linux-kernel - Re: [PATCH] led: ledtrig-transient: replace timer

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Date:   Mon, 24 Apr 2017 20:05:59 -0700
From:   David Lin <dtwlin@...gle.com>
To:     Jacek Anaszewski <jacek.anaszewski@...il.com>
Cc:     rpurdie@...ys.net, pavel@....cz, robh@...nel.org,
        Rom Lemarchand <romlem@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>, stable@...r.kernel.org,
        linux-leds@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] led: ledtrig-transient: replace timer_list with hrtimer

Hi Jacek,

On Mon, Apr 24, 2017 at 12:59 PM, Jacek Anaszewski
<jacek.anaszewski@...il.com> wrote:
>
> Hi David,
>
> Thanks for the patch.
>
> Unfortunately we cannot switch to using hr timers just like that
> without introducing side effects for many devices. We had similar
> attempt of increasing timer tirgger accuracy two years ago [0].
>
> In short words, for drivers that can sleep while setting brightness
> and/or are using a bus like I2C you will not be able to enforce
> 1ms delay period.
>
> I recommend you to go through the thread [0] so that we had
> a well defined ground for the discussion on how to address this
> issue properly.
>

I think I understand the background now, and would agree that not all
the LED driver require hrtimer as human eye can't probably tell
there's a 10ms variation in a blink. However, there's a need to
support hrtimer if the LED subsystem claims support the use case of
vibrator (please see Documentation/leds/ledtrig-transient.txt) as even
a 5ms of variation is perceivable to the user. I'm thinking if a
better interim solution is to introduce a
LEDS_TRIGGER_TRANSIENT_HRTIMER config to work with both timers in
compile time. Would you agree?

David