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SubjectRe: [PATCH v2 4/4] ACPI / button: Add document for ACPI control method lid device restrictions
[I just realised Ctrl+enter means "send" for gmail, see the end of the answers]

On Mon, Jul 11, 2016 at 1:42 PM, Benjamin Tissoires
<benjamin.tissoires@gmail.com> wrote:
> On Mon, Jul 11, 2016 at 5:20 AM, Zheng, Lv <lv.zheng@intel.com> wrote:
>> Hi,
>>
>>> From: Benjamin Tissoires [mailto:benjamin.tissoires@gmail.com]
>>> Subject: Re: [PATCH v2 4/4] ACPI / button: Add document for ACPI control
>>> method lid device restrictions
>>>
>>> Hi,
>>>
>>> On Thu, Jul 7, 2016 at 9:11 AM, Lv Zheng <lv.zheng@intel.com> wrote:
>>> > There are many AML tables reporting wrong initial lid state, and some of
>>> > them never reports lid state. As a proxy layer acting between, ACPI
>>> button
>>> > driver is not able to handle all such cases, but need to re-define the
>>> > usage model of the ACPI lid. That is:
>>> > 1. It's initial state is not reliable;
>>> > 2. There may not be open event;
>>> > 3. Userspace should only take action against the close event which is
>>> > reliable, always sent after a real lid close.
>>> > This patch adds documentation of the usage model.
>>> >
>>> > Link: https://lkml.org/2016/3/7/460
>>> > Link: https://github.com/systemd/systemd/issues/2087
>>> > Signed-off-by: Lv Zheng <lv.zheng@intel.com>
>>> > Cc: Bastien Nocera: <hadess@hadess.net>
>>> > Cc: Benjamin Tissoires <benjamin.tissoires@gmail.com>
>>> > Cc: linux-input@vger.kernel.org
>>> > ---
>>> > Documentation/acpi/acpi-lid.txt | 62
>>> +++++++++++++++++++++++++++++++++++++++
>>> > 1 file changed, 62 insertions(+)
>>> > create mode 100644 Documentation/acpi/acpi-lid.txt
>>> >
>>> > diff --git a/Documentation/acpi/acpi-lid.txt b/Documentation/acpi/acpi-
>>> lid.txt
>>> > new file mode 100644
>>> > index 0000000..7e4f7ed
>>> > --- /dev/null
>>> > +++ b/Documentation/acpi/acpi-lid.txt
>>> > @@ -0,0 +1,62 @@
>>> > +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".
>>> > +
>>> > +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, many 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
>>> > +simply so many examples always retuning "closed" as initial lid state.
>>> > +
>>> > +2. Restrictions of the lid state change notifications
>>> > +
>>> > +There are many AML tables never notifying when the lid device state is
>>> > +changed to "opened". But it is ensured that the AML tables always
>>> notify
>>> > +"closed" when the lid state is changed to "closed". This is normally used
>>> > +to trigger some system power saving operations on Windows. Since it is
>>> > +fully tested, this notification is reliable for all AML tables.
>>> > +
>>> > +3. Expections for the userspace users of the ACPI lid device driver
>>> > +
>>> > +The userspace programs should stop relying on
>>> > +/proc/acpi/button/lid/LID0/state to obtain the lid state. This file is only
>>> > +used for the validation purpose.
>>>
>>> I'd say: this file actually calls the _LID method described above. And
>>> given the previous explanation, it is not reliable enough on some
>>> platforms. So it is strongly advised for user-space program to not
>>> solely rely on this file to determine the actual lid state.
>> [Lv Zheng]
>> OK.
>>
>>>
>>> > +
>>> > +New userspace programs should rely on the lid "closed" notification to
>>> > +trigger some power saving operations and may stop taking actions
>>> according
>>> > +to the lid "opened" notification. A new input switch event -
>>> SW_ACPI_LID is
>>> > +prepared for the new userspace to implement this ACPI control method
>>> lid
>>> > +device specific logics.
>>>
>>> That's not entirely what we discussed before (to prevent regressions):
>>> - if the device doesn't have reliable LID switch state, then there
>>> would be the new input event, and so userspace should only rely on
>>> opened notifications.
>>> - if the device has reliable switch information, the new input event
>>> should not be exported and userspace knows that the current input
>>> switch event is reliable.
>>>
>>> Also, using a new "switch" event is a terrible idea. Switches have a
>>> state (open/close) and you are using this to forward a single open
>>> event. So using a switch just allows you to say to userspace you are
>>> using the "new" LID meaning, but you'll still have to manually reset
>>> the switch and you will have to document how this event is not a
>>> switch.
>>>
>>> Please use a simple KEY_LID_OPEN event you will send through
>>> [input_key_event(KEY_LID_OPEN, 1), input_sync(),
>>> input_key_event(KEY_LID_OPEN, 0), input_sync()], which userspace knows
>>> how to handle.
>> [Lv Zheng]
>> It should be KEY_LID_CLOSE.
>
> yep, sorry.
>
>> However I checked the KEY code definitions.
>> It seems their values are highly dependent on the HID specification.
>
> That was convenient enough when the code was written. Now, we can
> extend new keycodes as we want, as long as Dmitry agrees :)
>
>> I'm not sure which key code value should I use for this.
>> Could you point me out?
>>
>

I think using 0x277 (or 0x278 given that KEY_DATA is reusing
KEY_FASTREVERSE) would be fine.

>
>>>
>>> > +
>>> > +During the period the userspace hasn't been switched to use the new
>>> > +SW_ACPI_LID event, Linux users can use the following boot parameter
>>> to
>>> > +handle possible issues:
>>> > + button.lid_init_state=method:
>>> > + This is the default behavior of the Linux ACPI lid driver, Linux kernel
>>> > + reports the initial lid state using the returning value of the _LID
>>> > + control method.
>>> > + This can be used to fix some platforms if the _LID control method's
>>> > + returning value is reliable.
>>> > + button.lid_init_state=open:
>>> > + Linux kernel always reports the initial lid state as "opened".
>>> > + This may fix some platforms if the returning value of the _LID control
>>> > + method is not reliable.
>>>
>>> This worries me as there is no plan after "During the period the
>>> userspace hasn't been switched to use the new event".
>>>
>>> I really hope you'll keep sending SW_LID for reliable LID platforms,
>>> and not remove it entirely as you will break platforms.
>>
>> [Lv Zheng]
>> We won't remove SW_LID from the kernel :).
>>
>> And we haven't removed SW_LID from the acpi button driver.
>> We'll just stop sending "initial lid state" from acpi button driver, i.e., the behavior carried out by "button.lid_init_state=ignore".

That won't do for the very same use case than before. It makes sense
to boot a laptop while the LID is closed if you have an external
monitor plugged in (the docking station allows to have an extra power
button accessible when the lid is closed).

>>
>> Maybe it is not sufficient, after the userspace has been changed to support the new event, we should stop sending SW_LID from acpi button driver.

I'd say do not touch SW_LID at all (and allow users to tweak its
behavior for local fixes, which is what you currently have).
Just send the extra KEY_LID_CLOSE, no matter what.
And start listing which devices have troubles, and we can add those
into a hwdb file shipped with logind. I hope the systemd team will
agree with me.

Cheers,
Benjamin

>>
>> Cheers,
>> -Lv

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