Similar SoCs with different CPUs and interrupt bindings

[Geert Uytterhoeven <geert@linux-m68k.org>]

        Hi Rob, Krzysztof,

This is a topic that came up at the RISC-V BoF at Plumbers, and it was
suggested to bring it up with you.

The same SoC may be available with either RISC-V or other (e.g. ARM) CPU
cores (an example of this are the Renesas RZ/Five and RZ/G2UL SoCs).
To avoid duplication, we would like to have:
  - <riscv-soc>.dtsi includes <base-soc>.dtsi,
  - <arm-soc>.dtsi includes <base-soc>.dtsi.

Unfortunately RISC-V and ARM typically use different types of interrupt
controllers, using different bindings (e.g. 2-cell vs. 3-cell), and
possibly using different interrupt numbers.  Hence the interrupt-parent
and interrupts{-extended} properties should be different, too.

Possible solutions[1]:
  1. interrupt-map

  2. Use a SOC_PERIPHERAL_IRQ() macro in interrupts properties in
     <base-soc>.dtsi, with
       - #define SOC_PERIPHERAL_IRQ(nr, na) nr          // RISC-V
       - #define SOC_PERIPHERAL_IRQ(nr, na) GIC_SPI na  // ARM
     Note that the cpp/dtc combo does not support arithmetic, so even
     the simple case where nr = 32 + na cannot be simplified.

  3. Wrap inside RISCV() and ARM() macros, e.g.:

        RISCV(interrupts = <412 IRQ_TYPE_LEVEL_HIGH>;)
        ARM(interrupts = <GIC_SPI 380 IRQ_TYPE_LEVEL_HIGH>;)

     Cfr. ARM() and THUMB() in arch/arm/include/asm/unified.h, as used
     to express the same operation using plain ARM or ARM Thumb
     instructions.

Personally, I'm leaning towards the third solution, as it is the most
flexible, and allows us to extend to more than 2 interrupt controllers.

Note that this is actually not a new issue.  For years, ARM SoCs have
existed with multiple types of cores on the same die, using Cortex-A
cores for the application, and Cortex-R/SuperH/V850/... cores for
real-time and/or baseband operation.  So far this wasn't an issue, as
only the Cortex-A cores ran Linux, and we ignored the other cores (and
the related interrupt controllers and hierarchy) in DT.

What do you think?
Thanks for your comments!

[1] https://lore.kernel.org/lkml/20220815050815.22340-7-samuel@sholland.org

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds