linux-kernel - Re: Vibrations in input vs. LED was Re: [PATCH v2 0/3] led: ledtrig-transient: add support for hrtimer

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Message-ID: <CAKdAkRSfV2nHUMGJfkzHQcRK4sXOgREb0QZe2WQ6Hce0N8iSgQ@mail.gmail.com>
Date:   Tue, 19 Sep 2017 14:07:21 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Jacek Anaszewski <jacek.anaszewski@...il.com>
Cc:     Pavel Machek <pavel@....cz>,
        "linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
        David Lin <dtwlin@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Richard Purdie <rpurdie@...ys.net>,
        Hans de Goede <hdegoede@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh@...nel.org>,
        Rom Lemarchand <romlem@...gle.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "linux-leds@...r.kernel.org" <linux-leds@...r.kernel.org>
Subject: Re: Vibrations in input vs. LED was Re: [PATCH v2 0/3] led:
 ledtrig-transient: add support for hrtimer

On Tue, Sep 19, 2017 at 1:45 PM, Jacek Anaszewski
<jacek.anaszewski@...il.com> wrote:
> On 09/19/2017 12:29 AM, Dmitry Torokhov wrote:
>> On Mon, Sep 18, 2017 at 1:50 PM, Jacek Anaszewski
>> <jacek.anaszewski@...il.com> wrote:
>>> Hi,
>>>
>>> On 09/17/2017 08:22 PM, Pavel Machek wrote:
>>>> Hi!
>>>>
>>>>>> If your objection is that FF is not easily engaged from the shell -
>>>>>> yes, but I do not think that actual users who want to do vibration do
>>>>>> that via shell either. On the other hand, can you drop privileges and
>>>>>> still allow a certain process control your vibrator via LED interface?
>>>>>> With FF you can pass an FD to whoever you deem worthy and later revoke
>>>>>> access.
>>>>>>
>>>>>> IOW sysfs interfaces are nice for quick hacks, but when you want to
>>>>>> use them in real frameworks, where you need to think about proper
>>>>>> namespaces, isolation, etc, etc, other kinds of interfaces might suit
>>>>>> better.
>>>>>
>>>>> I'd leave the decision to the user. We could add a note to the
>>>>> Documentation/leds/ledtrig-transient.txt that force feedback interface
>>>>> should be preferable choice for driving vibrate devices.
>>>>
>>>> We don't want to leave decision to the user; because then we'll end up
>>>> with userland applications having to support _both_ interfaces.
>>>
>>> This state has lasted for five years now. I don't recall any
>>> complaints. Do you?
>>
>> I was telling Shuah 5 years ago that using LED for what she wanted was
>> not the right idea. I even made a patch for her implementing the
>> functionality they were after: https://lkml.org/lkml/2012/4/10/41
>>
>> Unfortunately this still somehow made it into the kernel. I guess the
>> angle of having LEDs auto-shutoff makes sense still; using it for
>> haptics does not.
>
> Thanks for the pointer.
>
>>>
>>>> Plus, it is not really your decision. Dmitry is maintainer of input
>>>> subsystem, input was doing force feedback for 10+ years, and he
>>>> already made a decision.
>>>
>>> It seems that you applied a fait accompli method here.
>>>
>>> Actually could you share what the decision is? AFAIK we're not
>>> discussing here any patch for the input subsystem?
>>
>> No, we are discussing if it makes sense encouraging 2nd interface for
>> haptic. We are also discussing whether it makes sense to implement new
>> features in LED subsystem that seem to be only beneficial for this
>> interface (I assume the "normal" LEDs do not need that kind of
>> precision?).
>
> As you noticed in one of the previous messages, thanks to the leds-gpio
> driver we can drive a wide range of devices from the level of
> LED subsystem.

Yes, you can create whatever. It goes normally as this: crap, we need
to ship something really soon, factory build is a week from now and we
have absolutely no time to think about proper interface. Hey, let's
quickly bolt on some quirk on an unrelated interface, it almost does
what we want. We do not care that out use case does not really fit
here, our custom one-off userspace will know how to deal with it.

> LED trigger mechanism makes it even more versatile,
> and, in this area, even somehow akin to the GPIO subsystem. In the
> future we could think of making this trigger mechanism accessible by
> the two and thus any initiative aiming at enhancing it shouldn't be
> discouraged.

If there is a use case that would benefit from this functionality:
certainly. Do you have one in mind?

Thanks.

-- 
Dmitry