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

[Phil Elwell <phil@xxxxxxxxxxxxxxx>]

Once more, with plain text, which I'd hoped the Android GMail client
would work out for itself.

On Thu, 13 Feb 2025, 18:53 Herve Codina, <herve.codina@xxxxxxxxxxx> wrote:
>
> Hi Phil,
>
> On Thu, 13 Feb 2025 17:57:37 +0000
> Phil Elwell <phil@xxxxxxxxxxxxxxx> wrote:
>
> > On Thu, 13 Feb 2025 at 17:45, Andrew Lunn <andrew@xxxxxxx> wrote:
> > >
> > > > > Or do you mean a custom board, which has a CPU, RP1 and the button and
> > > > > fan are directly on this custom board? You then want a board DTS which
> > > > > includes all these pieces?
> > > >
> > > > That depends on whether you count the Raspberry Pi 5 as a custom board.
> > >
> > > So you mean the Pi 5 board would itself make use of the resources the
> > > RP1 device has? They are not simply connected to headers for plugin
> > > boards, but used by the main board? Hence you want to describe them in
> > > the board .DTS file.
> >
> > That's correct. But even for plug-in devices, those which are on
> > non-discoverable buses need overlays to declare them, which causes a
> > problem when the overlay application happens before the kernel is
> > started.
> >
>
> Hum, I see.
>
> We worked on overlay usage on non-discoverable buses wired to a connector
> and we did a talk about issues we are facing on at Plumber [0].
>
> You can also find our big picture in [1] and a last contribution introducing
> export-symbols feature in [2]. export-symbols is also under discussion on
> some other threads.
>
> Also, we proposed the i2c bus extensions feature [3] whose goal is to allow
> an addon board to add devices on an i2c bus provided by a base board and
> wired to an connector the addon board is connected to.
>
> Maybe in your case, you can decouple resources (gpio, pwm) provided by the
> addon board and used by the base board using also nexus node.
>
> We use a nexus node [4] (not presented at the Plumbers talk because the idea
> came during 'out of talk' discussions in Plumbers) in order to allow our
> addon board to use resources provided by the base board.
>
> In your case, if I understood, you are in the other direction but why not
> using also a nexus node to decouple and translate resources in this other
> direction ?
>
> Don't know if this idea can help but feel free to ask for some more
> information if needed.

Nexus nodes look interesting - I see them as adding a layer of
abstraction such that, for example, boards can declare which of their
specific resources performs a common function so that clients can
treat them all the same. We do the same thing in a limited way by
using common labels on nodes, but this goes much further.

In the case of Pi 5 and RP1, I imagine you are proposing that the Pi 5
dtb declares the connector node and the overlay fills in the content
with references to its GPIO controller, PWM controller etc. However, I
think the overlay would also have to be board specific because it's
not possible to patch part of a property from an overlay, so you'd end
up overwriting the GPIO number as well as the controller reference.

What is needed to make this work is the ability to cope with
unresolved references in the base dtb, to be resolved as each overlay
is applied, with runtime checking that each reference is resolved
before it is used, all of which sounds like a nightmare. Plus, we
really don't want to have to change the way all our camera and display
overlays work on all Raspberry Pis just to accommodate somebody's idea
of how RP1 should be handled.

Besides, Occam's razor surely applies.

Phil