forked from Minki/linux
f9151fcfcd
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>
97 lines
4.9 KiB
Plaintext
97 lines
4.9 KiB
Plaintext
Special Usage Model of the ACPI Control Method Lid Device
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Copyright (C) 2016, Intel Corporation
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Author: Lv Zheng <lv.zheng@intel.com>
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Abstract:
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Platforms containing lids convey lid state (open/close) to OSPMs using a
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control method lid device. To implement this, the AML tables issue
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Notify(lid_device, 0x80) to notify the OSPMs whenever the lid state has
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changed. The _LID control method for the lid device must be implemented to
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report the "current" state of the lid as either "opened" or "closed".
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For most platforms, both the _LID method and the lid notifications are
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reliable. However, there are exceptions. In order to work with these
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exceptional buggy platforms, special restrictions and expections should be
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taken into account. This document describes the restrictions and the
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expections of the Linux ACPI lid device driver.
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1. Restrictions of the returning value of the _LID control method
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The _LID control method is described to return the "current" lid state.
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However the word of "current" has ambiguity, some buggy AML tables return
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the lid state upon the last lid notification instead of returning the lid
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state upon the last _LID evaluation. There won't be difference when the
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_LID control method is evaluated during the runtime, the problem is its
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initial returning value. When the AML tables implement this control method
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with cached value, the initial returning value is likely not reliable.
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There are platforms always retun "closed" as initial lid state.
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2. Restrictions of the lid state change notifications
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There are buggy AML tables never notifying when the lid device state is
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changed to "opened". Thus the "opened" notification is not guaranteed. But
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it is guaranteed that the AML tables always notify "closed" when the lid
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state is changed to "closed". The "closed" notification is normally used to
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trigger some system power saving operations on Windows. Since it is fully
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tested, it is reliable from all AML tables.
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3. Expections for the userspace users of the ACPI lid device driver
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The ACPI button driver exports the lid state to the userspace via the
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following file:
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/proc/acpi/button/lid/LID0/state
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This file actually calls the _LID control method described above. And given
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the previous explanation, it is not reliable enough on some platforms. So
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it is advised for the userspace program to not to solely rely on this file
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to determine the actual lid state.
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The ACPI button driver emits the following input event to the userspace:
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SW_LID
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The ACPI lid device driver is implemented to try to deliver the platform
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triggered events to the userspace. However, given the fact that the buggy
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firmware cannot make sure "opened"/"closed" events are paired, the ACPI
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button driver uses the following 3 modes in order not to trigger issues.
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If the userspace hasn't been prepared to ignore the unreliable "opened"
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events and the unreliable initial state notification, Linux users can use
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the following kernel parameters to handle the possible issues:
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A. button.lid_init_state=method:
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When this option is specified, the ACPI button driver reports the
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initial lid state using the returning value of the _LID control method
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and whether the "opened"/"closed" events are paired fully relies on the
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firmware implementation.
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This option can be used to fix some platforms where the returning value
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of the _LID control method is reliable but the initial lid state
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notification is missing.
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This option is the default behavior during the period the userspace
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isn't ready to handle the buggy AML tables.
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B. button.lid_init_state=open:
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When this option is specified, the ACPI button driver always reports the
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initial lid state as "opened" and whether the "opened"/"closed" events
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are paired fully relies on the firmware implementation.
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This may fix some platforms where the returning value of the _LID
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control method is not reliable and the initial lid state notification is
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missing.
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If the userspace has been prepared to ignore the unreliable "opened" events
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and the unreliable initial state notification, Linux users should always
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use the following kernel parameter:
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C. button.lid_init_state=ignore:
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When this option is specified, the ACPI button driver never reports the
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initial lid state and there is a compensation mechanism implemented to
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ensure that the reliable "closed" notifications can always be delievered
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to the userspace by always pairing "closed" input events with complement
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"opened" input events. But there is still no guarantee that the "opened"
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notifications can be delivered to the userspace when the lid is actually
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opens given that some AML tables do not send "opened" notifications
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reliably.
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In this mode, if everything is correctly implemented by the platform
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firmware, the old userspace programs should still work. Otherwise, the
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new userspace programs are required to work with the ACPI button driver.
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This option will be the default behavior after the userspace is ready to
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handle the buggy AML tables.
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