csu3_am335x/board-support/linux-4.9.59+gitAUTOINC+a75d8e9305-ga75d8e9305/Documentation/acpi/acpi-lid.txt

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.