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

[Dmitry Torokhov]

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.

Thanks.

-- 
Dmitry