linux-kernel - Re: [PATCH v7 5/5] PCI: of: Create device-tree PCI host bridge node

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Message-ID: <20250219173912.GA224527@bhelgaas>
Date: Wed, 19 Feb 2025 11:39:12 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: Herve Codina <herve.codina@...tlin.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>, Rob Herring <robh@...nel.org>,
	Saravana Kannan <saravanak@...gle.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, Lizhi Hou <lizhi.hou@....com>,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	linux-pci@...r.kernel.org,
	Allan Nielsen <allan.nielsen@...rochip.com>,
	Horatiu Vultur <horatiu.vultur@...rochip.com>,
	Steen Hegelund <steen.hegelund@...rochip.com>,
	Thomas Petazzoni <thomas.petazzoni@...tlin.com>
Subject: Re: [PATCH v7 5/5] PCI: of: Create device-tree PCI host bridge node

On Tue, Feb 04, 2025 at 08:35:00AM +0100, Herve Codina wrote:
> PCI devices device-tree nodes can be already created. This was
> introduced by commit 407d1a51921e ("PCI: Create device tree node for
> bridge").
> 
> In order to have device-tree nodes related to PCI devices attached on
> their PCI root bus (the PCI bus handled by the PCI host bridge), a PCI
> root bus device-tree node is needed. This root bus node will be used as
> the parent node of the first level devices scanned on the bus. On
> device-tree based systems, this PCI root bus device tree node is set to
> the node of the related PCI host bridge. The PCI host bridge node is
> available in the device-tree used to describe the hardware passed at
> boot.
> 
> On non device-tree based system (such as ACPI), a device-tree node for
> the PCI host bridge or for the root bus does not exist. Indeed, the PCI
> host bridge is not described in a device-tree used at boot simply
> because no device-tree are passed at boot.
> 
> The device-tree PCI host bridge node creation needs to be done at
> runtime. This is done in the same way as for the creation of the PCI
> device nodes. I.e. node and properties are created based on computed
> information done by the PCI core. Also, as is done on device-tree based
> systems, this PCI host bridge node is used for the PCI root bus.

This is a detailed low-level description of what this patch does.  Can
we include a high level outline of what the benefit is and why we want
this patch?

Based on 185686beb464 ("misc: Add support for LAN966x PCI device"), I
assume the purpose is to deal with some kind of non-standard PCI
topology, e.g., a single B/D/F function contains several different
pieces of functionality to be driven by several different drivers, and
we build a device tree description of those pieces and then bind those
drivers to the functionality using platform_device interfaces?

Bjorn