Re: [PATCH v6 00/10] Add support for RaspberryPi RP1 PCI device using a DT overlay

[Phil Elwell <phil@raspberrypi.com>]

On Mon, 17 Feb 2025 at 17:56, Herve Codina <herve.codina@bootlin.com> wrote:
>
> On Mon, 17 Feb 2025 17:03:34 +0000
> Phil Elwell <phil@raspberrypi.com> wrote:
>

<snip>

> > The job of the nexus node would be to translate a generic request for
> > a numbered resource to a specific request for an RP1 resource with
> > arbitrary properties. The arbitrary properties could be GPIO offsets,
> > which are board specific, while the node supplying the resource is
> > provided by the overlay. This means that an entry in the table,
> > described by a single property, could have contributions from the base
> > DT and the overlay, which is not possible since overlays overwrite
> > whole properties.
>
> Hum, I am a bit lost.
> Some DT example (base and overlay) could help me to understand.

I could, but I think it is becoming a distraction.

<snip>

> > I think I can see how that could be made to work for GPIOs. It looks
> > as though the GPIO subsystem is the only one making use of
> > of_parse_phandle_with_args_map. Interrupts seem to have an open-coded
> > equivalent, and iommus. What about I2C and PWM?
>
> Support for PWM has been recently accepted.
>   https://lore.kernel.org/all/ufl4kwrjyp4zid4muvghefevqc6hk3zyvxnsu72fxd4f46fzg6@hufkci2dzjid/
>
> For i2c, nexus node is not suitable.
>
> Nexus node works well when resources are indexed (gpio line in a gpio chip
> for instance). For bus controller there is no index.
> I mean we never refer a i2c bus controller using <&i2c-ctrl 12>.
>
> For i2c, I proposed i2c bus extension:
>   https://lore.kernel.org/all/20250205173918.600037-1-herve.codina@bootlin.com/

I don't see in principle why an address-cells of 0 should cause a
problem - it's a degenerate case, but it's still conceptually valid.

However, we seem to be having to invent a lot of new infrastructure -
some of it already supported by the kernel, some of it not - because
we have made the mistake of using a discoverable bus for a
point-to-point link on a PCB. I don't see how this is fundamentally
different to the USB-attached Ethernet controller on e.g. the Pi 3B.
Despite the fact that the Ethernet interface is discoverable, it has a
Device Tree declaration (see
arch/arm/boot/dts/broadcom/bcm283x-rpi-smsc9514.dtsi) in order that
the firmware can supply a MAC address. Yes, the RP1 DT declaration is
significantly larger, but size shouldn't matter for what seems to be
objections based on tenets.

Phil