linux-kernel - RE: [PATCH v2] pwm_backlight: Add device tree support for Low Threshold Brightness

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Date:	Tue, 25 Sep 2012 04:29:27 +0000
From:	"Philip, Avinash" <avinashphilip@...com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	"grant.likely@...retlab.ca" <grant.likely@...retlab.ca>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"rob@...dley.net" <rob@...dley.net>,
	"rpurdie@...ys.net" <rpurdie@...ys.net>,
	"thierry.reding@...onic-design.de" <thierry.reding@...onic-design.de>,
	"broonie@...nsource.wolfsonmicro.com" 
	<broonie@...nsource.wolfsonmicro.com>,
	"shawn.guo@...aro.org" <shawn.guo@...aro.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Nori, Sekhar" <nsekhar@...com>,
	"Hebbar, Gururaja" <gururaja.hebbar@...com>
Subject: RE: [PATCH v2] pwm_backlight: Add device tree support for Low
 Threshold Brightness

On Fri, Sep 21, 2012 at 23:13:39, Stephen Warren wrote:
> On 09/21/2012 12:03 AM, Philip, Avinash wrote:
> > Hi Stephen,
> > 
> > On Fri, Sep 21, 2012 at 10:46:45, Stephen Warren wrote:
> >> On 09/20/2012 10:51 PM, Philip, Avinash wrote:
> >>> Some backlights perform poorly when driven by a PWM with a short
> >>> duty-cycle. For such devices, the low threshold can be used to specify a
> >>> lower bound for the duty-cycle and should be chosen to exclude the
> >>> problematic range.
> >>>
> >>> This patch adds support for an optional low-threshold-brightness
> >>> property.
> >>
> >>> diff --git a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
> >>
> >>>  Optional properties:
> >>>    - pwm-names: a list of names for the PWM devices specified in the
> >>>                 "pwms" property (see PWM binding[0])
> >>> +  - low-threshold-brightness: brightness threshold low level. Low threshold
> >>> +    brightness set to value so that backlight present on low end of
> >>> +    brightness.
> >>
> >> For my education, why not just specify values above this value in the
> >> brightness-levels array; how do those two interact?
> > 
> > Please find details from 
> > https://lkml.org/lkml/2012/7/18/284
> 
> Hmm. That still doesn't really explain what this property does.
> 
> I'm going to guess that if this property is present, and values in the
> brightness-levels property get scaled between the
> low-threshold-brightness and 255 instead of being used directly.

This is correct.

> But then, in the email you linked to, what does "But brightness-levels won't
> be uniformly divided" mean?

For some panels, backlight would absent on low end of brightness due to low
percentage in duty_cycle. Consider following example where backlight absent
for brightness levels from 0 - 51.

pwms = <&pwm 0 50000>;
brightness-levels = <0 51 53 56 62 75 101 152 255>; 
default-brightness-level = <6>;

So in the example, brightness-levels are set to have values for backlight present.
Here levels are not uniformly divided.

But by providing,
brightness_threshold_level = <50>;

we can have a uniform division in brightness-levels as below

brightness-levels = <0 2 4 8 16 32 64 128 255>;


> Why would doing the calculation at run-time
> be any better than simply putting the correct values into
> brightness-levels in the first place?

I think you mean to provide the backlight-levels as in example
brightness-levels = <0 51 53 56 62 75 101 152 255>;

In that case, I still feel adding brightness_threshold_level would be a
better option.

> 
> Either way, the DT binding should explain exactly what this value is
> used for, and how it affects the interpretation of values in
> brightness-levels.

Is DT binding documentation a good place to explain this feature?
Initially Thierry suggested document option. So I left out.

Thanks
Avinash

> 

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