Date:   Wed, 26 Apr 2017 00:34:55 +0200
From:   Pavel Machek <pavel@....cz>
To:     David Lin <dtwlin@...gle.com>
Cc:     Jacek Anaszewski <jacek.anaszewski@...il.com>, rpurdie@...ys.net,
        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

On Mon 2017-04-24 20:05:59, David Lin wrote:
> 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

I believe we should fix the documentation. It is LED subsystem,
requirements are different, and we _already_ have haptic feedback
subsystem.
								Pavel

IOW, I suggest this: (hmm, and more led->LED is needed, and more
english fixes. Oh well.)

Signed-off-by: Pavel Machek <pavel@....cz>

diff --git a/Documentation/leds/ledtrig-transient.txt b/Documentation/leds/ledtrig-transient.txt
index 3bd38b4..c5cf475 100644
--- a/Documentation/leds/ledtrig-transient.txt
+++ b/Documentation/leds/ledtrig-transient.txt
@@ -16,17 +16,11 @@ set a timer to hold a state, however when user space application crashes or
 goes away without deactivating the timer, the hardware will be left in that
 state permanently.
 
-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.
-
 Transient trigger addresses the need for one shot timer activation. The
 transient trigger can be enabled and disabled just like the other leds
 triggers.
 
-When an led class device driver registers itself, it can specify all leds
+When an LED class device driver registers itself, it can specify all leds
 triggers it supports and a default trigger. During registration, activation
 routine for the default trigger gets called. During registration of an led
 class device, the LED state does not change.
@@ -144,7 +138,6 @@ repeat the following step as needed:
 	echo none > trigger
 
 This trigger is intended to be used for for the following example use cases:
- - Control of vibrate (phones, tablets etc.) hardware by user space app.
  - Use of LED by user space app as activity indicator.
  - Use of LED by user space app as a kind of watchdog indicator -- as
        long as the app is alive, it can keep the LED illuminated, if it dies


-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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