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

From: Rob Clark <robdclark@gmail.com>
To: Doug Anderson <dianders@chromium.org>
Cc: "Mark Rutland" <mark.rutland@arm.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Stéphane Marchesin" <marcheu@chromium.org>
Subject: Re: [PATCH v2 1/2] dt-bindings: add Starry KR122EA0SRA panel binding
Date: Fri, 10 Jun 2016 18:03:14 -0400	[thread overview]
Message-ID: <CAF6AEGsZxZuSVUCqVH=8FUOTMiWBVT52xQ+Qcaqt3yF=pRmf3w@mail.gmail.com> (raw)
In-Reply-To: <CAD=FV=XunjDGJYY1+-GPC+hN=kNxLiWbrtZD0_BO1ywerSLhfQ@mail.gmail.com>

On Fri, Jun 10, 2016 at 3:52 PM, Doug Anderson <dianders@chromium.org> wrote:
> Rob,
>
> On Fri, Jun 10, 2016 at 11:43 AM, Rob Clark <robdclark@gmail.com> wrote:
>> On Fri, Jun 10, 2016 at 1:02 PM, Douglas Anderson <dianders@chromium.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.

oh, hmm.. sad.. I was hoping that eDP would save us from dsi per-panel
driver hell.. I guess being able to use panel-simple is at least an
improvement.  But panel specific sequences is sounds like panel-simple
won't save us all the time :-(

> 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.

The theory is (although I think not true currently for most of the arm
drivers) that we should be reading back from hw the current config
from bootloader splash screen, to avoid a modeset (and conveniently
also the need to read edid) at boot.

BR,
-R

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