Re: [PATCH v2] PCI: of: Downgrade error message on missing of_root node

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

Hi Andrea,

+ CC Rob

On Thu, 6 Nov 2025 16:21:47 +0100
Andrea della Porta <andrea.porta@suse.com> wrote:

> Hi Herve,
> 
> On 13:18 Thu 06 Nov     , Herve Codina wrote:
> > Hi, Andrea, Bjorn,
> > 
> > On Thu, 6 Nov 2025 12:04:07 +0100
> > Andrea della Porta <andrea.porta@suse.com> wrote:
> >   
> > > [+cc Herve]
> > > 
> > > Hi Bjorn,
> > > 
> > > On 18:23 Wed 05 Nov     , Bjorn Helgaas wrote:  
> > > > [+cc Lizhi]
> > > > 
> > > > On Wed, Nov 05, 2025 at 07:33:40PM +0100, Andrea della Porta wrote:    
> > > > > When CONFIG_PCI_DYNAMIC_OF_NODES is enabled, an error message
> > > > > is generated if no 'of_root' node is defined.
> > > > > 
> > > > > On DT-based systems, this cannot happen as a root DT node is
> > > > > always present. On ACPI-based systems, this is not a true error
> > > > > because a DT is not used.
> > > > > 
> > > > > Downgrade the pr_err() to pr_info() and reword the message text
> > > > > to be less context specific.    
> > > > 
> > > > of_pci_make_host_bridge_node() is called in the very generic
> > > > pci_register_host_bridge() path.  Does that mean every boot of a
> > > > kernel with CONFIG_PCI_DYNAMIC_OF_NODES on a non-DT system will see
> > > > this message?    
> > > 
> > > This is the case, indeed. That's why downgrading to info seems sensible.
> > >   
> > > > 
> > > > This message seems like something that will generate user questions.
> > > > Or is this really an error, and we were supposed to have created
> > > > of_root somewhere but it failed?  If so, I would expect a message
> > > > where the of_root creation failed.    
> > > 
> > > Not really an error per se: on ACPI system we usually don't have DT, so
> > > this message just warns you that there will be no pci nodes created on it.
> > > Which, again, should be of no importance on ACPI.  
> > 
> > I my last understanding, all architecture (even x86) have the DT root node
> > set. This node is empty on architectures that don't use DT to describe
> > hardware at boot (ACPI for instance).  
> 
> This does not seem to be the case for all arch, see below.
> 
> > 
> > This DT node is needed for PCI board that will be described by a DT overlay.
> > LAN966x for instance.
> > 
> > On v6.18-rc1 kernel, I successfully used my LAN966x board on a ACPI system.
> > This means that of_root DT node was present on my system.
> >   
> > > 
> > > The only scenario in which this message is actually an error would be on
> > > ACPI system that use DT as a complement to make runtime overlay work,  
> > 
> > It is an error also if you use a PCI board that needs PCI DT nodes
> > (CONFIG_PCI_DYNAMIC_OF_NODES) Lan966x for instance.  
> 
> Yes, I was referring exactly to that.
> 
> >   
> > > i.e. the overlay approach for RP1 on RPi5 with ACPI fw. AFAIK this fw is
> > > more a PoC that something really widespread and currntly the overlay
> > > approach is in stand-by anyway (meaning no one will use it unless some
> > > major changes will be made to make it work). But there may be other
> > > situations in which this scenario could arise, I'm thinking about Bootlin's
> > > LAN966x driver which also uses runtime overlay to describe thw hw.
> > > On ACPI system the root DT node is not created because unflatten_device_tree()
> > > is not called.  
> > 
> > I am not so sure.
> > My LAN966x board is working on a x86 ACPI system.  
> 
> Indeed it depends on the architecture. On x86 an empty DT node is created,
> provided you have CONFIG_OF_EARLY_FLATTREE defined (which I guess you have,
> even if it's not in default config).

Indeed, I have CONFIG_OF_EARLY_FLATTREE = y.

> 
> On arm64, ACPI and DT are mutually exclusive, unless the DT is basically empty
> (i.e. only root node and chosen node). The DT root node is not automatically
> created if not provided at boot, though. This reinforces my idea of providing
> the only root node DT on arm as well, but I'm not entirely sure about 
> possible side effects.
> 

Isn't it possible to have the same kind of operations on ARM64 ACPI and on x86?

In order to have CONFIG_PCI_DYNAMIC_OF_NODES working on ACPI, we need a DT
node, even empty.

ARM64 ACPI without an empty DT node means that no PCI boards using a DT
description will work on this system.

Best regards,
Hervé