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1.. SPDX-License-Identifier: GPL-2.02.. include:: <isonum.txt>3 4=========================================================5Special Usage Model of the ACPI Control Method Lid Device6=========================================================7 8:Copyright: |copy| 2016, Intel Corporation9 10:Author: Lv Zheng <lv.zheng@intel.com>11 12Abstract13========14Platforms containing lids convey lid state (open/close) to OSPMs15using a control method lid device. To implement this, the AML tables issue16Notify(lid_device, 0x80) to notify the OSPMs whenever the lid state has17changed. The _LID control method for the lid device must be implemented to18report the "current" state of the lid as either "opened" or "closed".19 20For most platforms, both the _LID method and the lid notifications are21reliable. However, there are exceptions. In order to work with these22exceptional buggy platforms, special restrictions and exceptions should be23taken into account. This document describes the restrictions and the24exceptions of the Linux ACPI lid device driver.25 26 27Restrictions of the returning value of the _LID control method28==============================================================29 30The _LID control method is described to return the "current" lid state.31However the word of "current" has ambiguity, some buggy AML tables return32the lid state upon the last lid notification instead of returning the lid33state upon the last _LID evaluation. There won't be difference when the34_LID control method is evaluated during the runtime, the problem is its35initial returning value. When the AML tables implement this control method36with 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 notifications40==================================================41 42There are buggy AML tables never notifying when the lid device state is43changed to "opened". Thus the "opened" notification is not guaranteed. But44it is guaranteed that the AML tables always notify "closed" when the lid45state is changed to "closed". The "closed" notification is normally used to46trigger some system power saving operations on Windows. Since it is fully47tested, it is reliable from all AML tables.48 49Exceptions for the userspace users of the ACPI lid device driver50================================================================51 52The ACPI button driver exports the lid state to the userspace via the53following file::54 55 /proc/acpi/button/lid/LID0/state56 57This file actually calls the _LID control method described above. And given58the previous explanation, it is not reliable enough on some platforms. So59it is advised for the userspace program to not to solely rely on this file60to determine the actual lid state.61 62The ACPI button driver emits the following input event to the userspace:63 * SW_LID64 65The ACPI lid device driver is implemented to try to deliver the platform66triggered events to the userspace. However, given the fact that the buggy67firmware cannot make sure "opened"/"closed" events are paired, the ACPI68button 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 use72the 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 the76 initial lid state using the returning value of the _LID control method77 and whether the "opened"/"closed" events are paired fully relies on the78 firmware implementation.79 80 This option can be used to fix some platforms where the returning value81 of the _LID control method is reliable but the initial lid state82 notification is missing.83 84 This option is the default behavior during the period the userspace85 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 the89 initial lid state as "opened" and whether the "opened"/"closed" events90 are paired fully relies on the firmware implementation.91 92 This may fix some platforms where the returning value of the _LID93 control method is not reliable and the initial lid state notification is94 missing.95 96If the userspace has been prepared to ignore the unreliable "opened" events97and the unreliable initial state notification, Linux users should always98use the following kernel parameter:99 100C. button.lid_init_state=ignore:101 When this option is specified, the ACPI button driver never reports the102 initial lid state and there is a compensation mechanism implemented to103 ensure that the reliable "closed" notifications can always be delivered104 to the userspace by always pairing "closed" input events with complement105 "opened" input events. But there is still no guarantee that the "opened"106 notifications can be delivered to the userspace when the lid is actually107 opens given that some AML tables do not send "opened" notifications108 reliably.109 110 In this mode, if everything is correctly implemented by the platform111 firmware, the old userspace programs should still work. Otherwise, the112 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 to114 handle the buggy AML tables.115