Re: [RFC V3 2/3] drm/bridge: add a dummy panel driver to support lvds bridges

From: Thierry Reding <thierry.reding@gmail.com>
To: Ajay kumar <ajaynumb@gmail.com>
Cc: "linux-samsung-soc@vger.kernel.org"
	<linux-samsung-soc@vger.kernel.org>,
	Daniel Vetter <daniel.vetter@ffwll.ch>,
	sunil joshi <joshi@samsung.com>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	Andrzej Hajda <a.hajda@samsung.com>,
	Prashanth G <prashanth.g@samsung.com>,
	Ajay Kumar <ajaykumar.rs@samsung.com>
Subject: Re: [RFC V3 2/3] drm/bridge: add a dummy panel driver to support lvds bridges
Date: Mon, 19 May 2014 17:30:55 +0200	[thread overview]
Message-ID: <20140519153053.GB30978@ulmo> (raw)
In-Reply-To: <CAEC9eQN5Z+dGqNBDq_pkMu0AzASkrA9xPpEjKRWRoVYAq2L1bA@mail.gmail.com>

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On Sun, May 18, 2014 at 01:50:36PM +0530, Ajay kumar wrote:
> On Sun, May 18, 2014 at 2:44 AM, Thierry Reding
> <thierry.reding@gmail.com> wrote:
> > On Thu, May 15, 2014 at 05:10:16PM +0530, Ajay kumar wrote:
> >> On Thu, May 15, 2014 at 1:43 PM, Thierry Reding <thierry.reding@gmail.com> wrote:
> > [...]
> >> > I still don't see how controlling the enable GPIO from the panel will be
> >> > in any way better, or different for that matter, from simply enabling
> >> > the backlight and let the backlight driver handle the enable GPIO. Have
> >> > you tried to do that and found that it doesn't work?
> >> It works, but it gives me glitch when I try to configure video.
> >> This is because backlight_on(pwm-backlight) happens much before
> >> I configure video(inside drm), and while configuring video there would
> >> be a glitch,
> >> and that glitch would be visible to the user since backlight is already enabled.
> >
> > Okay, so this means that your backlight is turned on too early. Instead
> > of working around that problem by moving control of the backlight enable
> > GPIO from the backlight driver into the panel driver, the correct way to
> > fix it is to make sure the backlight stays off until video is ready.
> Ok. Please suggest an idea how to do the same!

I did post a patch[0] a long time ago that added a way to fix this for
pwm-backlight at least.

> I have already suggested a simple idea which conforms to a valid spec.
> Just because enable_gpio is already a part of pwm_bl.c, I somewhat feel
> like we are forcing people to handle enable_gpio inside pwm_bl.c.

And that's a good thing. That's what people will expect. Backlights are
exposed via sysfs, which is currently also the only way to control the
backlight from userspace. If the driver for that doesn't have everything
required to control the backlight how can userspace control it?

> Note that, pwm_bl can still work properly without enabling the backlight GPIO.
> And, I did point out to a valid datasheet from AUO, which clearly indicates why
> backlight enable GPIO should be a part of panel driver and not pwm_bl driver.

Just because some spec mentions the backlight enable pin in some panel
power up sequence diagram that doesn't mean we have to implement it as
part of the panel driver.

> I am not trying to say we should remove enable_gpio from pwm_bl.
> Provided that its already an optional property, and if someone wants
> to control it in a panel driver, what is wrong in doing so?

See above.

Thierry

[0]: https://lkml.org/lkml/2013/10/7/188

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