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

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Message-ID: <CADMs+9ag_PVeBXsTtFN7bxA50rXmFeVPCsaAQYR2gJxSzANSGQ@mail.gmail.com>
Date:	Fri, 10 Jun 2016 15:08:50 -0700
From:	Stéphane Marchesin <marcheu@...omium.org>
To:	Rob Clark <robdclark@...il.com>
Cc:	Doug Anderson <dianders@...omium.org>,
	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>
Subject: Re: [PATCH v2 1/2] dt-bindings: add Starry KR122EA0SRA panel binding

On Fri, Jun 10, 2016 at 3:03 PM, Rob Clark <robdclark@...il.com> wrote:
> On Fri, Jun 10, 2016 at 3:52 PM, Doug Anderson <dianders@...omium.org> wrote:
>> 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.
>
> 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 :-(

Yes, although you can probably make things mostly work with improper
sequencing and enough retries, a lot of ARM hw either requires
interesting sequencing, or has broken/unreliable DDC, which translates
into the use of panel simple on the sw side.

>
>> 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.

On some devices the firmware doesn't set any video mode, so there
isn't anything we can read back. That is our case :)

Stéphane


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