Re: [PATCH V7 11/12] Documentation: bridge: Add documentation for ps8622 DT properties

[Tomi Valkeinen <tomi.valkeinen@ti.com>]

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On 23/09/14 17:41, Thierry Reding wrote:
> On Tue, Sep 23, 2014 at 12:34:54PM +0200, Andrzej Hajda wrote:
>> On 09/23/2014 12:10 PM, Thierry Reding wrote:
>>> On Tue, Sep 23, 2014 at 11:43:47AM +0200, Andrzej Hajda wrote:
>>>> On 09/23/2014 10:35 AM, Thierry Reding wrote:
>>> [...]
>>>>> But I agree that it would be nice to unify bridges and encoders more. It
>>>>> should be possible to make encoder always a bridge (or perhaps even
>>>>> replace encoders with bridges altogether). Then once you're out of the
>>>>> DRM device everything would be a bridge until you get to a panel.
>>>> I agree that some sort of unification of bridges, (slave) encoders is a good
>>>> thing, this way we stay only with bridges and panels as receivers.
>>>> But we will still have to deal with the code like:
>>>>     if (on the other end of the link is panel)
>>>>         do panel framework specific things
>>>>     else
>>>>         do bridge framework specific things
>>>>
>>>> The code in both branches usually does similar things but due to framework
>>>> differences it is difficult to merge.
>>> That's because they are inherently different entities. You can perform
>>> operations on a panel that don't make sense for a bridge and vice-versa.
>>>
>>>> Ideally it would be best to get rid of such constructs. For example by
>>>> trying to
>>>> make frameworks per interface type exposed by device (eg. video_sink) and
>>>> not by device type (drm_bridge, drm_panel).
>>> But then you loose all information about the real type of device.
>> Driver knows type of its device anyway. Why should it know device
>> type of devices it is connected to? It is enough it knows how to talk to
>> other device.
>> Like in real world, why desktop PC should know it is connected to DVI
>> monitor or to
>> DVI/HDMI converter which is connected to TV?
> 
> TVs are much more standardized. There are things like DDC/CI which can
> be used to control the device. Or there's additional circuitry or
> control paths to change things like brightness. The same isn't true of
> panels.
> 
>>>  Or you
>>> have to create a common base class. And then you're still back to
>>> dealing with the two specific cases in many places, so the gain seems
>>> rather minimal.
>>
>> For example RGB panel and RGB/??? bridge have the same hardware input
>> interface.
>> Why shall they have different representation in kernel?
> 
> Because panels have different requirements than bridges. Panels for
> example require the backlight to be adjustable, bridges don't.

I agree that panels and bridges are different, but not from the video
source's point of view. A bridge/encoder just streams video data to the
next entity in the chain, according to the given configuration. It does
not matter if the next one is a panel or another bridge. The source
bridge doesn't care if the next entity has a backlight or not.

I don't know if we need a different representation for bridges and
panels. Thinking about backlight, the driver can just register the
backlight device if it needs one. There's no need to differentiate
bridges and panels for that. But maybe there are other reasons that
warrant different representations.

However, my current feeling is that there's no need for different
representations.

 Tomi



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