RE: [PATCH v2 4/4] ACPI / button: Add document for ACPI control method lid device restrictions

[Zheng, Lv]

Hi, Benjamin

> From: Benjamin Tissoires [mailto:benjamin.tissoires@xxxxxxxxx]
> Subject: Re: [PATCH v2 4/4] ACPI / button: Add document for ACPI control
> method lid device restrictions
> 
> On Tue, Jul 19, 2016 at 9:17 AM, Zheng, Lv <lv.zheng@xxxxxxxxx> wrote:
> > Hi, Dmitry
> >
> >> From: Dmitry Torokhov [mailto:dmitry.torokhov@xxxxxxxxx]
> >> Subject: Re: [PATCH v2 4/4] ACPI / button: Add document for ACPI
> control
> >> method lid device restrictions
> >>
> >> On Fri, Jul 08, 2016 at 11:17:39AM +0200, Benjamin Tissoires wrote:
> >> > Hi,
> >> >
> >> > On Thu, Jul 7, 2016 at 9:11 AM, Lv Zheng <lv.zheng@xxxxxxxxx>
> 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@xxxxxxxxx>
> >> > > Cc: Bastien Nocera: <hadess@xxxxxxxxxx>
> >> > > Cc: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxx>
> >> > > Cc: linux-input@xxxxxxxxxxxxxxx
> >> > > ---
> >> > >  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@xxxxxxxxx>
> >> > > +
> >> > > +
> >> > > +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.
> >> >
> >> > > +
> >> > > +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.
> >> >
> >> > > +
> >> > > +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.
> >>
> >> How about we leave the kernel alone and userspace (which would have
> to
> >> cope with the new KEY_LID_OPEN anyway) would just have to know
> that if
> >> switch's parent is PNP0C0D:00 (or phys is PNP0C0D/button/input0)
> then
> >> it
> >> can't trust the events and it needs additional heuristics.
> >
> > [Lv Zheng]
> > I found a problem with the key event approach.
> > And need your suggestions.
> >
> > Some AML tables invoke Notify(lid_device, ...) in several different places.
> > It may be invoked from different functions.
> >
> > Finally, it's not guaranteed that one "lid close" action can only trigger
> one key close notification.
> > If we use EV_KEY, then there should be many platforms triggering
> multiple "lid close" events to the user space.
> >
> > Original switch event based design can automatically eliminate the
> redundant events.
> >
> > Does input layer has an event type that can handle such situation?
> > Or shall ACPI button driver handle this?
> >
> 
> Keys also have some redundant event elimination, but it's as long as
> you are holding the key in the press (or released) position. So in
> your case, that would mean sending input_key(1), wait a little while
> other notifications are processed, and then sending input_key(0)
> (assuming each notification comes in with its own thread). Not sure
> you will gain anything from the new implementation you just sent with
> the rate-limit.

[Lv Zheng] 
However this key is virtual.
The multiple notifications are just triggered by the AML code.
The Notify(lid_device, xxx) may be invoked in a function.
And this function may be invoked multiple times by other control methods.
So I do not know when it is "released".

Using the feature of the keys, it sounds like that I should setup a timer.
When the state is changed, I should report input_key(1) and prepare the timer.
Then report the input_key(0) when the timer times out.
The side effect is the input_key(0) will be deferred.

I just refreshed the patch as v4 with an ACPI button driver internal workaround.
By adding a time_after() check which looks more lightweight than setting up a timer.
Could you also help to check if that solution is OK?
Thanks in advance.

Best regards
-Lv