Message-ID: <CAMEGJJ2_HVKfsE3P22baadbzxSDAX=yTr=m76YuXa5A2cJsJig@mail.gmail.com>
Date: Thu, 13 Feb 2025 21:12:43 +0000
From: Phil Elwell <phil@...pberrypi.com>
To: Herve Codina <herve.codina@...tlin.com>
Cc: Andrew Lunn <andrew@...n.ch>, Andrea della Porta <andrea.porta@...e.com>, Arnd Bergmann <arnd@...db.de>, 
	"maintainer:BROADCOM BCM7XXX ARM ARCHITECTURE" <bcm-kernel-feedback-list@...adcom.com>, bhelgaas@...gle.com, brgl@...ev.pl, 
	Catalin Marinas <catalin.marinas@....com>, Conor Dooley <conor+dt@...nel.org>, derek.kiernan@....com, 
	devicetree@...r.kernel.org, dragan.cvetic@....com, 
	Florian Fainelli <florian.fainelli@...adcom.com>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, krzk+dt@...nel.org, kw@...ux.com, 
	Linus Walleij <linus.walleij@...aro.org>, 
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, linux-clk@...r.kernel.org, 
	linux-gpio@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>, 
	"open list:PCI NATIVE HOST BRIDGE AND ENDPOINT DRIVERS" <linux-pci@...r.kernel.org>, 
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	Will Deacon <will@...nel.org>, Dave Stevenson <dave.stevenson@...pberrypi.com>
Subject: Re: [PATCH v6 00/10] Add support for RaspberryPi RP1 PCI device using
 a DT overlay

On Thu, 13 Feb 2025, 21:06 Herve Codina, <herve.codina@...tlin.com> wrote:
>
> Hi Phil,
>
> On Thu, 13 Feb 2025 20:15:06 +0000
> Phil Elwell <phil@...pberrypi.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@...tlin.com> wrote:
> > >
> > > Hi Phil,
> > >
> > > On Thu, 13 Feb 2025 17:57:37 +0000
> > > Phil Elwell <phil@...pberrypi.com> wrote:
> > >
> > > > On Thu, 13 Feb 2025 at 17:45, Andrew Lunn <andrew@...n.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.

I feel I've been misunderstood - I appreciate your ideas.

Perhaps it would help if you could outline how you think we could
apply your suggestions?

Thanks,

Phil

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