Re: [PATCH 2/3] arm64: dts: renesas: r8a77961: Add lvds0 device node

Hi Nikito,

On Thu, Dec 30, 2021 at 12:12:04AM +0300, Nikita Yushchenko wrote:

  Endpoints are meant to model a link between two ports, so an endpoint

quoted

shouldn't exist in isolation. The issue with creating named endpoints in
SoC files is that you can't tell there what remote devices may exist, so
the endpoint may or may not match the actual hardware design of a board.
I think it's better to create endpoints on both sides together in
overlays.

https://lore.kernel.org/linux-renesas-soc/20211229193135.28767-2-laurent.pinchart+renesas@ideasonboard.com/T/#t (local)

What I don't like here is: details of particular SoC (such as "panel gets video from port@1 of &lvds1) 
leak into per-panel DT fragment.

This limits possibilities to share DT fragments between different use cases. In the patch pointed by the 
above URL, you have to reference both board and panel in the dts file name.

I'd prefer to make each DT fragment to use only either entities defined in that fragment itself, or 
defined "interface entities" between this and "neighbor" DT fragment.

Such as:
- SoC's DT fragment defines a named port/endpoint to export video stream at
- board's DT fragment defines a named panel node corresponding to panel plugged into board's physical 
connector, and connects endpoints with SoC's video export,
- panel's DT fragment extends the panel node from board with video mode information for this particular 
panel.

And similar for backlight, power, and whatever else exposed on the physical panel connector.

So for the board's physical connector there is a set of board-DT-provided entities for use by DT 
fragment of whatever component plugged to the connector, without direct references to final SoC 
interfaces. And possibility to reuse DT fragments between boards, and probably have a library of DT 
fragments for hardware currently available in the market, usable with different boards where that 
hardware can be connected.

I agree it's annoying, but we'll have a similar problem, just the other
way around, with an endpoint defined in the SoC dtsi. Many R-Car SoCs
have two LVDS encoders, and you can attach a panel to either of them.
Some boards use LVDS0, some boards use LVDS1, and some boards could even
use both.

A real solution for this problem will require a new concept. The "DT
connector" proposal is related to this problem space. There's also a
proprietary implementation in the Rapsberry Pi boot loader of a
mechanism to support parametrized overlays ([2] and [3], or [4] for an
example of how a panel reset or backlight GPIO can be parametrized).

[1] https://lwn.net/Articles/689783/
[2] https://www.raspberrypi.com/documentation/computers/configuration.html#part3
[3] https://github.com/raspberrypi/firmware/blob/master/boot/overlays/README#L122
[4] https://github.com/raspberrypi/firmware/blob/master/boot/overlays/README#L312

-- 
Regards,

Laurent Pinchart