ACPI / button: Add document for ACPI control method lid device restrictions

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.