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

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
[off-list ref] wrote:

quoted

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 <redacted>
---
 .../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.

quoted

+
+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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