Re: [REGRESSION] PCI: Dynamic OF node creation hangs on invalid bridge configuration

[Herve Codina]

Hi Angel, Bjorn,

On Thu, 3 Sep 2026 18:17:44 -0500
Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:

> [+to Herve, Lizhi for PCI dynamic OF node stuff]
> 
> On Sun, Aug 23, 2026 at 02:14:25AM +0000, Angel J wrote:
> > Hello,
> > 
> > This regression was introduced in v6.17-rc1 and remains in the dynamic
> > PCI OF node code. Linux 6.18 fails very early during boot on my Dell XPS
> > 8940 when CONFIG_PCI_DYNAMIC_OF_NODES=y. The display goes black before
> > any useful console or pstore output appears, and the machine remains
> > hung.  
> 
> Oof, sorry, that's a terrible failure to debug.  Thanks for your
> persistence in chasing it.
> 
> I don't think 49d63971f963 ("misc: rp1: RaspberryPi RP1 misc driver")
> is a likely culprit by itself because there's just nothing there that
> looks like it would relate to a Dell XPS 8940.
> 
> > Disabling CONFIG_PCI_DYNAMIC_OF_NODES makes the same kernel boot normally.
> > Linux v6.19-rc5 also boots because the RP1 driver stopped selecting that
> > option, not because the dynamic PCI OF node code changed.
> > 
> > Hardware:
> > 
> >   System: Dell XPS 8940, board 0K3CM7, BIOS 2.27.1 (2025-04-03)
> >   CPU: Intel Core i7-11700 (Rocket Lake)
> >   Triggering device: 0000:00:00.0, Intel 8086:4c43
> >   Firmware boot: UEFI, ACPI-based x86_64
> > 
> > Test results:
> > 
> >   Linux 6.12.103: good
> >   Linux v6.17-rc1: bad
> >   Linux 6.18.44, CONFIG_PCI_DYNAMIC_OF_NODES=y: bad
> >   Linux 6.18.44, CONFIG_PCI_DYNAMIC_OF_NODES=n: good
> >   Linux 6.18.44, CONFIG_PCI_DYNAMIC_OF_NODES=y, skip invalid bridge: good
> >   Linux v6.19-rc4: bad
> >   Linux v6.19-rc5: good
> >   Linux 4621c338d33f: bad
> >   Linux e55feea3a03a: good
> > 
> > The first-parent boundary between the last two tests is:
> > 
> >   e55feea3a03a ("Merge tag 'soc-fixes-6.19' ...")
> > 
> > That merge contains ce26f588c831 ("misc: rp1: drop overlay support"), which
> > removes:
> > 
> >   select PCI_DYNAMIC_OF_NODES
> > 
> > from MISC_RP1. The generated configurations differed only by:
> > 
> >   -CONFIG_PCI_DYNAMIC_OF_NODES=y
> >   +# CONFIG_PCI_DYNAMIC_OF_NODES is not set
> > 
> > CONFIG_MISC_RP1=m in both. The known-bad 4621c338d33f and Linux 6.18.44
> > both boot after disabling PCI_DYNAMIC_OF_NODES (and MISC_RP1, whose Kconfig
> > select otherwise forces it on).
> > 
> > There is no RP1 device in this machine, and its driver does not probe.
> > MISC_RP1 affects this x86 system only because its Kconfig select enables
> > PCI_DYNAMIC_OF_NODES globally.
> > 
> > I narrowed the hang to dynamic OF node creation for 0000:00:00.0
> > (8086:4c43), the Intel Rocket Lake-S integrated memory controller. On
> > this machine it reports PCI class 0604 and header type 1, but it is bound
> > to icl_uncore and is not a usable forwarding bridge:
> > 
> >   Bus: primary=ff, secondary=ff, subordinate=ff
> >   I/O and memory bridge windows: invalid/all ones
> >   kernel: bridge configuration invalid ([bus ff-ff]), reconfiguring

Is this PCI logs reported with PCI_DYNAMIC_OF_NODES=y or PCI_DYNAMIC_OF_NODES=n
or always whatever the PCI_DYNAMIC_OF_NODES Kconfig value ?

A PCI class 0604 is a PCI-to-PCI bridge.

Something in kernel code prevent the scan of this PCI-to-PCI bridge even if the
kernel tries to reconfigure it. Indeed, the device is "Intel Rocket Lake-S
integrated memory controller" and not a PCI-to-PCI bridge

The of_pci_make_dev_node() is called by the PCI core [1] to create OF node for
bridges.

of_pci_make_dev_node() will set (device_type = "pci" in the created node).

But if the device it is not a bridge, this is obviously wrong (even if I don't
understand why it hangs the system).

Maybe the test done at [1] should be improved to detect those wrong bridges.
and skip the of_pci_make_dev_node() call when a wrong bridge is detected.

[1] https://elixir.bootlin.com/linux/v7.2/source/drivers/pci/bus.c#L355


Best regards,
Hervé