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

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Message-ID: <20120926164949.GA24041@avionic-0098.mockup.avionic-design.de>
Date:	Wed, 26 Sep 2012 18:49:49 +0200
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Stephen Warren <swarren@...dotorg.org>
Cc:	"Philip, Avinash" <avinashphilip@...com>,
	"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>,
	"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 Wed, Sep 26, 2012 at 09:27:51AM -0600, Stephen Warren wrote:
> On 09/25/2012 10:35 PM, Philip, Avinash wrote:
> > On Tue, Sep 25, 2012 at 11:49:14, Stephen Warren wrote:
> >> On 09/24/2012 10:29 PM, Philip, Avinash wrote:
> >>> 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.
> >>
> >> So why not just change the values so they /are/ what you want? After
> >> all, it's just data and you can put whatever values you want there. What
> >> is preventing you from doing this?
> > 
> > brightness_threshold_level was added to explore lth_brightness support already
> > present in non-DT case.
> 
> I understand that. Given my discussion above, I would advocate removing
> lth_brightness from the non-DT case rather than adding it to the DT
> case, since it seems entirely pointless.

It is still required for the case where brightness levels are not used.
So we can't drop it right away. I agree however that we should plan to
get rid of the max_brightness and lth_brightness eventually. Since the
DT bindings don't use it yet we should keep only the brightness levels.
Once all users have been converted we can rename max_brightness to
something like num_levels and remove lth_brightness. dft_brightness can
probably be renamed to default_level.

Thierry

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