Re: [PATCH 0/6] PCI: dwc: Revert Link Up IRQ support

[Lukas Wunner <lukas@wunner.de>]

On Tue, Nov 11, 2025 at 11:51:00AM +0100, Niklas Cassel wrote:
> Revert all patches related to pcie-designware Root Complex Link Up IRQ
> support.
> 
> While this fake hotplugging was a nice idea, it has shown that this feature
> does not handle PCIe switches correctly:
[...]
> During the initial scan, PCI core doesn't see the switch and since the Root
> Port is not hot plug capable, the secondary bus number gets assigned as the
> subordinate bus number. This means, the PCI core assumes that only one bus
> will appear behind the Root Port since the Root Port is not hot plug
> capable.
> 
> This works perfectly fine for PCIe endpoints connected to the Root Port,
> since they don't extend the bus. However, if a PCIe switch is connected,
> then there is a problem when the downstream busses starts showing up and
> the PCI core doesn't extend the subordinate bus number after initial scan
> during boot.

In principle it is possible to set the is_hotplug_bridge flag on the
bridge to force allocation of more buses.  We've already got a quirk
to set the flag on a Conventional PCI bridge whose hotplug capability
cannot be discovered otherwise (see quirk_hotplug_bridge() in
drivers/pci/quirks.c).

Thanks,

Lukas