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

[Herve Codina <herve.codina@bootlin.com>]

Hi Phil,

On Thu, 13 Feb 2025 20:15:06 +0000
Phil Elwell <phil@raspberrypi.com> wrote:

> 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@bootlin.com> wrote:
> >
> > Hi Phil,
> >
> > On Thu, 13 Feb 2025 17:57:37 +0000
> > Phil Elwell <phil@raspberrypi.com> wrote:
> >  
> > > On Thu, 13 Feb 2025 at 17:45, Andrew Lunn <andrew@lunn.ch> 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.

Just to be clear, my comments were not there to tell you how RP1 should
work. I just proposed ideas without trying to force anything and I can
fully understand that ideas proposed don't feed your needs.

Sorry if my approach was misunderstood.

Best regards,
Hervé