Re: [PATCH v4 0/7] Support inhibiting input devices

Andrzej Pietrasiewicz
2020-06-10T13:12:49Z

Hi All,

W dniu 10.06.2020 o 12:38, Rafael J. Wysocki pisze:
> On Wed, Jun 10, 2020 at 11:50 AM Hans de Goede <hdegoede@redhat.com> wrote:
>>
>> Hi All,
>>
>> On 6/8/20 1:22 PM, Andrzej Pietrasiewicz wrote:
>>> This is a quick respin of v3, with just two small changes, please see
>>> the changelog below.
>>>
>>> Userspace might want to implement a policy to temporarily disregard input
>>> from certain devices.
>>>
>>> An example use case is a convertible laptop, whose keyboard can be folded
>>> under the screen to create tablet-like experience. The user then must hold
>>> the laptop in such a way that it is difficult to avoid pressing the keyboard
>>> keys. It is therefore desirable to temporarily disregard input from the
>>> keyboard, until it is folded back. This obviously is a policy which should
>>> be kept out of the kernel, but the kernel must provide suitable means to
>>> implement such a policy.
>>
>> First of all sorry to start a somewhat new discussion about this
>> while this patch set is also somewhat far along in the review process,
>> but I believe what I discuss below needs to be taken into account.
>>
>> Yesterday I have been looking into why an Asus T101HA would not stay
>> suspended when the LID is closed. The cause is that the USB HID multi-touch
>> touchpad in the base of the device starts sending events when the screen
>> gets close to the touchpad (so when the LID is fully closed) and these
>> events are causing a wakeup from suspend. HID multi-touch devices
>> do have a way to tell them to fully stop sending events, also disabling
>> the USB remote wakeup the device is doing. The question is when to tell
>> it to not send events though ...
>>
>> So now I've been thinking about how to fix this and I believe that there
>> is some interaction between this problem and this patch-set.
>>
>> The problem I'm seeing on the T101HA is about wakeups, so the question
>> which I want to discuss is:
>>
>> 1. How does inhibiting interact with enabling /
>> disabling the device as a wakeup source ?
>>
>> 2. Since we have now made inhibiting equal open/close how does open/close
>> interact with a device being a wakeup source ?
>>
>> And my own initial (to be discussed) answers to these questions:
>>
>> 1. It seems to me that when a device is inhibited it should not be a
>> wakeup source, so where possible a input-device-driver should disable
>> a device's wakeup capabilities on suspend if inhibited
>
> If "inhibit" means "do not generate any events going forward", then
> this must also cover wakeup events, so I agree.

I agree, too.

>
>> 2. This one is trickier I don't think we have really clearly specified
>> any behavior here. The default behavior of most drivers seems to be
>> using something like this in their suspend callback:
>>
>> if (device_may_wakeup(dev))
>> enable_irq_wake(data->irq);
>> else if (input->users)
>> foo_stop_receiving_events(data);
>>
>> Since this is what most drivers seem to do I believe we should keep
>> this as is and that we should just clearly document that if the
>> input_device has users (has been opened) or not does not matter
>> for its wakeup behavior.
>>
>> Combining these 2 answers leads to this new pseudo code template
>> for an input-device's suspend method:
>>
>> /*
>> * If inhibited we have already disabled events and
>> * we do NOT want to setup the device as wake source.
>> */
>> if (input->inhibited)
>> return 0;

Right, if a device is inhibited it shouldn't become a wakeup source,
because that would contradict the purpose of being inhibited.

>>
>> if (device_may_wakeup(dev))
>> enable_irq_wake(data->irq);

What would it mean to become a wakeup source if there are no users,
or nobody has ever opened the device? There are no interested
input handlers (users) so what's the point of becoming a wakeup
source? Why would the system need to wake up?

>> else if (input->users)
>> foo_stop_receiving_events(data);
>>
>> ###
>
> Sounds reasonable to me.
>
>> A different, but related issue is how to make devices actually use the
>> new inhibit support on the builtin keyboard + touchpad when say the lid
>> is closed. Arguably this is an userspace problem, but it is a tricky
>> one. Currently on most modern Linux distributions suspend-on-lid-close
>> is handled by systemd-logind and most modern desktop-environments are
>> happy to have logind handle this for them.
>>
>> But most knowledge about input devices and e.g. heurisitics to decide
>> if a touchpad is internal or external are part of libinput. Now we could
>> have libinput use the new inhibit support (1), but then when the lid
>> closes we get race between whatever process is using libinput trying
>> to inhibit the touchpad (which must be done before to suspend to disable
>> it as wakeup source) and logind trying to suspend the system.
>>
>> One solution here would be to move the setting of the inhibit sysfs
>> attr into logind, but that requires adding a whole bunch of extra
>> knowledge to logind which does not really belong there IMHO.
>>
>> I've been thinking a bit about this and to me it seems that the kernel
>> is in the ideal position to automatically inhibit some devices when
>> some EV_SW transitions from 0->1 (and uninhibit again on 1->0). The
>> issue here is to chose on which devices to enable this. I believe
>> that the auto inhibit on some switches mechanism is best done inside
>> the kernel (disabled by default) and then we can have a sysfs
>> attr called auto_inhibit_ev_sw_mask which can be set to e.g.
>> (1 << SW_LID) to make the kernel auto-inhibit the input-device whenever
>> the lid is closed, or to ((1 << SW_LID) | (1 << SW_TABLET_MODE)) to
>> inhibit both when the lid is closed or when switched to tablet mode.
>
> I agree that the kernel is the right place to handle this, but it
> requires some extra knowledge about dependencies between devices.
>
> It'd be kind of like power resources in ACPI, so for each state of a
> "master" device (in principle, there may be more states of it than
> just two) there would be a list of "dependent" intput devices that
> need to be inhibited when the "master" device goes into that state.
>
>> This could then be combined with a userspace utility run from an
>> udev rule which makes the actual decision what auto_inhibit_ev_sw_mask
>> should be set for a given input device.
>>
>> This will put the mechanism for what we want inside the kernel and
>> leaves the policy on which switches we want this for out of the
>> kernel.
>>
>> Note adding this new auto_inhibit_ev_sw_mask sysfs attr falls
>> somewhat outside the context of this patchset and could be done
>> as a follow up to this patch-set.

Yes, please ;)

But I do believe that we need to
>> figure out how (non ChromeOS) userspace can / will use the new inhibit
>> interface before merging it.

Of course.

Regards,

Andrzej