Special Usage Model of the ACPI Control Method Lid Device
  
  Copyright (C) 2016, Intel Corporation
  Author: Lv Zheng <lv.zheng@intel.com>
  
  
  Abstract:
  
  Platforms containing lids convey lid state (open/close) to OSPMs using a
  control method lid device. To implement this, the AML tables issue
  Notify(lid_device, 0x80) to notify the OSPMs whenever the lid state has
  changed. The _LID control method for the lid device must be implemented to
  report the "current" state of the lid as either "opened" or "closed".
  
  For most platforms, both the _LID method and the lid notifications are
  reliable. However, there are exceptions. In order to work with these
  exceptional buggy platforms, special restrictions and expections should be
  taken into account. This document describes the restrictions and the
  expections of the Linux ACPI lid device driver.
  
  
  1. Restrictions of the returning value of the _LID control method
  
  The _LID control method is described to return the "current" lid state.
  However the word of "current" has ambiguity, some buggy AML tables return
  the lid state upon the last lid notification instead of returning the lid
  state upon the last _LID evaluation. There won't be difference when the
  _LID control method is evaluated during the runtime, the problem is its
  initial returning value. When the AML tables implement this control method
  with cached value, the initial returning value is likely not reliable.
  There are platforms always retun "closed" as initial lid state.
  
  2. Restrictions of the lid state change notifications
  
  There are buggy AML tables never notifying when the lid device state is
  changed to "opened". Thus the "opened" notification is not guaranteed. But
  it is guaranteed that the AML tables always notify "closed" when the lid
  state is changed to "closed". The "closed" notification is normally used to
  trigger some system power saving operations on Windows. Since it is fully
  tested, it is reliable from all AML tables.
  
  3. Expections for the userspace users of the ACPI lid device driver
  
  The ACPI button driver exports the lid state to the userspace via the
  following file:
    /proc/acpi/button/lid/LID0/state
  This file actually calls the _LID control method described above. And given
  the previous explanation, it is not reliable enough on some platforms. So
  it is advised for the userspace program to not to solely rely on this file
  to determine the actual lid state.
  
  The ACPI button driver emits the following input event to the userspace:
    SW_LID
  The ACPI lid device driver is implemented to try to deliver the platform
  triggered events to the userspace. However, given the fact that the buggy
  firmware cannot make sure "opened"/"closed" events are paired, the ACPI
  button driver uses the following 3 modes in order not to trigger issues.
  
  If the userspace hasn't been prepared to ignore the unreliable "opened"
  events and the unreliable initial state notification, Linux users can use
  the following kernel parameters to handle the possible issues:
  A. button.lid_init_state=method:
     When this option is specified, the ACPI button driver reports the
     initial lid state using the returning value of the _LID control method
     and whether the "opened"/"closed" events are paired fully relies on the
     firmware implementation.
     This option can be used to fix some platforms where the returning value
     of the _LID control method is reliable but the initial lid state
     notification is missing.
     This option is the default behavior during the period the userspace
     isn't ready to handle the buggy AML tables.
  B. button.lid_init_state=open:
     When this option is specified, the ACPI button driver always reports the
     initial lid state as "opened" and whether the "opened"/"closed" events
     are paired fully relies on the firmware implementation.
     This may fix some platforms where the returning value of the _LID
     control method is not reliable and the initial lid state notification is
     missing.
  
  If the userspace has been prepared to ignore the unreliable "opened" events
  and the unreliable initial state notification, Linux users should always
  use the following kernel parameter:
  C. button.lid_init_state=ignore:
     When this option is specified, the ACPI button driver never reports the
     initial lid state and there is a compensation mechanism implemented to
     ensure that the reliable "closed" notifications can always be delievered
     to the userspace by always pairing "closed" input events with complement
     "opened" input events. But there is still no guarantee that the "opened"
     notifications can be delivered to the userspace when the lid is actually
     opens given that some AML tables do not send "opened" notifications
     reliably.
     In this mode, if everything is correctly implemented by the platform
     firmware, the old userspace programs should still work. Otherwise, the
     new userspace programs are required to work with the ACPI button driver.
     This option will be the default behavior after the userspace is ready to
     handle the buggy AML tables.