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

[Benjamin Tissoires <benjamin.tissoires@gmail.com>]

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.

> +
> +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.

Cheers,
Benjamin

> --
> 1.7.10
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