog timer is provided that implements the Linux-standard
  watchdog timer interface.  It has three module parameters that can be
  used to control it:
  
    modprobe ipmi_watchdog timeout=<t> pretimeout=<t> action=<action type>
        preaction=<preaction type> preop=<preop type> start_now=x
        nowayout=x ifnum_to_use=n
  
  ifnum_to_use specifies which interface the watchdog timer should use.
  The default is -1, which means to pick the first one registered.
  
  The timeout is the number of seconds to the action, and the pretimeout
  is the amount of seconds before the reset that the pre-timeout panic will
  occur (if pretimeout is zero, then pretimeout will not be enabled).  Note
  that the pretimeout is the time before the final timeout.  So if the
  timeout is 50 seconds and the pretimeout is 10 seconds, then the pretimeout
  will occur in 40 second (10 seconds before the timeout).
  
  The action may be "reset", "power_cycle", or "power_off", and
  specifies what to do when the timer times out, and defaults to
  "reset".
  
  The preaction may be "pre_smi" for an indication through the SMI
  interface, "pre_int" for an indication through the SMI with an
  interrupts, and "pre_nmi" for a NMI on a preaction.  This is how
  the driver is informed of the pretimeout.
  
  The preop may be set to "preop_none" for no operation on a pretimeout,
  "preop_panic" to set the preoperation to panic, or "preop_give_data"
  to provide data to read from the watchdog device when the pretimeout
  occurs.  A "pre_nmi" setting CANNOT be used with "preop_give_data"
  because you can't do data operations from an NMI.
  
  When preop is set to "preop_give_data", one byte comes ready to read
  on the device when the pretimeout occurs.  Select and fasync work on
  the device, as well.
  
  If start_now is set to 1, the watchdog timer will start running as
  soon as the driver is loaded.
  
  If nowayout is set to 1, the watchdog timer will not stop when the
  watchdog device is closed.  The default value of nowayout is true
  if the CONFIG_WATCHDOG_NOWAYOUT option is enabled, or false if not.
  
  When compiled into the kernel, the kernel command line is available
  for configuring the watchdog:
  
    ipmi_watchdog.timeout=<t> ipmi_watchdog.pretimeout=<t>
  	ipmi_watchdog.action=<action type>
  	ipmi_watchdog.preaction=<preaction type>
  	ipmi_watchdog.preop=<preop type>
  	ipmi_watchdog.start_now=x
  	ipmi_watchdog.nowayout=x
  
  The options are the same as the module parameter options.
  
  The watchdog will panic and start a 120 second reset timeout if it
  gets a pre-action.  During a panic or a reboot, the watchdog will
  start a 120 timer if it is running to make sure the reboot occurs.
  
  Note that if you use the NMI preaction for the watchdog, you MUST NOT
  use the nmi watchdog.  There is no reasonable way to tell if an NMI
  comes from the IPMI controller, so it must assume that if it gets an
  otherwise unhandled NMI, it must be from IPMI and it will panic
  immediately.
  
  Once you open the watchdog timer, you must write a 'V' character to the
  device to close it, or the timer will not stop.  This is a new semantic
  for the driver, but makes it consistent with the rest of the watchdog
  drivers in Linux.
  
  
  Panic Timeouts
  --------------
  
  The OpenIPMI driver supports the ability to put semi-custom and custom
  events in the system event log if a panic occurs.  if you enable the
  'Generate a panic event to all BMCs on a panic' option, you will get
  one event on a panic in a standard IPMI event format.  If you enable
  the 'Generate OEM events containing the panic string' option, you will
  also get a bunch of OEM events holding the panic string.
  
  
  The field settings of the events are:
  * Generator ID: 0x21 (kernel)
  * EvM Rev: 0x03 (this event is formatting in IPMI 1.0 format)
  * Sensor Type: 0x20 (OS critical stop sensor)
  * Sensor #: The first byte of the panic string (0 if no panic string)
  * Event Dir | Event Type: 0x6f (Assertion, sensor-specific event info)
  * Event Data 1: 0xa1 (Runtime stop in OEM bytes 2 and 3)
  * Event data 2: second byte of panic string
  * Event data 3: third byte of panic string
  See the IPMI spec for the details of the event layout.  This event is
  always sent to the local management controller.  It will handle routing
  the message to the right place
  
  Other OEM events have the following format:
  Record ID (bytes 0-1): Set by the SEL.
  Record type (byte 2): 0xf0 (OEM non-timestamped)
  byte 3: The slave address of the card saving the panic
  byte 4: A sequence number (starting at zero)
  The rest of the bytes (11 bytes) are the panic string.  If the panic string
  is longer than 11 bytes, multiple messages will be sent with increasing
  sequence numbers.
  
  Because you cannot send OEM events using the standard interface, this
  function will attempt to find an SEL and add the events there.  It
  will first query the capabilities of the local management controller.
  If it has an SEL, then they will be stored in the SEL of the local
  management controller.  If not, and the local management controller is
  an event generator, the event receiver from the local management
  controller will be queried and the events sent to the SEL on that
  device.  Otherwise, the events go nowhere since there is nowhere to
  send them.
  
  
  Poweroff
  --------
  
  If the poweroff capability is selected, the IPMI driver will install
  a shutdown function into the standard poweroff function pointer.  This
  is in the ipmi_poweroff module.  When the system requests a powerdown,
  it will send the proper IPMI commands to do this.  This is supported on
  several platforms.
  
  There is a module parameter named "poweroff_powercycle" that may
  either be zero (do a power down) or non-zero (do a power cycle, power
  the system off, then power it on in a few seconds).  Setting
  ipmi_poweroff.poweroff_control=x will do the same thing on the kernel
  command line.  The parameter is also available via the proc filesystem
  in /proc/sys/dev/ipmi/poweroff_powercycle.  Note that if the system
  does not support power cycling, it will always do the power off.
  
  The "ifnum_to_use" parameter specifies which interface the poweroff
  code should use.  The default is -1, which means to pick the first one
  registered.
  
  Note that if you have ACPI enabled, the system will prefer using ACPI to
  power off.