linux-kernel - Re: [PATCH v2 1/4] dt-bindings: pwm-backlight: add pwm-delay-us property

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Date:   Thu, 6 Jul 2017 20:23:26 +0200
From:   Enric Balletbo Serra <eballetbo@...il.com>
To:     Rob Herring <robh+dt@...nel.org>
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>,
        Pavel Machek <pavel@....cz>,
        Richard Purdie <rpurdie@...ys.net>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Heiko Stuebner <heiko@...ech.de>,
        Linux PWM List <linux-pwm@...r.kernel.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Guenter Roeck <groeck@...omium.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        huang lin <hl@...k-chips.com>
Subject: Re: [PATCH v2 1/4] dt-bindings: pwm-backlight: add pwm-delay-us property

Hi Rob,

2017-07-06 19:07 GMT+02:00 Rob Herring <robh+dt@...nel.org>:
> On Fri, Jun 30, 2017 at 6:21 AM, Enric Balletbo i Serra
> <enric.balletbo@...labora.com> 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>
>> ---
>> Changes since v1:
>>  - As suggested by Daniel Thompson
>>    - Do not assume power-on delay and power-off delay will be the same
>>
>> v1: https://lkml.org/lkml/2017/6/28/219
>>
>>  Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt | 6 ++++++
>>  1 file changed, 6 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
>> index 764db86..49b037e 100644
>> --- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
>> +++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
>> @@ -17,6 +17,11 @@ 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.
>> +                  The 1st cell is the pre-delay in micro seconds.
>> +                  The 2nd cell is the post-delay in micro seconds.
>
> pre and post imply a time before and after a certain event, but these
> are for 2 different events. These are more like an enable/on delay and
> disable/off delay which probably should be separate properties. What
> happens when we need the opposite sequence or a different sequence?
> Maybe some panel requires the PWM to be 0 until some time after
> enabling.
>

Maybe, Only I can say that the panels I checked always first enable
the PWM and then set the ENABLE signal, but of course I didn't check
all the panels.

Would be more acceptable have enable-delay-us and disable-delay-us proprieties?

> I don't understand why you even need a post delay. The PWM can be set
> to 0 while enabled, right? So if the PWM is set to 0 while enabled and
> then disable the backlight, then there's no delay. Plus this doesn't
> make much sense to me electrically either. The PWM duty cycle is going
> to be completely async to the enable line change. The PWM state could
> go from 1 to 0 at any point in time relative to the enable line
> change.
>

To be honest I'm also not sure why is required the post delay, some
panels specify a 0 but others specifies a minimum value between you
off the panel and disable the PWM. The only reason I added the post
delay is because the different datasheets specifies it, I don't have a
use case that the post delay is used to fix something.

Thanks,
 Enric

> These issues are the problem with generic bindings. Adding 1 property
> is no big deal, but then what happens with the next variation. These
> timing constraints need to be able to be implied by the panel's
> compatible.
>
> Rob