1 .. SPDX-License-Identifier: GPL-2.0 2 .. include:: <isonum.txt> 3 4 ========================================================= 5 Special 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 12 Abstract 13 ======== 14 Platforms containing lids convey lid state (open/close) to OSPMs 15 using a control method lid device. To implement this, the AML tables issue 16 Notify(lid_device, 0x80) to notify the OSPMs whenever the lid state has 17 changed. The _LID control method for the lid device must be implemented to 18 report the "current" state of the lid as either "opened" or "closed". 19 20 For most platforms, both the _LID method and the lid notifications are 21 reliable. However, there are exceptions. In order to work with these 22 exceptional buggy platforms, special restrictions and exceptions should be 23 taken into account. This document describes the restrictions and the 24 exceptions of the Linux ACPI lid device driver. 25 26 27 Restrictions of the returning value of the _LID control method 28 ============================================================== 29 30 The _LID control method is described to return the "current" lid state. 31 However the word of "current" has ambiguity, some buggy AML tables return 32 the lid state upon the last lid notification instead of returning the lid 33 state 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 35 initial returning value. When the AML tables implement this control method 36 with cached value, the initial returning value is likely not reliable. 37 There are platforms always return "closed" as initial lid state. 38 39 Restrictions of the lid state change notifications 40 ================================================== 41 42 There are buggy AML tables never notifying when the lid device state is 43 changed to "opened". Thus the "opened" notification is not guaranteed. But 44 it is guaranteed that the AML tables always notify "closed" when the lid 45 state is changed to "closed". The "closed" notification is normally used to 46 trigger some system power saving operations on Windows. Since it is fully 47 tested, it is reliable from all AML tables. 48 49 Exceptions for the userspace users of the ACPI lid device driver 50 ================================================================ 51 52 The ACPI button driver exports the lid state to the userspace via the 53 following file:: 54 55 /proc/acpi/button/lid/LID0/state 56 57 This file actually calls the _LID control method described above. And given 58 the previous explanation, it is not reliable enough on some platforms. So 59 it is advised for the userspace program to not to solely rely on this file 60 to determine the actual lid state. 61 62 The ACPI button driver emits the following input event to the userspace: 63 * SW_LID 64 65 The ACPI lid device driver is implemented to try to deliver the platform 66 triggered events to the userspace. However, given the fact that the buggy 67 firmware cannot make sure "opened"/"closed" events are paired, the ACPI 68 button driver uses the following 3 modes in order not to trigger issues. 69 70 If the userspace hasn't been prepared to ignore the unreliable "opened" 71 events and the unreliable initial state notification, Linux users can use 72 the following kernel parameters to handle the possible issues: 73 74 A. 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 87 B. 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 96 If the userspace has been prepared to ignore the unreliable "opened" events 97 and the unreliable initial state notification, Linux users should always 98 use the following kernel parameter: 99 100 C. 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.
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