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

From: Laurent Pinchart
Date: Thu Aug 28 2014 - 08:18:39 EST

Hi Boris,

On Wednesday 27 August 2014 09:52:35 Boris BREZILLON wrote:
> On Tue, 26 Aug 2014 01:39:21 +0200 Laurent Pinchart wrote:
> > On Thursday 21 August 2014 19:26:33 Boris BREZILLON wrote:
> >> On Thu, 21 Aug 2014 19:08:53 +0200
> >> Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx> 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.
> >
> > The DRM connector object was initially meant to model a physical user-
> > accessible connector on a board (VGA, DVI, HDMI, ...) and the properties
> > of the monitor plugged into it. It has then been (ab)used to represent
> > panels, as they're similar to monitors.
> >
> > In your case the VGA and HDMI connectors should be modeled as DRM
> > connectors, the RGB to HDMI encoder as a DRM encoder, and the LCDC as a
> I don't have any VGA connector (or I'm missing something :-)),

My bad.

> but I have an LCD panel and an RGB to HDMI encoder connected on the same RGB
> connector.

There's no such thing as an RGB connector in DRM. Your SoC has a parallel RGB
video output (I assume it's a DPI bus). From a DRM point of view, that bus
corresponds to the output of the CRTC.

> > As DRM hardcodes the pipeline model to CRTC -> encoder -> connector, you
> > will also need a DRM encoder in the VGA path. I suppose your board has a
> > VGA DAC, that's the component you should expose as a DRM encoder (even if
> > it can't be controlled and doesn't limit the valid modes).
> Actually, my problem is that both devices are connected on the same RGB
> connector, and thus share the same display mode (resolution, HSYNC,
> VSYNC, RGB output mode, ...).
> This means that all remote devices have to agree on a specific mode if
> we want to mirror the display on several output devices, otherwise we
> must disable one of the output devices.

That's not really a problem. From a DRM perspective you need to model your
device as

,------. ,---------------. ,-----------------.
| CRTC | -+--> | Dummy Encoder | ----> | Panel Connector |
`------´ | `---------------´ `-----------------´
| ,---------------. ,-----------------.
\--> | HDMI Encoder | ----> | HDMI Connector |
`---------------´ `-----------------´

The HDMI pipeline is pretty straightforward.

You have told me that the panel has a parallel RGB input without any encoder
in the panel pipeline (by the way, which panel model are you using ?).
However, DRM requires an encoder in every pipeline. You will thus need to
instantiate a dummy encoder. One option would be to set the encoder and
respectively, as that's what userspace usually expects for panels. That
doesn't reflect the reality in your case though, so creating a new
DRM_MODE_CONNECTOR_DPI type might be needed, possibly to be used with

As neither encoder can modify the mode, the same mode will be output on the
two connectors.

> >>>>> 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.


Laurent Pinchart

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at