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Date:   Sat, 12 May 2018 10:35:59 +0200
From:   Pavel Machek <pavel@ucw.cz>
To:     Jacek Anaszewski <jacek.anaszewski@gmail.com>
Cc:     Baolin Wang <baolin.wang@linaro.org>, robh+dt@kernel.org,
        mark.rutland@arm.com, xiaotong.lu@spreadtrum.com,
        broonie@kernel.org, linux-leds@vger.kernel.org,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/2] leds: Add Spreadtrum SC27xx breathing light
 controller driver
Message-ID: <20180512083559.GB8944@amd>
References: <f393ca5a99059e7086307b99fc435777fe09018b.1525757122.git.baolin.wang@linaro.org>
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Hi!

> >>I disagree here. We already had the same discussion at the occasion
> >>of the patch [0] and it turned out to be a dead-end [1]. Now we have
> >>neither the driver nor the generic pattern interface.
> >>
> >>We also already have some older LED class drivers that implement custom
> >>pattern interfaces (e.g. drivers/leds/leds-lm3533.c) and the same
> >>approach can be applied in this case.
> >
> >Please don't. It was mistake to implement custom pattern interfaces
> >back then, it is still mistake now.
>=20
> It turned out to be really hard to cover all known pattern generator
> implementations with generic interface. Sure, it would be nice to have
> one, but the whole discussion around [0] only unveiled the diversity of
> parameters to cover. And still new devices appear on the market.
>=20
> We would have to propose a set of pattern schemes and allow to
> add new ones to it.

I believe that what I'm proposing below is close enough to universal.

> >If we really need solution now, I'd recommend "pattern" file with
> >
> >"<delta time> <brightness> <delta time> <brightness>".
> >
> >In this specific case, hardware only supports patterns in this format:
> >
> >low_time 0 rise_time 255 high_time 255 fall_time 0
> >
> >so driver would simply -EINVAL on anything else.
>=20
> I'm fine with the pattern file, but the pattern format would have
> to be defined in the per-driver ABI documentation. It wouldn't much
> differ from the custom pattern approach though, unless I'm missing some
> gain of having pattern setting in a uniformly named single sysfs file
> (with semantics differing from driver to driver).

I'm proposing "<delta time> <brightness> ..." sysfs file. It certainly
covers this hardware, it would be enough to cover the Qualcomm Pulse
generator (IIRC), and it would cover most uses cases of Nokia N900's
LED.

Yes, we would need to document limitations of each chip. But it should
be easily possible to run pattern designed for Spreadtrum on N900,
even if it would not work the other way around.

(If someone really wants to run complex patterns on simple hardware,
we can provide software emulation using same file format. I believe I
still have that patch somewhere.)

									Pavel
--=20
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo=
g.html

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