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

[Boris BREZILLON <boris.brezillon@free-electrons.com>]

On Thu, 21 Aug 2014 15:16:08 +0200
Thierry Reding <thierry.reding@gmail.com> wrote:

> 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@gmail.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@gmail.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@atmel.com> wrote:
> > > > > > 
> > > > > > > Hi Boris,
> > > > > > > 
> > > > > > > You can add
> > > > > > > 
> > > > > > > Tested-by: Ludovic Desroches <ludovic.desroches@atmel.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?

I take the first valid one :D (which indeed is not really reliable when
you have several output devices). I guess marking one output as the
primary output in the DT is not an acceptable solution either because
it describes a configuration rather than an HW capability.

Still opened to any suggestions regarding this issue ;-).

> 
> > > 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?

Sure, it should be pretty easy too.

I also had a look at the component framework, but I'm not sure it
applies in this context. AFAIU it is designed to delay a master device
registration until all it's slaves are ready to act upon.


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