Re: [PATCH V7 11/12] Documentation: bridge: Add documentation for ps8622 DT properties

From: Thierry Reding <thierry.reding@gmail.com>
To: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"linux-samsung-soc@vger.kernel.org"
	<linux-samsung-soc@vger.kernel.org>,
	Sean Paul <seanpaul@google.com>,
	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>,
	Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	Ajay kumar <ajaynumb@gmail.com>,
	Prashanth G <prashanth.g@samsung.com>,
	Ajay Kumar <ajaykumar.rs@samsung.com>
Subject: Re: [PATCH V7 11/12] Documentation: bridge: Add documentation for ps8622 DT properties
Date: Tue, 23 Sep 2014 12:01:40 +0200	[thread overview]
Message-ID: <20140923100139.GT30514@ulmo> (raw)
In-Reply-To: <54214008.4040105@ti.com>

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On Tue, Sep 23, 2014 at 12:40:24PM +0300, Tomi Valkeinen wrote:
> On 23/09/14 11:35, Thierry Reding wrote:
> 
> > Well, a display controller is never going to attach to a panel directly.
> 
> With parallel RGB, that (almost) happens. There's voltage level shifting
> probably in the middle, but I don't see anything else there.

The level shifting could be considered an encoder. Anyway, I didn't mean
physically attach to a panel but rather in DRM. You'll always need an
encoder and connector before you go to the panel.

> > But I agree that it would be nice to unify bridges and encoders more. It
> > should be possible to make encoder always a bridge (or perhaps even
> > replace encoders with bridges altogether). Then once you're out of the
> > DRM device everything would be a bridge until you get to a panel.
> 
> What exactly is a bridge and what is an encoder? Those are DRM
> constructs, aren't they?

Yes. I think bridges are mostly a superset of encoders.

> As I see it, a video pipeline consist of a video source (display
> controller usually), a chain of encoders (all of which may not be
> strictly "encoders", they could be level shifters, muxes, ESD protection
> devices or such), and either a display device like a panel or a
> connector to outside world.

Well, the panel itself is attached to a connector. The connector is
really what userspace uses to control the output and indirectly the
panel.

> Am I right that in DRM world the encoder is the first device in the
> display chain after the display controller,

Yes.

>                                             and the next is a bridge?

A bridge or a connector. Typically it would be a connector, but that's
only if you don't have bridges in between.

> That sounds totally artificial, and I hope we don't reflect that in the
> DT side in any way.

Yes, they are software concepts and I'm not aware of any of it being
exposed in DT. A display controller is usually implemented as DRM CRTC
object and outputs (DSI, eDP, HDMI) as encoder (often with a connector
tied to it).

Thierry

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