Re: [PATCH] drm/panel: panel-simple: Support panel-dpi

From: Sam Ravnborg <sam@ravnborg.org>
To: Thierry Reding <thierry.reding@gmail.com>
Cc: devicetree@vger.kernel.org, eben@raspberrypi.org,
	Maxime Ripard <maxime.ripard@bootlin.com>,
	dri-devel@lists.freedesktop.org,
	Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
	Rob Herring <robh+dt@kernel.org>,
	Sean Paul <seanpaul@chromium.org>,
	Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
	Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Subject: Re: [PATCH] drm/panel: panel-simple: Support panel-dpi
Date: Fri, 8 Mar 2019 18:12:12 +0100	[thread overview]
Message-ID: <20190308171212.GA22502@ravnborg.org> (raw)
In-Reply-To: <20190308133924.GA2559@ulmo>

Hi Thierry.
On Fri, Mar 08, 2019 at 02:39:24PM +0100, Thierry Reding wrote:
> On Fri, Mar 08, 2019 at 02:01:48PM +0100, Sam Ravnborg wrote:
> > > One thing that's not clear to me is whether or not we want to allow
> > > video timings to be specified in DT. I used to think that we didn't,
> > > because the video timings are implied by the specific compatible string
> > > (which we already determined is mandatory anyway),
> > 
> > We often have two users of the timings for a simple panel.
> > First we have the bootloader that may present something on the
> > panel - next step it then the kernel.
> > 
> > Bootloaders such as U-boot and barebox supports devicetree.
> > So with the timings specified in the devicetree there are three
> > users that can use the timings, and it is simple to share the
> > timing specifications.
> 
> I think this is not true in practice. As far as I know U-Boot and Linux
> don't share the device tree. So we wouldn't actually be sharing the
> video timings, we'd be duplicating them. And whether we duplicate them
> in code or DT isn't really all that different.
U-boot copies selected DT files from the kernel to U-boot.
barebox has a (sanitized?) set of DT files from the kernel, with
barebox specifics added on top of it. This receives updates from the kernel
more or less for each kernel rc.
The origin of the DT files are the kernel, but they keep copies
for various reasons. And their update process from the kernel differs.

In other words - barebox uses the kernel DT files in practice.
U-boot in practice have their own copy (as I see it - did not look to close)

> > As it is now one has to patch the kernel to add a panel to panel-simple,
> > and add timing to device tree to let barebox use it.
> > 
> > So it would be good once and for all to have the rules specified.
> > And the preferred solution is to have timing in the devicetree
> > so we can use it both in the kernel and in the bootloaders.
> 
> This is *exactly* the same argument that I've heard many times before.
> And it is still overly simplistic. Video timings are just one part of
> the description of the panel. In most cases you need at least also a
> power sequence.
There are panels that are compatibe with panel-simple and there are the
other panels.
My comment is solely for the panel-simple compatible panels, where
we already know stuff like power sequence and such.

There are today a lot of panels in that group and in my tree there
is patches waiting to add another three panels.
This is panels that "just works" with barebox with timings(*) specified
in the DT and for the kernel requires a patch to panle-simple.

There are obviously a lot of panels that have additioanl requirements,
but then this is not "panle-simple" compatible panels and outside the
scope of my request.

I hope this clarifies it.

	Sam
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