linux-kernel - Re: [PATCH] backlight: pwm_bl: disable PWM when 'duty

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Date:	Tue, 7 Jun 2016 11:02:18 +0300
From:	Tomi Valkeinen <tomi.valkeinen@...com>
To:	Lothar Waßmann <LW@...O-electronics.de>
CC:	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Jingoo Han <jingoohan1@...il.com>,
	Lee Jones <lee.jones@...aro.org>,
	Thierry Reding <thierry.reding@...il.com>,
	<linux-fbdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-pwm@...r.kernel.org>
Subject: Re: [PATCH] backlight: pwm_bl: disable PWM when 'duty_cycle' is zero

On 07/06/16 09:49, Lothar Waßmann wrote:
> Hi,
> 
> On Mon, 6 Jun 2016 15:02:21 +0300 Tomi Valkeinen wrote:
>> Hi,
>>
>> On 06/06/16 13:44, Lothar Waßmann wrote:
>>> 'brightness' is usually an index into a table of duty_cycle values,
>>> where the value at index 0 may well be non-zero
>>> (tegra30-apalis-eval.dts and tegra30-colibri-eval-v3.dts are real-life
>>> examples).
>>> Thus brightness == 0 does not necessarily mean that the PWM output
>>> will be inactive.
>>> Check for 'duty_cycle == 0' rather than 'brightness == 0' to decide
>>> whether to disable the PWM.
>>
>> The binding doc does say:
>>
>>   - brightness-levels: Array of distinct brightness levels. Typically these
>>       are in the range from 0 to 255, but any range starting at 0 will do.
>>       The actual brightness level (PWM duty cycle) will be interpolated
>>       from these values. 0 means a 0% duty cycle (darkest/off), while the
>>       last value in the array represents a 100% duty cycle (brightest).
>>
> So, what should I do, when I need a range of levels that doesn't start
> at 0? E.g. if the brightness is inverse proportional to the PWM duty
> cycle.

That's a question to the PWM/backlight maintainers, but I think in the
addition of your patch, the binding doc needs to be changed, as it
doesn't hold true after your patch.

 Tomi



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