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

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Date:   Mon, 9 Apr 2018 17:54:33 +0200
From:   Sebastian Reichel <sebastian.reichel@...labora.co.uk>
To:     Pavel Machek <pavel@....cz>
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,

On Tue, Apr 03, 2018 at 12:49:08PM +0200, Pavel Machek wrote:
> > +enum ti_lmu_bl_type {
> > +	TI_LMU_BL,  /* backlight userspace interface */
> > +	TI_LMU_LED, /* led userspace interface */
> > +};
> ...
> > +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.

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.

-- Sebastian

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