Hi Robin,

On Wed, Sep 21, 2022 at 11:20 AM Robin Murphy [off-list ref] wrote:

On 2022-09-21 08:46, Geert Uytterhoeven wrote:

quoted

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.

4. Put all the "interrupts" properties in the SoC-specific DTSI at the
same level as the interrupt controller to which they correspond. Works
out of the box with no horrible mystery macros, and is really no more or
less error-prone than any other approach. Yes, it means replicating a
bit of structure and/or having labels for everything (many of which may
be wanted anyway), but that's not necessarily a bad thing for
readability anyway. Hierarchical definitions are standard FDT practice
and should be well understood, so this is arguably the simplest and
least surprising approach :)

Thanks for the suggestion!

It does mean we have to update 3 .dtsi files when adding support
for a new device. As long as all DT changes go through the same (soc)
tree, we can easily manage the dependencies.

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

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