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