Date:	Thu, 21 Aug 2014 15:16:08 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Boris BREZILLON <boris.brezillon@...e-electrons.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, Aug 21, 2014 at 03:06:00PM +0200, Boris BREZILLON wrote:
> 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 :-).

Yeah, I guess that's one way to do it. But it's tricky to get right when
you have several outputs. Which one should be considered the primary and
trigger fbdev creation?

> > 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

Those look interesting. Any chance you could look into how to do the
same without resorting to notifiers?

Thierry

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