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

From: Andreas Kemnade <andreas@kemnade.info>
To: Pavel Machek <pavel@ucw.cz>
Cc: Daniel Thompson <daniel.thompson@linaro.org>,
	lee.jones@linaro.org, jingoohan1@gmail.com,
	jacek.anaszewski@gmail.com, dmurphy@ti.com, robh+dt@kernel.org,
	mark.rutland@arm.com, b.zolnierkie@samsung.com,
	dri-devel@lists.freedesktop.org, linux-leds@vger.kernel.org,
	devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-fbdev@vger.kernel.org,
	"H. Nikolaus Schaller" <hns@goldelico.com>
Subject: Re: [PATCH 1/2] backlight: lm3630a: add an enable gpio for the HWEN pin
Date: Sun, 15 Sep 2019 19:34:29 +0200	[thread overview]
Message-ID: <20190915193429.5feef312@aktux> (raw)
In-Reply-To: <20190915165204.GA4857@bug>

Hi,

On Sun, 15 Sep 2019 18:52:04 +0200
Pavel Machek <pavel@ucw.cz> wrote:

> Hi!
> 
> > > > > 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.  
> 
> Dunno.. if the power consumption does not matter, why does the chip have the enable
> pin in the first place, and why do we bother supporting it? We could hardcode the
> pin to enabled as well..

Well, I agree having the pin and no power saving seems not to make
sense. Two points here: I think it is a good idea to properly describe
the hardware in the devicetree. What to do with that information is
another thing.
A problem is that at the moment I cannot easily measure consumption
of the chip. Hmm, even testing a solution which disables the pin while
the chip is not in use, is not so easy.
But wait...
I could use a wrong gpio but one that I can easily monitor to check if
the pin is toggled. And set the real pin to high by some other means.
And then use the real gpio to check if timings are correct (waiting
enough after enabling the chip, e.g.

Regards,
Andreas