1.. SPDX-License-Identifier: GPL-2.0
2.. include:: <isonum.txt>
3
4=========================================================
5Special Usage Model of the ACPI Control Method Lid Device
6=========================================================
7
8:Copyright: |copy| 2016, Intel Corporation
9
10:Author: Lv Zheng <lv.zheng@intel.com>
11
12Abstract
13========
14Platforms containing lids convey lid state (open/close) to OSPMs
15using a control method lid device. To implement this, the AML tables issue
16Notify(lid_device, 0x80) to notify the OSPMs whenever the lid state has
17changed. The _LID control method for the lid device must be implemented to
18report the "current" state of the lid as either "opened" or "closed".
19
20For most platforms, both the _LID method and the lid notifications are
21reliable. However, there are exceptions. In order to work with these
22exceptional buggy platforms, special restrictions and exceptions should be
23taken into account. This document describes the restrictions and the
24exceptions of the Linux ACPI lid device driver.
25
26
27Restrictions of the returning value of the _LID control method
28==============================================================
29
30The _LID control method is described to return the "current" lid state.
31However the word of "current" has ambiguity, some buggy AML tables return
32the lid state upon the last lid notification instead of returning the lid
33state upon the last _LID evaluation. There won't be difference when the
34_LID control method is evaluated during the runtime, the problem is its
35initial returning value. When the AML tables implement this control method
36with cached value, the initial returning value is likely not reliable.
37There are platforms always return "closed" as initial lid state.
38
39Restrictions of the lid state change notifications
40==================================================
41
42There are buggy AML tables never notifying when the lid device state is
43changed to "opened". Thus the "opened" notification is not guaranteed. But
44it is guaranteed that the AML tables always notify "closed" when the lid
45state is changed to "closed". The "closed" notification is normally used to
46trigger some system power saving operations on Windows. Since it is fully
47tested, it is reliable from all AML tables.
48
49Exceptions for the userspace users of the ACPI lid device driver
50================================================================
51
52The ACPI button driver exports the lid state to the userspace via the
53following file::
54
55  /proc/acpi/button/lid/LID0/state
56
57This file actually calls the _LID control method described above. And given
58the previous explanation, it is not reliable enough on some platforms. So
59it is advised for the userspace program to not to solely rely on this file
60to determine the actual lid state.
61
62The ACPI button driver emits the following input event to the userspace:
63  * SW_LID
64
65The ACPI lid device driver is implemented to try to deliver the platform
66triggered events to the userspace. However, given the fact that the buggy
67firmware cannot make sure "opened"/"closed" events are paired, the ACPI
68button driver uses the following 3 modes in order not to trigger issues.
69
70If the userspace hasn't been prepared to ignore the unreliable "opened"
71events and the unreliable initial state notification, Linux users can use
72the following kernel parameters to handle the possible issues:
73
74A. button.lid_init_state=method:
75   When this option is specified, the ACPI button driver reports the
76   initial lid state using the returning value of the _LID control method
77   and whether the "opened"/"closed" events are paired fully relies on the
78   firmware implementation.
79
80   This option can be used to fix some platforms where the returning value
81   of the _LID control method is reliable but the initial lid state
82   notification is missing.
83
84   This option is the default behavior during the period the userspace
85   isn't ready to handle the buggy AML tables.
86
87B. button.lid_init_state=open:
88   When this option is specified, the ACPI button driver always reports the
89   initial lid state as "opened" and whether the "opened"/"closed" events
90   are paired fully relies on the firmware implementation.
91
92   This may fix some platforms where the returning value of the _LID
93   control method is not reliable and the initial lid state notification is
94   missing.
95
96If the userspace has been prepared to ignore the unreliable "opened" events
97and the unreliable initial state notification, Linux users should always
98use the following kernel parameter:
99
100C. button.lid_init_state=ignore:
101   When this option is specified, the ACPI button driver never reports the
102   initial lid state and there is a compensation mechanism implemented to
103   ensure that the reliable "closed" notifications can always be delivered
104   to the userspace by always pairing "closed" input events with complement
105   "opened" input events. But there is still no guarantee that the "opened"
106   notifications can be delivered to the userspace when the lid is actually
107   opens given that some AML tables do not send "opened" notifications
108   reliably.
109
110   In this mode, if everything is correctly implemented by the platform
111   firmware, the old userspace programs should still work. Otherwise, the
112   new userspace programs are required to work with the ACPI button driver.
113   This option will be the default behavior after the userspace is ready to
114   handle the buggy AML tables.
115