linux-kernel - Re: [PATCH v2] pwm-backlight: fix the panel power sequence

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Date:	Thu, 05 Nov 2015 10:40:30 +0100
From:	Philipp Zabel <p.zabel@...gutronix.de>
To:	YH Huang <yh.huang@...iatek.com>
Cc:	linux-pwm@...r.kernel.org, linux-fbdev@...r.kernel.org,
	Jingoo Han <jingoohan1@...il.com>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	linux-kernel@...r.kernel.org,
	Thierry Reding <thierry.reding@...il.com>,
	linux-mediatek@...ts.infradead.org,
	Sascha Hauer <kernel@...gutronix.de>,
	Matthias Brugger <matthias.bgg@...il.com>,
	yingjoe.chen@...iatek.com, Lee Jones <lee.jones@...aro.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2] pwm-backlight: fix the panel power sequence

Am Mittwoch, den 04.11.2015, 09:47 +0800 schrieb YH Huang:
> On Tue, 2015-11-03 at 12:08 +0100, Philipp Zabel wrote:
> > Hi YH,
> > 
> > Am Dienstag, den 03.11.2015, 16:11 +0800 schrieb YH Huang:
> > > > The reasoning is that devices where there is no phandle link pointing to
> > > > the backlight (for example from a simple-panel node), we should keep the
> > > > current default behaviour (enable during probe).
> > > 
> > > I have a little problem for the current default behaviour.
> > > Should we enable during probe?
> > 
> > Here I mean enabling the backlight (at the end of the probe function),
> > not enabling the GPIO already when requesting it.
> > 
> > > Before this patch ( http://patchwork.ozlabs.org/patch/324690/ ),
> > > we disable "enable-gpio" in the probe function.
> > 
> > While before this patch the GPIO would be initialized in the disabled
> > state, the call to backlight_update_status at the end of the probe
> > function would still enable the backlight afterwards.
> 
> Based on this, could we disable it initially and update in the
> backlight_update_status function?
> 
> Like this,
> 
> if (pb->enable_gpio) {
> 	if (phandle &&
> 	    gpiod_get_direction(pb->enable_gpio) == GPIOF_DIR_OUT &&
> 	    gpiod_get_value(pb->enable_gpio) == 1)
> 		gpiod_direction_output(pb->enable_gpio, 1);

The gpiod_direction_output call is a no-op, since the direction is
already output and the value is already 1.
Also, I propose to set initial blanking to FB_BLANK_POWERDOWN in this
case, and wait for the panel driver to enable the backlight at the
appropriate time.

regards
Philipp

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