linux-kernel - Re: [PATCHv4 08/10] backlight: add TI LMU backlight driver

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Message-ID: <20180410063820.GA5793@amd>
Date:   Tue, 10 Apr 2018 08:38:20 +0200
From:   Pavel Machek <pavel@....cz>
To:     Sebastian Reichel <sebastian.reichel@...labora.co.uk>
Cc:     Milo Kim <Milo.Kim@...com>, Lee Jones <lee.jones@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Tony Lindgren <tony@...mide.com>,
        Jingoo Han <jingoohan1@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org, linux-fbdev@...r.kernel.org,
        devicetree@...r.kernel.org, linux-omap@...r.kernel.org
Subject: Re: [PATCHv4 08/10] backlight: add TI LMU backlight driver

Hi!

> > > +static int ti_lmu_bl_add_device(struct ti_lmu_bank *lmu_bank)
> > > +{
> > > +	switch (lmu_bank->type) {
> > > +	case TI_LMU_BL:
> > > +		return ti_lmu_bl_register_backlight(lmu_bank);
> > > +	case TI_LMU_LED:
> > > +		return ti_lmu_bl_register_led(lmu_bank);
> > > +	default:
> > > +		return -EINVAL;
> > > +	}
> > > +}
> > 
> > Ok, this is somehow unusual/crazy. Single driver with two
> > interfaces.
> > 
> > Do we need the LED interface for something?
> >
> > If yes, I believe reasonable solution would be to always provide LED
> > interface, and then have "backlight-trigger" which that would provide
> > backlight interface for arbitrary LED.
> 
> Userspace expects keyboard backlight to be exposed via the LED
> subsystem and display backlight via the backlight subsystem.

Ok.

> I considered always exposing the banks via the LED subsystem and
> using a generic backlight driver. That brings its own problems,
> since there is a dependency between the display and the backlight.
> This is described in DT using a phandle. Getting the right backlight
> device from the phandle will become very tricky with this approach.

I believe we have to do this.

Virtually any LED can be used as a backlight, and we don't really want
to add two personalities to all the LED drivers.

And it should not be too bad: LED will just have default trigger,
which will say this LED corresponds to this display device. I believe
someone wanted to do that for USB/ethernet activity.

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

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