Re: [PATCH 13/20] drm/bridge: Add RGB to VGA bridge support

[Maxime Ripard <maxime.ripard@free-electrons.com>]

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Hi Rob,

On Mon, May 16, 2016 at 09:07:18AM -0500, Rob Herring wrote:
> On Mon, May 16, 2016 at 7:47 AM, Maxime Ripard
> <maxime.ripard@free-electrons.com> wrote:
> > Some boards have an entirely passive RGB to VGA bridge, based on either
> > DACs or resistor ladders.
> >
> > Those might or might not have an i2c bus routed to the VGA connector in
> > order to access the screen EDIDs.
> >
> > Add a bridge that doesn't do anything but expose the modes available on the
> > screen, either based on the EDIDs if available, or based on the XGA
> > standards.
> >
> > Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
> > ---
> >  .../bindings/display/bridge/dumb-vga.txt           |  40 +++++
> >  drivers/gpu/drm/bridge/Kconfig                     |   6 +
> >  drivers/gpu/drm/bridge/Makefile                    |   1 +
> >  drivers/gpu/drm/bridge/dumb-vga.c                  | 186 +++++++++++++++++++++
> >  4 files changed, 233 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/display/bridge/dumb-vga.txt
> >  create mode 100644 drivers/gpu/drm/bridge/dumb-vga.c
> >
> > diff --git a/Documentation/devicetree/bindings/display/bridge/dumb-vga.txt b/Documentation/devicetree/bindings/display/bridge/dumb-vga.txt
> > new file mode 100644
> > index 000000000000..757f04de97f3
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/display/bridge/dumb-vga.txt
> > @@ -0,0 +1,40 @@
> > +Passive RGB to VGA bridge
> > +-------------------------
> > +
> > +This binding is aimed for entirely passive RGB to VGA bridges that do not
> > +require any configuration.
> 
> No reset or enable lines or regulators?

On the three designs I've seen:
  - One doesn't need any of these
  - One has a GPIO enable pin but with it's resistor not populated
  - One has a regulator controlled by a GPIO

So I guess in most cases, we don't need anything, but we still might
to cover all cases.

> > +
> > +Required properties:
> > +
> > +- compatible: Should be "dumb-vga-bridge"
> > +
> > +Optional properties:
> > +
> > +- ddc-i2c-bus: phandle to the i2c bus used to communicate with the monitor
> 
> The i2c bus is a property of the connector, not the bridge chip, so it
> belongs in a VGA connector node (which we already have binding for).
> 
> I think for the really "dumb" (or transparent case), you should only
> have a connector. For anything more complex, then you should have both
> a DAC and connector node.

So, in essence, something like:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/arm/boot/dts/r8a7794-alt.dts#n37
 
> On the driver side, I think we could handle multiple connectors with a
> single driver. Ideally, that would work with any DRM driver that has a
> connector node.

I'm not sure what you mean by that. I really think it should be a
separate driver in order to share the common code that so far the
current implementations put in some driver specific code (even though
it might be completely generic).

Thanks!
Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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