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

[Bjorn Helgaas <helgaas@kernel.org>]

[+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
> 
> A minimal dynamic node for this device boots. Adding only
> device_type="pci" makes it hang. Full PCI dynamic OF node generation also
> boots when only 0000:00:00.0 is skipped.
> 
> I also tested a generic guard in of_pci_make_dev_node(). It reads
> PCI_PRIMARY_BUS and skips node creation if the primary bus does not match
> pdev->bus->number, the secondary bus is not greater than the primary bus,
> or the secondary bus is greater than the subordinate bus. Linux 6.18.44
> boots with CONFIG_PCI_DYNAMIC_OF_NODES=y and logs:
> 
>   pci 0000:00:00.0: skipping dynamic OF node for invalid bridge
> 
> The regression was introduced by 49d63971f963 in v6.17-rc1. Commit
> ce26f588c831 made v6.19-rc5 boot by removing MISC_RP1's select of
> PCI_DYNAMIC_OF_NODES. That avoids the failing path in the tested
> configuration but does not change of_pci_make_dev_node().
> 
> I do not have a log from the failed boots. No output appeared with
> earlyprintk=efi,keep, keep_bootcon, ignore_loglevel, loglevel=8,
> initcall_debug, and EFI pstore enabled. Blacklisting both GPU drivers and
> disabling the Intel IOMMU did not make the bad kernel boot.
> 
> Should of_pci_make_dev_node() reject devices with invalid bridge bus
> configuration, or should 8086:4c43 use a device-specific quirk? I can
> prepare and test a patch for either approach. For Linux 6.18 stable,
> would that fix be preferable to backporting ce26f588c831 and its
> dependencies?

A quirk doesn't sound right because AFAICS there isn't any hardware
defect with the 00:00.0 bridge.

> #regzbot introduced: 49d63971f963
> #regzbot title: PCI_DYNAMIC_OF_NODES early boot hang on ACPI x86
> 
> Thanks,
> Angel J
> 
> 
> 
> 
>