Hello Geert,
On 14/02/2025 at 10:42:04 +01, Geert Uytterhoeven <geert+renesas@xxxxxxxxx> wrote:
> make dtbs_check:
>
> arch/arm/boot/dts/renesas/r9a06g032-rzn1d400-db.dtb: serial@40060000: compatible: 'oneOf' conditional failed, one must be fixed:
> ['renesas,r9a06g032-uart', 'renesas,rzn1-uart', 'snps,dw-apb-uart'] is too long
> ...
>
> As per commit 72b0505f0830df95 ("dt: serial: Add Renesas RZ/N1 binding
> documentation"), the RZ/N1 UART is a modified Synopsys DesignWare UART.
> The modifications only relate to DMA, so you could actually use the
> controller with the Synopsys compatible string if you are not using DMA,
> but you should not do so. Hence the first three UARTs (which don't
> support DMA) were added with a "snps,dw-apb-uart" fallback, to use the
> existing Synopsys DesignWare UART support.
>
> Since support for the RZ/N1-specific compatible value was added to the
> driver a long time ago (commit 2ff5fa7f742ab0c6 ("serial: 8250_dw: Add
> compatible string for Renesas RZ/N1 UART") in v4.19), the extra
> compatible value can be dropped safely.
>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
> ---
> Alternatively, the bindings[1] could be augmented with an extra section:
>
> - items:
> - enum:
> - renesas,r9a06g032-uart
> - renesas,r9a06g033-uart
> - const: renesas,rzn1-uart
> - const: snps,dw-apb-uart # RZ/N1 without DMA
>
> and perhaps extra logic to prohibit the dmas property when both
> renesas,rzn1-uart and snps,dw-apb-uart are present.
I must say that I prefer this secondary approach, which feels more
accurate. I won't block the one that your proposed below for sure, but I
feel like it is more relevant to add this third entry in the bindings
rather than removing it from the DT. Either way, fine by me.
>
> Given the complexity of the latter, I went for the simple solution.
>
> [1] Documentation/devicetree/bindings/serial/snps-dw-apb-uart.yaml
Thanks,
Miquèl
