linux-kernel - Re: [PATCH 1/2] dt-bindings: pwm-backlight: Add pwm-delay-us property

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Date:   Wed, 28 Jun 2017 16:15:22 +0200
From:   Enric Balletbo Serra <eballetbo@...il.com>
To:     Pavel Machek <pavel@....cz>
Cc:     Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Lee Jones <lee.jones@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Rob Herring <robh+dt@...nel.org>,
        Richard Purdie <rpurdie@...ys.net>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        linux-pwm@...r.kernel.org, linux-fbdev@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Guenter Roeck <groeck@...omium.org>,
        huang lin <hl@...k-chips.com>
Subject: Re: [PATCH 1/2] dt-bindings: pwm-backlight: Add pwm-delay-us property

Hi Daniel, Pavel,

2017-06-28 15:30 GMT+02:00 Pavel Machek <pavel@....cz>:
> On Wed 2017-06-28 13:16:31, Enric Balletbo i Serra wrote:
>> From: huang lin <hl@...k-chips.com>
>>
>> Add a pwm-delay-us property to specify the delay between setting an
>> initial (non-zero) PWM value and enabling the backlight, and also the
>> delay between disabling the backlight and setting PWM value to 0.
>
>
>
>> Signed-off-by: huang lin <hl@...k-chips.com>
>> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@...labora.com>
>> ---
>>  Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
>> index 764db86..f75b08f 100644
>> --- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
>> +++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
>> @@ -17,6 +17,9 @@ Optional properties:
>>                 "pwms" property (see PWM binding[0])
>>    - enable-gpios: contains a single GPIO specifier for the GPIO which enables
>>                    and disables the backlight (see GPIO binding[1])
>> +  - pwm-delay-us: delay between setting an initial (non-zero) PWM value and
>> +                  enabling the backlight, and also the delay between disabling
>> +                  the backlight and setting PWM value to 0.
>>
>
>> If is safe to assume power-on delay and power-off delay will be the same?
>>
>> I've only took a quick look but several backlight controllers support asymetric power-on/off sequences..

Daniel, right the ones I checked are symmetric but asymmetric timings
are possible I guess, so I think now that specify the two delays is
more accurate, maybe the binding should be something like this?

  pwm-delay-us = <delay_before_on  delay_after_off>;

> I understood it as "you set PWM and it takes a while for display to
> light up"
> but that's not correct. Changelog from second patch makes it
> clear. Please
> clarify it here, too.

Pavel, oh, your dude is interesting ...

That's the idea, the sequence is:
  Power on, you set the PWM signal, wait a bit and set the LED_EN signal.
  Power off, you clear the LED_EN signal, wait a bit and stop the PWM signal.

Note that the patch inverts the sequence, before this patch first you
set LED_EN signal and then the PWM signal

I assumed that the sequence was wrong but maybe I'm mistaken and there
are some panels that follow the original sequence. On the few panels I
checked the power on/off sequence is how I described above, i.e. see
[1] p. 17, the sequence is first PWM and then LED_EN. I'll take a look
at other panel datasheets, or if you know one, could you provide me
the datasheet?

Thanks,

[1] http://www.jxlcd.com/Upload/PicFiles/N116BGE-L41.pdf

>
>                                                                 Pavel
> --
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html