linux-kernel - Re: [PATCH v6 1/5] pwm-backlight: enable/disable the PWM before/after LCD enable toggle.

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Date:   Mon, 26 Mar 2018 16:18:05 +0100
From:   Daniel Thompson <daniel.thompson@...aro.org>
To:     Enric Balletbo Serra <eballetbo@...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>,
        Rob Herring <robh+dt@...nel.org>,
        Heiko Stuebner <heiko@...ech.de>,
        Thierry Reding <thierry.reding@...il.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Richard Purdie <rpurdie@...ys.net>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Linux PWM List <linux-pwm@...r.kernel.org>,
        linux-fbdev@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Guenter Roeck <groeck@...omium.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
        linux-leds@...r.kernel.org,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        kernel@...labora.com
Subject: Re: [PATCH v6 1/5] pwm-backlight: enable/disable the PWM
 before/after LCD enable toggle.

On Mon, Mar 26, 2018 at 04:40:21PM +0200, Enric Balletbo Serra wrote:
> Hi Pavel,
> 
> 2018-03-26 12:42 GMT+02:00 Pavel Machek <pavel@....cz>:
> > On Mon 2018-03-26 11:52:09, 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)
> >
> > Ok, but this changes behaviour for other panels, too. Are you sure you
> > are not breaking one of those?
> 
> I can't say that I am 100% sure because I didn't find all the
> datasheets of all the panels supported in the kernel. But all the
> datasheets I checked specifies this sequence as valid. In general I
> think that doesn't really matter, but I know that at least the
> B116XTN02 panel requires enable first the PWM, wait 10ms and then
> enable BL_EN to avoid garbage. So the other way around is not valid
> for this panel. That's the reason for this patchset.

This is certainly a patch that could cause regressions... but it would
be a very odd panel that *likes* to be exposed to all the weird edges
that might occur whilst the PWM stablizes and a panel that *needs* to
see weird edges to work seems even less likely (since it could act
different with each SoC).

So whilst the patch is not absolutely cast iron guaranteed free of
risk, I think it is well enough argued for.

Naturally I may change my position *very* quickly on receipt of the
first bug report ;-)


Daniel.