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

[Zheng, Lv]

Hi,

> 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 Fri, Jul 8, 2016 at 7:51 PM, Dmitry Torokhov
> <dmitry.torokhov@xxxxxxxxx> wrote:
> > 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.
> >
> 
> I really wished we could leave the kernel alone, but some platform
> need fixes: we are using an EV_SW, and on those platform, we only get
> the close event, which means it gets ignored by the input layer.
> Those platform have a form factor which makes the situation acceptable
> (tablet with cover, or very low cost laptops).
> However, switching away from a different EV_SW and tell userspace to
> ignore it would break systems when you are using a docking station.
> The acpi would provide fake values for the LID switch and this will
> screw the session if you are working on a docking station with an
> external monitor and the LID closed.
> 
> My initial suggestion was:
> - detect in the kernel whether the ACPI LID information was judged as
> reliable
> - if reliable keep things as it
> - if not reliable add an extra KEY_LID_CLOSE to notify userspace that
> the SW_LID is not reliable (it will emulate the LID open events) and
> that it will only get true close events.
[Lv Zheng] 
The problem is:
ACPI subsystem has no idea if it is reliable.

The only possible mean to get this awareness is:
1. Waiting for the user report,
2. Check the AML tables and confirm if this is the problem,
3. Add a quirk in the kernel.
As there could be many such kind of "unreliable tables", the quirk list will be endless.
That's the motivation of this discussion:
We need to find a way, end the need of introducing such kind of endless quirk list into the kernel.

The kernel parameter "button.lid_init_state" is a solution to eliminate the need of writing such an endless quirk table.
However, it doesn't fix anything.
Some unwanted power consumptions are caused by the userspace behavior.
And it is not fixable in the kernel. That's the motivation for us to discuss here.

> 
> After further thoughts, I think we can simply add the extra key, and
> have an hwdb entry in logind to enumerate the devices only relying on
> the close event. If the userspace is not updated, we can tell user to
> use the button.lid_init_state=open quirk to simulate the behavior
> logind should expect.
> 
> That means only adding one extra key (and its events) in the kernel
> and let userspace decide what to do.

[Lv Zheng] 
Yes. All sounds reasonable.

But TBH, I really do not know how can ACPI subsystem determine the following stuffs automatically:
1. if _LID initial state is reliable or not,
2. will open event be sent by the platform or not.

So my choice will be:
Leaving this to be determined by the users.
We can have a kernel parameter, switching acpi button driver between SW_LID and KEY_LID_CLOSE.
IMO, this should be sufficient for the vendors.

Thanks and best regards
-Lv