[U-Boot] [PATCH v1 3/5] colibri_t20: fix display configuration

[Stephen Warren]

On 09/14/2016 09:20 AM, Marcel Ziswiler wrote:
> On Mon, 2016-09-12 at 12:18 -0600, Stephen Warren wrote:
>> On 09/09/2016 10:10 AM, Marcel Ziswiler wrote:
>>>
>>> Without this patch the following error will be shown:
>>>
>>> stdio_add_devices: Video device failed (ret=-22)
>>>
>>> As commit ec5507707a1d1e84056a6c864338f95f6118d3ca (video: tegra: Move
>>> to using simple-panel and pwm-backlight) states the Colibri T20 needs
>>> updating too which this patch finally attempts doing.
>>>
>>> Please note that the current U-Boot implementation requires a dummy
>>> GPIO e.g. for a fixed backlight regulator to be explicitly defined in
>>> order to work unlike in the Linux kernel where this is taken care of
>>> automatically.

>>> +			compatible = "regulator-fixed";
>>> +			reg = <0>;
>>> +			regulator-name = "Dummy";
>>> +			/* Dummy N/C */
>>> +			gpio = <&gpio TEGRA_GPIO(V, 7)
>>> GPIO_ACTIVE_HIGH>;
 >>
>> This is wrong. If that GPIO isn't actually part of the backlight, the DT
>> should not say that it is. The gpio property is optional according to
>> the DT binding documentation, so this shouldn't be needed.
>
> Well, I guess then it's lying.

Does "it" mean the binding? Please note that the binding defines how the 
DT should be structured and how code interpreting the DT should operate. 
The binding isn't derived from the code, but rather the code is derived 
from the binding.

 > If I leave it away I get the following:
>
> stdio_add_devices: Video device failed (ret=-38)
>
> And it won't quite work.

That sounds like a bug in the U-Boot regulator driver. I believe you 
should fix that, rather than working around the bug in DT.