linux-kernel - Re: [PATCH v4 00/11] drm: add support for Atmel HLCDC Display Controller

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Message-ID: <20140821192633.69f6b21b@bbrezillon>
Date:	Thu, 21 Aug 2014 19:26:33 +0200
From:	Boris BREZILLON <boris.brezillon@...e-electrons.com>
To:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	Ludovic Desroches <ludovic.desroches@...el.com>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Andrew Victor <linux@...im.org.za>,
	David Airlie <airlied@...ux.ie>,
	dri-devel@...ts.freedesktop.org, linux-pwm@...r.kernel.org,
	Samuel Ortiz <sameo@...ux.intel.com>,
	Lee Jones <lee.jones@...aro.org>,
	Rob Clark <robdclark@...il.com>,
	Mark Rutland <mark.rutland@....com>,
	devicetree@...r.kernel.org, Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	linux-kernel@...r.kernel.org, Rob Herring <robh+dt@...nel.org>,
	Bo Shen <voice.shen@...el.com>,
	Kumar Gala <galak@...eaurora.org>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 00/11] drm: add support for Atmel HLCDC Display
 Controller

Hi Laurent,

On Thu, 21 Aug 2014 19:08:53 +0200
Laurent Pinchart <laurent.pinchart@...asonboard.com> wrote:

[...]

> > >> 
> > >> While this could be acceptable when all drivers are statically linked
> > >> in the kernel, it might be problematic when you're using modules,
> > >> meaning that you won't be able to display anything on your LCD panel
> > >> until your HDMI bridge module has been loaded.
> > > 
> > > No. HDMI should be using proper hotplugging anyway, hence it should be
> > > always be loaded anyway. You're in for a world of pain if you think you
> > > can run DRM with a driver that's composed of separate kernel modules.
> > 
> > I was talking about the external RGB to HDMI encoder, should the driver
> > for this encoder (which is not on On Chip block) be compiled
> > statically too ?
> 
> Given the move to multiplatform kernels we need to aim for as few modules 
> compiled in as possible. I'd say this includes HDMI encoders, panels and 
> display controllers.
> 
> > > Also if you don't want to use deferred probe, then you're in for the
> > > full hotplugging panel dance and that implies that you need to fix a
> > > bunch of things in DRM (one being the framebuffer console instantiation
> > > that I referred to in the other thread).
> > 
> > For now, I wait until there is a device connected on the RGB connector
> > (connector status set to connector_status_connected) before creating an
> > fbdev. It might not be the cleanest way to solve this issue, but it
> > works :-).
> 
> Do you create a new drm_encoder at runtime for the HDMI encoder when it 
> appears ? I thought the DRM core and API were not able to correctly cope with 
> that.

I haven't started to work on the HDMI encoder yet, and ATM I only have
a single connector (which is true from an HW POV), which is then bound
to an LCD panel (the only type of remote endpoint I currently support).

BTW, I wonder how my use case should be represented in the DRM
subsystem. As I said, from an HW POV I only have one RGB (or whatever
name you choose for it) connector. But on such kind of connectors you
can connect several output devices (panels, encoders, ...).
And in my case I have 2 devices on the same RGB connector: a panel and
an RGB to HDMI converter.

> 
> > > You also can't be using the current device tree bindings because they all
> > > assume a dependency from the display controller/output to the panel. For
> > > hotplugging you'd need the dependency the other way around (the panel
> > > needs to refer to the output by phandle).
> > 
> > Here [1] is a proposal for notification support in the drm_panel
> > infrastructure (which is not that complicated), and here [2] is how
> > I use it in my atmel-hlcdc driver to generate hotplug events.
> 
> Is there a way we could use the component framework for that ? I know that 
> partial notification isn't supported at the moment, but Russell agreed it was 
> a real use case that should be implemented at some point.

I'll give it a try.

Best Regards,

Boris



-- 
Boris Brezillon, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com
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