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

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Message-ID: <20250220092514.444e90e4@bootlin.com>
Date: Thu, 20 Feb 2025 09:25:14 +0100
From: Herve Codina <herve.codina@...tlin.com>
To: Bjorn Helgaas <helgaas@...nel.org>
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

Hi Bjorn,

On Wed, 19 Feb 2025 11:39:12 -0600
Bjorn Helgaas <helgaas@...nel.org> wrote:

> 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?
> 

What do you think if I add the following at the end of the commit log?

   With this done, hardware available in complex PCI device can be
   described by a device-tree overlay loaded by the PCI device driver
   on non device-tree based systems. For instance, the LAN966x PCI device
   introduced by commit 185686beb464 ("misc: Add support for LAN966x
   PCI device") can be available on x86 systems.

Best regards,
Hervé