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Date:   Sun, 17 Sep 2017 18:41:09 +0200
From:   Jacek Anaszewski <>
To:     Dmitry Torokhov <>
Cc:     Pavel Machek <>,
        "" <>,
        David Lin <>,
        Jonathan Corbet <>,
        Richard Purdie <>,
        Hans de Goede <>,
        Greg Kroah-Hartman <>,
        Rob Herring <>,
        Rom Lemarchand <>,
        "" <>,
        lkml <>,
        "" <>
Subject: Re: Vibrations in input vs. LED was Re: [PATCH v2 0/3] led:
 ledtrig-transient: add support for hrtimer

On 09/16/2017 12:30 AM, Dmitry Torokhov wrote:
> On Fri, Sep 15, 2017 at 2:55 PM, Jacek Anaszewski
> <> wrote:
>> On 09/15/2017 08:34 PM, Dmitry Torokhov wrote:
>>> On Thu, Sep 14, 2017 at 1:58 PM, Pavel Machek <> wrote:
>>>> On Thu 2017-09-14 21:31:31, Jacek Anaszewski wrote:
>>>>> Hi David and Pavel,
>>>>> On 09/13/2017 10:20 PM, Pavel Machek wrote:
>>>>>> Hi!
>>>>>>> These patch series add the LED_BRIGHTNESS_FAST flag support for
>>>>>>> ledtrig-transient to use hrtimer so that platforms with high-resolution timer
>>>>>>> support can have better accuracy in the trigger duration timing. The need for
>>>>>>> this support is driven by the fact that Android has removed the timed_ouput [1]
>>>>>>> and is now using led-trigger for handling vibrator control which requires the
>>>>>>> timer to be accurate up to a millisecond. However, this flag support would also
>>>>>>> allow hrtimer to co-exist with the ktimer without causing warning to the
>>>>>>> existing drivers [2].
>>>>>> NAK.
>>>>>> LEDs do not need extra overhead, and vibrator control should not go
>>>>>> through LED subsystem.
>>>>>> Input subsystem includes support for vibrations and force
>>>>>> feedback. Please use that instead.
>>>>> I think that most vital criterion here is the usability of the
>>>>> interface. If it can be harnessed for doing the work seemingly
>>>>> unrelated to the primary subsystem's purpose, that's fine.
>>>>> Moreover, it is extremely easy to use in comparison to the force
>>>>> feedback one.
>>>> Well, no.
>>>> Kernel is supposed to provide hardware abstraction, that means it
>>>> should hide differences between different devices.
>>>> And we already have devices using input as designed. We don't want to
>>>> have situation where "on phones A, D and E, vibrations are handled via
>>>> input, while on B, C and F, they are handled via /sys/class/leds".
>>>> If we want to have discussion "how to make vibrations in input
>>>> easier to use", well that's fair. But I don't think it is particulary hard.
>>> I would like to know more about why you find the FF interface hard,
>> led-transient trigger can be activated using only following bash
>> commands:
>> # echo 1 > state
>> # echo 1000 > duration
>> # while [ 1 ]; do echo 1 > activate; sleep 3; done
>> Could you share sample sequence of commands to use ff driver?
> Cut what you need from this:
> 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.
However only if following conditions are met:
- force feedback driver supports gpio driven devices
- there is sample application in tools/input showing how to
  setup gpio driven vibrate device with use of ff interface
- it will be possible to setup vibrate interval with 1ms accuracy,
  similarly to what the discussed patch allows to do

>>> given that for rumble you need calls - one ioctl to set up rumble
>>> parameters, and a write to start the playback. The FF core should take
>>> care of handling duration of the effect, ramping it up and decaying,
>>> if desired, and we make sure to automatically stop effects when
>>> userspace closes the fd (because of ordinary exit or crash or FD being
>>> revoked).
>>>> If we want to say "lets move all vibrations from input to LED
>>>> subsystem"... I don't think that is good idea, but its a valid
>>>> discussion. Some good reasons would be needed.
>>>> But having half devices use one interface and half use different one
>>>> is just bad...
>>> Completely agree here. I just merged PWM vibra driver from Sebastian
>>> Reichel, we already had regulator-haptic driver, do we need gpio-based
>>> one? Or make regulator-based one working with fixed regulators?
>> Just to clarify: the background of this discussion is the question
>> whether we should remove the following lines from
>> Documentation/leds/ledtrig-transient.txt:
>> -As a specific example of this use-case, let's look at vibrate feature on
>> -phones. Vibrate function on phones is implemented using PWM pins on SoC or
>> -PMIC. There is a need to activate one shot timer to control the vibrate
>> -feature, to prevent user space crashes leaving the phone in vibrate mode
>> -permanently causing the battery to drain.
>> whether we should remove the following use case example from
>> In effect Pavel has objections to increasing ledtrig-transient
>> interval accuracy by adding hr_timer support to it, because vibrate
>> devices, as one of the use cases, can benefit from it.
>> So there are two issues:
>> 1. Addition of hr_timer support to LED trigger.
>> 2. Removal of vibrate devices use case from ledtrig-transient doc.
>> I am in favour of 1. and against 2. since we're not gaining anything
>> by hiding information about some kernel functionality when it will
>> still be there. It also doesn't define the location of any vibrate
>> device drivers, since sheer leds-gpio driver can be used for that
>> purpose.
> I would say that while leds-gpio can be used to do whatever (you can
> wire PS/2 mouse to a set of gpios and probably manage to drive it
> through a set of leds-gpio devices) it does not make it right
> interface for everything. We do have a standard interface for haptic
> (via rumble FF), at this moment I see no reason why we need another
> one. It just confuses userspace as it now needs to implement multiple
> interfaces, depending on the system it runs. Android expects that HAL
> will take care of hiding all of that from their upper layers and does
> not really pay close attention to kernel ABIs, but I think we should.

Generally I agree, provided that user will be able to achieve the
same using force feedback device as in case of LED transient
trigger, with the same easy guidance on how to employ it to do this
particular task with given hardware configuration.

Best regards,
Jacek Anaszewski

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