Re: [PATCH v6 2/3] dt-bindings: display: atmel: optional video-interface of endpoints

[Peter Rosin <peda@axentia.se>]

On 2018-08-03 12:32, Sakari Ailus wrote:
> Hi Peter,
> 
> Thanks for cc'ing me.

You're welcome, but Laurent is the one to "blame" for hooking you
up... :-)

> On Fri, Aug 03, 2018 at 12:11:23PM +0200, Peter Rosin wrote:
>> With bus-type/bus-width properties in the endpoint nodes, the video-
>> interface of the connection can be specified for cases where the
>> heuristic fails to select the correct output mode. This can happen
>> e.g. if not all RGB pins are routed on the PCB; the driver has no
>> way of knowing this, and needs to be told explicitly.
>>
>> This is critical for the devices that have the "conflicting output
>> formats" issue (SAM9N12, SAM9X5, SAMA5D3), since the most significant
>> RGB bits move around depending on the selected output mode. For
>> devices that do not have the "conflicting output formats" issue
>> (SAMA5D2, SAMA5D4), this is completely irrelevant.
>>
>> Acked-by: Boris Brezillon <boris.brezillon@bootlin.com>
>> Reviewed-by: Rob Herring <robh@kernel.org>
>> Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
>> Signed-off-by: Peter Rosin <peda@axentia.se>
>> ---
>>  .../devicetree/bindings/display/atmel/hlcdc-dc.txt | 30 ++++++++++++++++++++++
>>  1 file changed, 30 insertions(+)
>>
>> diff --git a/Documentation/devicetree/bindings/display/atmel/hlcdc-dc.txt b/Documentation/devicetree/bindings/display/atmel/hlcdc-dc.txt
>> index 82f2acb3d374..10e8707d1443 100644
>> --- a/Documentation/devicetree/bindings/display/atmel/hlcdc-dc.txt
>> +++ b/Documentation/devicetree/bindings/display/atmel/hlcdc-dc.txt
>> @@ -15,6 +15,14 @@ Required children nodes:
>>   to external devices using the OF graph reprensentation (see ../graph.txt).
>>   At least one port node is required.
>>  
>> +Optional properties in grandchild nodes:
>> + Any endpoint grandchild node may specify a desired video interface
>> + according to ../../media/video-interfaces.txt, specifically
>> + - bus-type: must be <0>.
> 
> bus-type won't tell anything in this case, you could simply drop it. If
> your hardware only supports parallel or Bt.656 bus then the code parsing
> the properties can (and should) know the bus type as well.
> 
> Looking at your implementation, bus-type is not being used. This supports
> the suggestion.

I only added the "bus-type = <0>;" in order to disallow any other
value for the bus-type to make it clear that any non-zero value would
be a bug in the device-tree and not in the driver...

> The V4L2 implementation is about to change as well and there's a bit of
> discussion ongoing related to the bus-type property:
> 
> <URL:https://www.spinics.net/lists/linux-media/msg137980.html>
> <URL:https://www.spinics.net/lists/linux-media/msg137979.html>

...but I can certainly drop "bus-type = <0>;" from the binding if it
is going away anyway. However, I wonder why it's ok to silently drop
the autodetect meaning of zero? What about old device-trees that do
have that zero there? Are those suddenly not conforming? Wouldn't a
deprecation notice be better so that old existing device-trees can
be parsed/understood even when using modern bindings documentation?

Cheers,
Peter

> 
>> + - bus-width: recognized values are <12>, <16>, <18> and <24>, and
>> +   override any output mode selection heuristic, forcing "rgb444",
>> +   "rgb565", "rgb666" and "rgb888" respectively.
>> +
>>  Example:
>>  
>>  	hlcdc: hlcdc@f0030000 {
>> @@ -50,3 +58,25 @@ Example:
>>  			#pwm-cells = <3>;
>>  		};
>>  	};
>> +
>> +Example 2: With a video interface override to force rgb565; as above
>> +but with these changes/additions:
>> +
>> +	&hlcdc {
>> +		hlcdc-display-controller {
>> +			pinctrl-names = "default";
>> +			pinctrl-0 = <&pinctrl_lcd_base &pinctrl_lcd_rgb565>;
>> +
>> +			port@0 {
>> +				#address-cells = <1>;
>> +				#size-cells = <0>;
>> +				reg = <0>;
>> +
>> +				hlcdc_panel_output: endpoint@0 {
>> +					reg = <0>;
>> +					bus-type = <0>;
>> +					bus-width = <16>;
>> +				};
>> +			};
>> +		};
>> +	};
>