ACPI / button: Add document for ACPI control method lid device restrictions

This patch adds documentation for the usage model of the control method lid
device on some buggy platforms.

Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Acked-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

diff --git a/Documentation/acpi/acpi-lid.txt b/Documentation/acpi/acpi-lid.txt
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+Special Usage Model of the ACPI Control Method Lid Device
+
+Copyright (C) 2016, Intel Corporation
+Author: Lv Zheng <lv.zheng@intel.com>
+
+
+Abstract:
+
+Platforms containing lids convey lid state (open/close) to OSPMs using a
+control method lid device. To implement this, the AML tables issue
+Notify(lid_device, 0x80) to notify the OSPMs whenever the lid state has
+changed. The _LID control method for the lid device must be implemented to
+report the "current" state of the lid as either "opened" or "closed".
+
+For most platforms, both the _LID method and the lid notifications are
+reliable. However, there are exceptions. In order to work with these
+exceptional buggy platforms, special restrictions and expections should be
+taken into account. This document describes the restrictions and the
+expections of the Linux ACPI lid device driver.
+
+
+1. Restrictions of the returning value of the _LID control method
+
+The _LID control method is described to return the "current" lid state.
+However the word of "current" has ambiguity, some buggy AML tables return
+the lid state upon the last lid notification instead of returning the lid
+state upon the last _LID evaluation. There won't be difference when the
+_LID control method is evaluated during the runtime, the problem is its
+initial returning value. When the AML tables implement this control method
+with cached value, the initial returning value is likely not reliable.
+There are platforms always retun "closed" as initial lid state.
+
+2. Restrictions of the lid state change notifications
+
+There are buggy AML tables never notifying when the lid device state is
+changed to "opened". Thus the "opened" notification is not guaranteed. But
+it is guaranteed that the AML tables always notify "closed" when the lid
+state is changed to "closed". The "closed" notification is normally used to
+trigger some system power saving operations on Windows. Since it is fully
+tested, it is reliable from all AML tables.
+
+3. Expections for the userspace users of the ACPI lid device driver
+
+The ACPI button driver exports the lid state to the userspace via the
+following file:
+  /proc/acpi/button/lid/LID0/state
+This file actually calls the _LID control method described above. And given
+the previous explanation, it is not reliable enough on some platforms. So
+it is advised for the userspace program to not to solely rely on this file
+to determine the actual lid state.
+
+The ACPI button driver emits the following input event to the userspace:
+  SW_LID
+The ACPI lid device driver is implemented to try to deliver the platform
+triggered events to the userspace. However, given the fact that the buggy
+firmware cannot make sure "opened"/"closed" events are paired, the ACPI
+button driver uses the following 3 modes in order not to trigger issues.
+
+If the userspace hasn't been prepared to ignore the unreliable "opened"
+events and the unreliable initial state notification, Linux users can use
+the following kernel parameters to handle the possible issues:
+A. button.lid_init_state=method:
+   When this option is specified, the ACPI button driver reports the
+   initial lid state using the returning value of the _LID control method
+   and whether the "opened"/"closed" events are paired fully relies on the
+   firmware implementation.
+   This option can be used to fix some platforms where the returning value
+   of the _LID control method is reliable but the initial lid state
+   notification is missing.
+   This option is the default behavior during the period the userspace
+   isn't ready to handle the buggy AML tables.
+B. button.lid_init_state=open:
+   When this option is specified, the ACPI button driver always reports the
+   initial lid state as "opened" and whether the "opened"/"closed" events
+   are paired fully relies on the firmware implementation.
+   This may fix some platforms where the returning value of the _LID
+   control method is not reliable and the initial lid state notification is
+   missing.
+
+If the userspace has been prepared to ignore the unreliable "opened" events
+and the unreliable initial state notification, Linux users should always
+use the following kernel parameter:
+C. button.lid_init_state=ignore:
+   When this option is specified, the ACPI button driver never reports the
+   initial lid state and there is a compensation mechanism implemented to
+   ensure that the reliable "closed" notifications can always be delievered
+   to the userspace by always pairing "closed" input events with complement
+   "opened" input events. But there is still no guarantee that the "opened"
+   notifications can be delivered to the userspace when the lid is actually
+   opens given that some AML tables do not send "opened" notifications
+   reliably.
+   In this mode, if everything is correctly implemented by the platform
+   firmware, the old userspace programs should still work. Otherwise, the
+   new userspace programs are required to work with the ACPI button driver.
+   This option will be the default behavior after the userspace is ready to
+   handle the buggy AML tables.