linux-kernel - Re: [PATCH v3 1/2] leds: core: Introduce generic pattern interface

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Date:   Tue, 17 Jul 2018 23:07:00 +0200
From:   Pavel Machek <pavel@....cz>
To:     Jacek Anaszewski <jacek.anaszewski@...il.com>
Cc:     David Lechner <david@...hnology.com>,
        Baolin Wang <baolin.wang@...aro.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Mark Brown <broonie@...nel.org>,
        Linux LED Subsystem <linux-leds@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/2] leds: core: Introduce generic pattern interface

Hi!

> >>- different hardware means via which brightness is set (MMIO, I2C, SPI,
> >>   PWM and other pulsed fashion based protocols),
> >>- the need for deferring brightness setting to a workqueue task to
> >>   allow for setting LED brightness from atomic context,
> >>- contention on locks
> >
> >I disagree here. Yes, it would be hard to synchronize blinking down to
> >microseconds, but it would be easy to get synchronization right down
> >to miliseconds and humans will not be able to tell the difference.
> 
> There have been problems with blink interval stability close to
> 1ms, and thus there were some attempts of employing hr timers,
> which in turn introduced a new class of issues related to
> system performance etc.

Yeah, well. This is LED subsystem. Noone should program blink
intervals at 1 msec.

> >>For the LEDs driven by the same chip it would make more sense
> >>to allow for synchronization, but it can be achieved on driver
> >>level, with help of some subsystem level interface to indicate
> >>which LEDs should be synchronized.
> >>
> >>However, when we start to pretend that we can synchronize the
> >>devices, we must answer how accurate we can be. The accuracy
> >>will decrease as blink frequency rises. We'd need to define
> >>reliability limit.
> >
> >We don't need _that_ ammount of overengineering. We just need to
> >synchronize them well enough :-).
> 
> Well, it would be disappointing for the users to realize that
> they don't get the effect advertised by the ABI documentation.

Linux is always best-effort, w.r.t. timing. And we can do well enough
that user will not see anything bad on "normal" systems.

> >>We've had few attempts of approaching the subject of synchronized
> >>blinking but none of them proved to be good enough to be merged.
> >
> >I'm sure interested person could do something like that in less than
> >two weeks fulltime... It is not rocket science, just a lot of work in
> >kernel...
> >
> >But patterns are few years overdue and I believe we should not delay
> >them any further.
> >
> >So... I guess I agree with Jacek in the end :-).
> 
> How about taking Baolin's patches as of v5? Later, provided that
> the pattern trigger yet to be implemented will create pattern file
> on activation, we'll need to initialize default-trigger DT property,
> to keep the interface unchanged.

I have yet to look at the v5 of patches. But I agree that we do not
need to design synchronization at this moment.

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

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