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 retun "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