linux-kernel - Re: [RESEND PATCH v5 1/5] pwm-backlight: enable/disable the PWM before/after LCD enable toggle.

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Date:   Mon, 19 Mar 2018 19:28:55 +0100
From:   Enric Balletbo i Serra <enric.balletbo@...labora.com>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Lee Jones <lee.jones@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jingoo Han <jingoohan1@...il.com>,
        Rob Herring <robh+dt@...nel.org>, Pavel Machek <pavel@....cz>,
        Heiko Stuebner <heiko@...ech.de>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Richard Purdie <rpurdie@...ys.net>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        linux-pwm@...r.kernel.org, linux-fbdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, groeck@...omium.org,
        linux-rockchip@...ts.infradead.org, kernel@...labora.com
Subject: Re: [RESEND PATCH v5 1/5] pwm-backlight: enable/disable the PWM
 before/after LCD enable toggle.

Hi Thierry,

Thanks for your ack.

On 19/03/18 17:58, Thierry Reding wrote:
> On Fri, Feb 16, 2018 at 06:40:30PM +0100, Enric Balletbo i Serra wrote:
>> Before this patch the enable signal was set before the PWM signal and
>> vice-versa on power off. This sequence is wrong, at least, it is on
>> the different panels datasheets that I checked, so I inverted the sequence
>> to follow the specs.
>>
>> For reference the following panels have the mentioned sequence:
>>   - N133HSE-EA1 (Innolux)
>>   - N116BGE (Innolux)
>>   - N156BGE-L21 (Innolux)
>>   - B101EAN0 (Auo)
>>   - B101AW03 (Auo)
>>   - LTN101NT05 (Samsung)
>>   - CLAA101WA01A (Chunghwa)
>>
>> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@...labora.com>
>> Acked-by: Daniel Thompson <daniel.thompson@...aro.org>
>> Acked-by: Jingoo Han <jingoohan1@...il.com>
>> ---
>> Changes since v4:
>>  - Rebase on top of mainline.
>>  - Add the acks from Daniel Thompson and Jingoo Han.
>> Changes since v3:
>>  - List the part numbers for the panel checked (Daniel Thompson)
>> Changes since v2:
>>  - Add this as a separate patch (Thierry Reding)
>> Changes since v1:
>>  - None
>>
>>  drivers/video/backlight/pwm_bl.c | 9 +++++----
>>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> I'm surprised that panels even care about this. Do you see actual
> breakage if these are the other way around?
> 

At least the B116XTN02 requires enable first the PWM, wait 10ms and then BL_EN
to avoid garbage. I did not observe the issue on other panels but the datasheets
I checked specifies this sequence as correct.

> Anyway, this seems to me as legit as the other way around, so:
> 
> Acked-by: Thierry Reding <thierry.reding@...il.com>
>