[PATCH v5 2/5] drm/bridge: Add RGB to VGA bridge support

[Archit Taneja]

On 10/06/2016 12:51 PM, Maxime Ripard wrote:
> Hi Archit,
>
> On Mon, Oct 03, 2016 at 04:40:57PM +0530, Archit Taneja wrote:
>> Hi Maxime,
>>
>> On 09/30/2016 08:07 PM, Maxime Ripard 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.
>>>
>>> Acked-by: Rob Herring <robh at kernel.org>
>>> Signed-off-by: Maxime Ripard <maxime.ripard at free-electrons.com>
>>> ---
>>> .../bindings/display/bridge/rgb-to-vga-bridge.txt  |  48 +++++
>>> drivers/gpu/drm/bridge/Kconfig                     |   7 +
>>> drivers/gpu/drm/bridge/Makefile                    |   1 +
>>> drivers/gpu/drm/bridge/rgb-to-vga.c                | 229 +++++++++++++++++++++
>>> 4 files changed, 285 insertions(+)
>>> create mode 100644 Documentation/devicetree/bindings/display/bridge/rgb-to-vga-bridge.txt
>>> create mode 100644 drivers/gpu/drm/bridge/rgb-to-vga.c
>>>
>>> diff --git a/Documentation/devicetree/bindings/display/bridge/rgb-to-vga-bridge.txt b/Documentation/devicetree/bindings/display/bridge/rgb-to-vga-bridge.txt
>>> new file mode 100644
>>> index 000000000000..a8375bc1f9cb
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/display/bridge/rgb-to-vga-bridge.txt
>>> @@ -0,0 +1,48 @@
>>> +Dumb RGB to VGA bridge
>>> +----------------------
>>> +
>>> +This binding is aimed for dumb RGB to VGA bridges that do not require
>>> +any configuration.
>>> +
>>> +Required properties:
>>> +
>>> +- compatible: Must be "rgb-to-vga-bridge"
>>
>> I'd talked to Laurent on IRC if he's okay with this. And I guess you to
>> had discussed it during XDC too. He's suggested that it'd be better to
>> have the compatible string as "simple-vga-dac".
>
> I just wished this bikeshedding had taken place publicly and be
> actually part of that discussion, but yeah, ok.

Sorry about that. I'd pinged him for an Ack, the discussion went
more than that :)

>
>> Some of the reasons behind having this:
>>
>> - We don't need to specify "rgb" in the compatible string since most
>> simple VGA DACs can only work with an RGB input.
>
> Ok.
>
>> - Also, with "dac" specified in the string, we don't need to
>> specifically mention "bridge" in the string. Also, bridge is a drm
>> specific term.
>>
>> - "simple" is considered because it's an unconfigurable bridge, and it
>> might be misleading for other VGA DACs to not use "vga-dac".
>
> All those "simple" bindings are just the biggest lie we ever
> told. It's simple when you introduce it, and then grows into something
> much more complicated than a non-simple implementation.

"simple" here is supposed to mean that it's an unconfigurable RGB to
VGA DAC. This isn't supposed to follow the simple-panel model, where
you add the "simple-panel" string in the compatible node, along with
you chip specific compatible string.

In other words, this driver shouldn't be touched again in the future :)
If someone wants to write a RGB to VGA driver which is even
slightly configurable, they'll need to write a new bridge driver.

Thanks,
Archit

>
>> What do you think about this? If you think it's good, would it be
>> possible for you to change this? I guess it's okay for the rest of
>> the patch to stay the same.
>
> I'll update and respin the serie.
>
> Thanks,
> Maxime
>

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