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

[architt@codeaurora.org (Archit Taneja)]

On 10/07/2016 02:44 PM, Maxime Ripard wrote:
> On Fri, Oct 07, 2016 at 10:27:31AM +0530, Archit Taneja wrote:
>>
>>
>> On 10/07/2016 02:34 AM, Laurent Pinchart wrote:
>>> Hi Sean,
>>>
>>> On Thursday 06 Oct 2016 15:53:28 Sean Paul wrote:
>>>> On Thu, Oct 6, 2016 at 1:27 PM, Laurent Pinchart wrote:
>>>>> On Thursday 06 Oct 2016 17:09:57 Archit Taneja wrote:
>>>>>> On 10/06/2016 12:51 PM, Maxime Ripard wrote:
>>>>>>> On Mon, Oct 03, 2016 at 04:40:57PM +0530, Archit Taneja wrote:
>>>>>>>> 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@kernel.org>
>>>>>>>>> Signed-off-by: Maxime Ripard <maxime.ripard@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.tx
>>>>>>>>> t
>>>>>>>>> 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.
>>>>>>>>> tx
>>>>>>>>> t @@ -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.
>>>>>
>>>>> I agree with Maxime, I don't like the word "simple". My preference would
>>>>> be "vga-dac" for a lack of a better qualifier than "simple" to describe
>>>>> the fact that the device requires no configuration. My only concern with
>>>>> "vga-dac" is that we would restrict usage of that compatible string for a
>>>>> subset of VGA DACs, with more complex devices not being compatible with
>>>>> "vga-dac" even though they are VGA DACs. That's a problem I can live with
>>>>> though.
>>>>
>>>> While we're bikeshedding (feel free to ignore my input on this), I
>>>> think Maxime's initial "dumb" qualifier was better than "simple".
>>>
>>> I think I agree.
>>>
>>>> I think "passive" also gets the point across better than "simple", which
>>>> we've already established as something else in drm.
>>>
>>> To my electrical engineer's ear, passive refers to a component or combination
>>> of components that is not capable of power gain. The resistors ladder VGA DAC
>>> that Maxime is trying to support is a passive system, but the ADV7123 VGA DAC
>>> that equally requires no configuration is an active component.
>>
>> If no one has any more objections within the next day, I'll pull in
>> Maxime's v5 RGB to VGA bridge driver, and change the compatible to
>> "dumb-vga-dac".
>
> That works for me. You'll probably want to update the Kconfig and file
> name to match though.

Queued to drm-misc, with the changes suggested by you and Laurent.

Thanks,
Archit

>
> Maxime
>

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