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

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Message-ID: <50631EF7.6040908@wwwdotorg.org>
Date:	Wed, 26 Sep 2012 09:27:51 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	"Philip, Avinash" <avinashphilip@...com>
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 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.
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