linux-kernel - Re: [PATCH v2 2/4] pwm-backlight: add support for pwm-delay-us property

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Date:   Thu, 6 Jul 2017 10:57:26 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Pavel Machek <pavel@....cz>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        Lee Jones <lee.jones@...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>,
        Heiko Stuebner <heiko@...ech.de>, linux-pwm@...r.kernel.org,
        linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        groeck@...omium.org, linux-rockchip@...ts.infradead.org,
        huang lin <hl@...k-chips.com>
Subject: Re: [PATCH v2 2/4] pwm-backlight: add support for pwm-delay-us
 property

On 06/07/17 10:24, Thierry Reding wrote:
> On Thu, Jul 06, 2017 at 10:17:18AM +0100, Daniel Thompson wrote:
>> On 06/07/17 10:12, Pavel Machek wrote:
>>> On Thu 2017-07-06 10:01:32, Thierry Reding wrote:
>>>> On Fri, Jun 30, 2017 at 01:21:07PM +0200, Enric Balletbo i Serra wrote:
>>>>> From: huang lin <hl@...k-chips.com>
>>>>>
>>>>> Some panels (i.e. N116BGE-L41), in their power sequence specifications,
>>>>> request a delay between set the PWM signal and enable the backlight and
>>>>> between clear the PWM signal and disable the backlight. Add support for
>>>>> the new pwm-delay-us property to meet the timing.
>>>>>
>>>>> Note that this patch inverts current sequence. Before this patch the
>>>>> enable signal was set before the PWM signal and vice-versa on power off.
>>>>>
>>>>> I assumed that this sequence was wrong, at least it is on different panel
>>>>> datasheets that I checked, so I inverted the sequence to follow:
>>>>>
>>>>>     On power on, set the PWM signal, wait, and set the LED_EN signal.
>>>>>     On power off, clear the LED_EN signal, wait, and stop the PWM signal.
>>>>
>>>> I think this should be two separate patches to make it easier to revert
>>>> the inverted sequence should it prove to regress on other panels.
>>>
>>> Don't make this overly complex. This is trivial. No need to split it
>>> into more patches.
>>
>> Agree. IMHO getting the code that reads the (optional) new parameter correct
>> is the best way to manage risk of regression since in most cases the delay
>> will be skipped anyway.
> 
> The potential regression that I'm referring to would be caused by
> inversing the sequence (GPIO enable -> PWM enable). That's completely
> unrelated to the delays introduced by this patch. Many boards use this
> driver and they've been running with the old sequence for many years.
> Granted, it's fairly unlikely to regress, but it's still a possibility.
> 
> Given that both changes are logically separate, I think separate patches
> are totally appropriate. I also don't think that this would overly
> complicate things.

... and you are right on both counts!

Thanks for the detailed reply.


Daniel.