Re: [PATCH RFC] dt-bindings: arm: renesas: Document Renesas Falcon sub-boards

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Hi Geert,

On 05/03/2021 13:37, Geert Uytterhoeven wrote:
> Add device tree bindings documentation for the Renesas R-Car V3U Falcon
> CSI/DSI and Ethernet sub-boards.  These are plugged into the Falcon
> BreakOut board to form the full Falcon board stack.

Aha, so this answers my questions this week ;-)

> Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
> ---
> Marked as RFC
> 
> The Falcon board stack consists of 4 boards:
>   1. CPU board, containing the R-Car V3U SoC, and core system parts like
>      RAM, console, eMMC,
>   2. BreakOut board, providing power, an Ethernet PHY, and a backplane
>      where boards 1, 3, and 4 are plugged in,
>   3. CSI/DSI sub-board, providing 2 GMSL displays and 12 GMSL cameras,
>   4. Ethernet sub-board, providing 6 Ethernet PHYs.
> 
> As the BreakOut board provides power, the CPU board cannot be used
> without the BreakOut board.  However, both the CSI/DSI and Ethernet
> sub-boards are optional.  So that means we have to support 4 stacks of
> board combinations (1+2, 1+2+3, 1+2+4, 1+2+3+4).

Presumably however - of course 2 could itself also be optional, with
/another/ board being designed to fit in place. Which I assume is why 1
is separated at all.

But that only comes when someone adds the DTB for that 'alternative'
breakout.

> That sounds like a good target for fdtoverlay, right?
> 
> For now[1] the Falcon include hierarchy looks like this (supporting only
> the full stack 1+2+3+4):
> 
>     r8a779a0-falcon.dts
>     |-- r8a779a0-falcon-cpu.dtsi
>     |   `-- r8a779a0.dtsi
>     |-- r8a779a0-falcon-csi-dsi.dtsi
>     `-- r8a779a0-falcon-ethernet.dtsi
> 
> Traditionally, we augmented the "model" and "compatible" properties of
> the root node in each additional layer:
> 
>     r8a779a0.dtsi:
> 	compatible = "renesas,r8a779a0";
> 
>     r8a779a0-falcon-cpu.dtsi:
> 	model = "Renesas Falcon CPU board";
> 	compatible = "renesas,falcon-cpu", "renesas,r8a779a0";
> 
>     r8a779a0-falcon.dts:
> 	model = "Renesas Falcon CPU and Breakout boards based on r8a779a0";
> 	compatible = "renesas,falcon-breakout", "renesas,falcon-cpu", "renesas,r8a779a0";
> 
> (Note: I haven't done that yet for the CSI/DSI and Ethernet sub-boards)
> 
> With a stack of 4 boards, some optional, this becomes a bit cumbersome.
> But it is still doable when using .dts and .dtsi files, by just adding 3
> more r8a779a0-falcon*.dts files.
> 
> So we can add model/compatible properties to the sub-boards:
> 
>     r8a779a0-falcon-csi-dsi.dtsi
> 	model = "Renesas Falcon CSI/DSI sub-board";
> 	compatible = "renesas,falcon-csi-dsi";
> 
>     r8a779a0-falcon-ethernet.dtsi:
> 	model = "Renesas Falcon Ethernet sub-board";
> 	compatible = "renesas,falcon-ethernet";
> 
> and update r8a779a0-falcon*dts to augment the properties.
> 
> However, this is currently not possible when using overlays, as applying
> an overlay would override the properties in the underlying DTB, not
> augment them.
> 
> Questions:
>   a. Should we document all possible combinations in the bindings file?
>      After this patch, we only have 1, 1+2, and 1+2+3+4 documented.

Will '1' on it's own be of any use? Or will there always need to be a
'2'? (which might be undefined / other board / '5').




>   b. How to handle "model" and "compatible" properties for (sub)boards?
>      Perhaps fdtoverlay could combine the "model" and "compatible"
>      properties in the root nodes?  However, that is not always desired.
> 

Combining the models could get ... long and repetitive couldn't it ?
I guess this is one advantage of having a separate .dts file and setting
it explicitly...

I guess there's no way to easily pass that in as a parameter of sorts :-(



> Thanks for your comments!

I'm looking forward to seeing this in action, and develop further.

Especially for cleaning up some of the GMSL add on boards we have.

--
Kieran


> 
> [1] [PATCH v2 0/3] arm64: dts: renesas: falcon: Add I2C EEPROMs and sub-boards
>     https://lore.kernel.org/linux-renesas-soc/20210304153257.4059277-1-geert+renesas@xxxxxxxxx
> ---
>  Documentation/devicetree/bindings/arm/renesas.yaml | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/arm/renesas.yaml b/Documentation/devicetree/bindings/arm/renesas.yaml
> index 5fd0696a9f91f383..08ba12632f52c317 100644
> --- a/Documentation/devicetree/bindings/arm/renesas.yaml
> +++ b/Documentation/devicetree/bindings/arm/renesas.yaml
> @@ -296,6 +296,13 @@ properties:
>            - const: renesas,falcon-cpu
>            - const: renesas,r8a779a0
>  
> +      - items:
> +          - const: renesas,falcon-breakout # Falcon BreakOut board (RTP0RC779A0BOB0010S)
> +          - const: renesas,falcon-csi-dsi # Falcon CSI/DSI sub-board (RTP0RC779A0DCS0010S)
> +          - const: renesas,falcon-ethernet # Falcon Ethernet sub-board (RTP0RC779A0ETS0010S)
> +          - const: renesas,falcon-cpu
> +          - const: renesas,r8a779a0
> +
>        - description: RZ/N1D (R9A06G032)
>          items:
>            - enum:
> 




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