linux-kernel - Re: [PATCH 1/2] backlight: lm3630a: add an enable gpio for the HWEN pin

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Date:   Tue, 10 Sep 2019 21:06:48 +0200
From:   Andreas Kemnade <andreas@...nade.info>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     lee.jones@...aro.org, jingoohan1@...il.com,
        jacek.anaszewski@...il.com, pavel@....cz, dmurphy@...com,
        robh+dt@...nel.org, mark.rutland@....com, b.zolnierkie@...sung.com,
        dri-devel@...ts.freedesktop.org, linux-leds@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fbdev@...r.kernel.org,
        "H. Nikolaus Schaller" <hns@...delico.com>
Subject: Re: [PATCH 1/2] backlight: lm3630a: add an enable gpio for the HWEN
 pin

On Tue, 10 Sep 2019 11:21:56 +0100
Daniel Thompson <daniel.thompson@...aro.org> wrote:

[...]
> > > Is this needed?
> > > 
> > > This is a remove path, not a power management path, and we have no idea
> > > what the original status of the pin was anyway?
> > >   
> > 
> > Looking at Ishdn on page 5 of the datasheet, switching it off everytime
> > possible seems not needed. We would need to call chip_init() everytime
> > we enable the gpio or live with default values.
> > Therefore I did decide to not put it into any power management path.
> > But switching it on and not switching it off feels so unbalanced.   
> 
> Either the power consumed by the controller when strings aren't lit up
> matters, in which case the driver should implement proper power
> management or it doesn't matter and changing the pin state isn't needed.
> 
> I'm happy with either of the above but this looks like a third way,
> where eager users could hack in a bit of extra power management by
> forcing drivers to unbind. 
> 
I think I will take the simple way. I am quite sure that the power
consumption with HWEN on and leds off does not matter. If someone
later comes up and finds out that I misread the datasheet, things
are prepared to be improved.
At least the hardware can be properly described in the devicetree.

Regards,
Andreas

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