Date:	Thu, 21 Aug 2014 15:06:00 +0200
From:	Boris BREZILLON <boris.brezillon@...e-electrons.com>
To:	Thierry Reding <thierry.reding@...il.com>
Cc:	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>,
	Laurent Pinchart <laurent.pinchart@...asonboard.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

On Thu, 21 Aug 2014 11:52:03 +0200
Thierry Reding <thierry.reding@...il.com> wrote:

> On Thu, Aug 21, 2014 at 11:41:59AM +0200, Boris BREZILLON wrote:
> > On Thu, 21 Aug 2014 11:04:07 +0200
> > Thierry Reding <thierry.reding@...il.com> wrote:
> > 
> > > On Thu, Aug 21, 2014 at 10:37:06AM +0200, Boris BREZILLON wrote:
> > > > Hi Ludovic,
> > > > 
> > > > On Thu, 21 Aug 2014 10:16:19 +0200
> > > > Ludovic Desroches <ludovic.desroches@...el.com> wrote:
> > > > 
> > > > > Hi Boris,
> > > > > 
> > > > > You can add
> > > > > 
> > > > > Tested-by: Ludovic Desroches <ludovic.desroches@...el.com>
> > > > 
> > > > Thanks for testing this driver.
> > > > 
> > > > > 
> > > > > Only one issue but not related to your patches, you can't display
> > > > > quickly the bootup logo since the panel detection takes too much
> > > > > time.
> > > > 
> > > > Yes, actually this is related to the device probe order: the
> > > > hlcdc-display-controller device is probed before the simple-panel, thus
> > > > nothing is detected on the RGB connector (I use of_drm_find_panel to
> > > > check for panel availability) when the display controller is
> > > > instantiated. I rely on the default polling infrastructure provided by
> > > > the DRM/KMS framework which polls for a new connector every 10s, and
> > > > this is far more than you kernel boot time.
> > > > 
> > > > Do anyone see a solution to reduce this delay (without changing the
> > > > polling interval). I thought we could add a notifier infrastructure to
> > > > the DRM panel framework, but I'm not sure this is how you want things
> > > > done...
> > > 
> > > Other drivers return -EPROBE_DEFER when a panel hasn't been registered
> > > yet. This will automatically take care of ordering things in a way that
> > > DRM/KMS will only be initialized after the panel has been probed.
> > 
> > Actually I'd like to avoid doing this with a deferred probe, because,
> > AFAIU, the remote endpoint is not tightly linked with the display
> > controller driver (I mean the display controller can still be
> > initialized without having a display connected on it).
> > Moreover the atmel dev kit I'm using has an HDMI bridge connected on
> > the same RGB connector and I'd like to use it in a near future.
> > Returning -EPROBE_DEFER in case of several devices connected on the
> > same connector implies that I'll have to wait for all the remote
> > end-points to be available before my display controller could be
> > instantiated.
> > 
> > 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 ?

> 
> 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 :-).

> 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.

Let me know if you want me to submit a proper patch series...

Best Regards,

Boris

[1]http://code.bulix.org/scq4g3-86804
[2]http://code.bulix.org/7dg501-86805

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