linux-kernel - Re: [PATCH 0/4] Add a generic driver for LED-based backlight

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Message-ID: <20190705102959.cmqhzpzikqjsydlx@holly.lan>
Date:   Fri, 5 Jul 2019 11:29:59 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Pavel Machek <pavel@....cz>
Cc:     Jean-Jacques Hiblot <jjhiblot@...com>, jacek.anaszewski@...il.com,
        robh+dt@...nel.org, mark.rutland@....com, lee.jones@...aro.org,
        jingoohan1@...il.com, dmurphy@...com, linux-leds@...r.kernel.org,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        tomi.valkeinen@...com
Subject: Re: [PATCH 0/4] Add a generic driver for LED-based backlight

On Fri, Jul 05, 2019 at 12:14:34PM +0200, Pavel Machek wrote:
> On Mon 2019-07-01 17:14:19, Jean-Jacques Hiblot wrote:
> > This series aims to add a led-backlight driver, similar to pwm-backlight,
> > but using a LED class device underneath.
> > 
> > A few years ago (2015), Tomi Valkeinen posted a series implementing a
> > backlight driver on top of a LED device:
> > https://patchwork.kernel.org/patch/7293991/
> > https://patchwork.kernel.org/patch/7294001/
> > https://patchwork.kernel.org/patch/7293981/
> > 
> > The discussion stopped because Tomi lacked the time to work on it.
> > 
> > This series takes it from there and implements the binding that was
> > discussed in https://patchwork.kernel.org/patch/7293991/. In this new
> > binding the backlight device is a child of the LED controller instead of
> > being another platform device that uses a phandle to reference a LED.
> 
> Other option would be to have backlight trigger. What are
> advantages/disadvantages relative to that?

I spent a little time thinking about that during the recent round of
reviews.

My rough thoughts were that LED triggers would be a good way to
handle it in the kernel code and would trivially solve a backlight
composed of multiple LED devices (since all could attach to the same
trigger). However I think it would be difficult to use the existing DT
bindings for triggers to express things like brightness curves and to
handle systems with multiple heads.

I'm not *too* worried about conflict with the existing backlight trigger
(which isn't actually a backlight... just a hook into the framebuffer
code to operate a binary LED). People like Daniel Vetter are
labouring diligently to get rid of the notifier it depends on so turning
it into a proper backlight device would probably help with that effort.

Anyhow... having thought the above I then shook myself a bit and
figured it was more important to focus on sane bindings that on what
the kernel does under the covers to realize them ;-) and decided to keep
quiet until the next set of bindings is proposed.

However... since you asked...

Daniel.