linux-kernel - Re: [PATCH v2 1/2] dt-bindings: add Starry KR122EA0SRA panel binding

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Date:	Fri, 10 Jun 2016 12:52:41 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Rob Clark <robdclark@...il.com>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	Mark Rutland <mark.rutland@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Stéphane Marchesin <marcheu@...omium.org>
Subject: Re: [PATCH v2 1/2] dt-bindings: add Starry KR122EA0SRA panel binding

Rob,

On Fri, Jun 10, 2016 at 11:43 AM, Rob Clark <robdclark@...il.com> wrote:
> On Fri, Jun 10, 2016 at 1:02 PM, Douglas Anderson <dianders@...omium.org> wrote:
>> The Starry KR122EA0SRA is a 12.2", 1920x1200 TFT-LCD panel connected
>> using eDP interfaces.
>
> so drive-by comment... but shouldn't eDP be probe-able?  Not sure why
> we need panel drivers or DT bindings?

I was wondering about that too.  As far as I can tell:

1. We need a panel driver because that appears to be what owns a
reference to the backlight / panel power regulator and that part is
not auto-probable.

2. As far as I could tell, there is no way to declare a generic
(unspecified) panel in the device tree.  Everyone seems to include
"simple-panel" in their compatible string but as far as I can tell
nothing in the kernel looks at it.

3. In theory, all the info specified here should match the EDID
exactly and thus (as you said) be probable.  However, it sounds like
(for power sequencing reasons) there might be reasons why you'd want
to know exactly what panel was present beforehand.  You might need to
power the panel and backlight in very specific sequences, for
instance.  I'm not sure it's always 100% possible in all embedded
designs to read the EDID before you know how the sequencing should
work (but, of course, I'm a NOOB).

4. Reading the EDID can be slow.  If you happen to know all the info
on the panel beforehand you can significantly speed up boot speed,
notably how fast you can get something on the screen.

Anyway, maybe someone else who actually knows what they're talking
about will chime in.  ;)

-Doug