Re: [PATCHv5 2/3] PCI: allow all bus devices to use the same slot

[Bjorn Helgaas <helgaas@kernel.org>]

On Tue, Feb 17, 2026 at 08:08:35AM -0800, Keith Busch wrote:
> From: Keith Busch <kbusch@kernel.org>
> 
> A pcie hotplug slot applies to the entire subordinate bus. Thus, pciehp
> only allocates a single hotplug_slot for the bridge to that bus. The pci
> slot, though, would only match to functions on device 0, meaning all
> device beyond that are not matched to any slot even though they share
> it. A slot reset will break all the missing devices because the handling
> skips them.

I tweaked this a bit to (a) refer to the "secondary" bus instead of
"subordinate" to avoid confusion with the PCI "subordinate" term
("pci_dev->subordinate" already has this confusion but is harder to
fix), and (b) mention ARI, which I think is what causes the problem
you're solving:

+ A PCIe hotplug slot applies to the entire secondary bus. Thus, pciehp only
+ allocates a single hotplug_slot for the bridge to that bus. The existing
+ PCI slot, though, would only match to functions on device 0, meaning any
+ devices beyond that, e.g., ARI functions, are not matched to any slot even
+ though they share it.

> For example, ARI devices with more than 8 functions fail because their state is
> not properly handled, nor is the attached driver notified of the reset. In the
> best case, the device will appear unresponsive to the driver, resulting in
> unexpected errors. A worse possibility may panic the kernel if in flight
> transactions trigger hardware reported errors like this real observation:

> @@ -222,6 +233,13 @@ static struct pci_slot *get_slot(struct pci_bus *parent, int slot_nr)
>   * consist solely of a dddd:bb tuple, where dddd is the PCI domain of the
>   * %struct pci_bus and bb is the bus number. In other words, the devfn of
>   * the 'placeholder' slot will not be displayed.
> + *
> + * Bus-wide slots:
> + * For PCIe hotplug, the physical slot encompasses the entire subordinate
> + * bus, not just a single device number. Pass @slot_nr == PCI_SLOT_ALL_DEVICES
> + * to create a slot that matches all devices on the bus. Unlike placeholder
> + * slots, bus-wide slots go through normal slot lookup and reuse existing
> + * slots if present.

+ * For PCIe hotplug, the physical slot encompasses the entire secondary
+ * bus, not just a single device number. If the device supports ARI and ARI
+ * Forwarding is enabled in the upstream bridge, a multi-function device
+ * may include functions that appear to have several different device
+ * numbers, i.e., PCI_SLOT() values.  Pass @slot_nr == PCI_SLOT_ALL_DEVICES

>   */
>  struct pci_slot *pci_create_slot(struct pci_bus *parent, int slot_nr,
>  				 const char *name,

> +++ b/include/linux/pci.h
> @@ -72,12 +72,18 @@
>  /* return bus from PCI devid = ((u16)bus_number) << 8) | devfn */
>  #define PCI_BUS_NUM(x) (((x) >> 8) & 0xff)
>  
> +/*
> + * PCI_SLOT_ALL_DEVICES indicates a slot that covers all devices on the bus.
> + * Used for PCIe hotplug where the physical slot is the entire subordinate bus.

+ * Used for PCIe hotplug where the physical slot is the entire secondary bus,
+ * and, if ARI Forwarding is enabled, functions may appear to be on multiple
+ * devices.

Let me know if I got any of this wrong.

Bjorn