Re: [PATCH v2 1/3] PCI: Preserve the initial bus number in pci_dev

[Mario Limonciello]

On 10/23/26 12:47, Bjorn Helgaas wrote:
On Tue, Oct 20, 2026 at 09:04:40AM -0500, Mario Limonciello wrote:
Bridges that need bus reassignment are scanned in two passes. Save
the firmware secondary bus number during the first pass, before the
bridge is disabled, and carry it to the bus created in the second
pass. Initialize pci_dev from this preserved bus number so drivers
can match firmware records after PCI bus renumbering.

Handle pci_alloc_dev(NULL) as well; some in-tree callers use it to
allocate a standalone pci_dev.

Signed-off-by: Mario Limonciello <mario.limoncie...@amd.com>
---
  drivers/pci/probe.c | 9 +++++++++
  include/linux/pci.h | 3 +++
  2 files changed, 12 insertions(+)

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 27008e2ea5afc..4ece4b70cdc15 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -1012,6 +1012,7 @@ static int pci_register_host_bridge(struct 
pci_host_bridge *bridge)
        bus->sysdata = bridge->sysdata;
        bus->ops = bridge->ops;
        bus->number = bus->busn_res.start = bridge->busnr;
+       bus->initial_bus_number = bus->number;
  #ifdef CONFIG_PCI_DOMAINS_GENERIC
        if (bridge->domain_nr == PCI_DOMAIN_NR_NOT_SET)
                bus->domain_nr = pci_bus_find_domain_nr(bus, parent);
@@ -1232,6 +1233,7 @@ static struct pci_bus *pci_alloc_child_bus(struct pci_bus 
*parent,
  
  	/* Set up the primary, secondary and subordinate bus numbers */
        child->number = child->busn_res.start = busnr;
+       child->initial_bus_number = busnr;
        child->primary = parent->busn_res.start;
        child->busn_res.end = 0xff;
  
@@ -1470,6 +1472,7 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev,
                        child = pci_add_new_bus(bus, dev, secondary);
                        if (!child)
                                goto out;
+                       child->initial_bus_number = secondary;
                        child->primary = primary;
                        pci_bus_insert_busn_res(child, secondary, subordinate);
                        child->bridge_ctl = bctl;
@@ -1491,6 +1494,8 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, 
struct pci_dev *dev,
                 * do in the second pass.
                 */
                if (!pass) {
+                       if (!dev->initial_secondary_bus_number)
+                               dev->initial_secondary_bus_number = secondary;
                        if (pcibios_assign_all_busses() || broken)
  
  				/*
@@ -1526,6 +1531,9 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, 
struct pci_dev *dev,
                        child = pci_add_new_bus(bus, dev, next_busnr);
                        if (!child)
                                goto out;
+                       if (dev->initial_secondary_bus_number)
+                               child->initial_bus_number =
+                                       dev->initial_secondary_bus_number;
                        pci_bus_insert_busn_res(child, next_busnr,
                                                bus->busn_res.end);
                }
@@ -2508,6 +2516,7 @@ struct pci_dev *pci_alloc_dev(struct pci_bus *bus)
        INIT_LIST_HEAD(&dev->bus_list);
        dev->dev.type = &pci_dev_type;
        dev->bus = pci_bus_get(bus);
+       dev->initial_bus_number = bus ? bus->initial_bus_number : 0;
        dev->driver_exclusive_resource = (struct resource) {
                .name = "PCI Exclusive",
                .start = 0,
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 5ffcf5ce8c966..f6b377dcf7a14 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -363,6 +363,8 @@ struct pci_dev {
        struct pci_slot *slot;          /* Physical slot this device is in */
  
  	unsigned int	devfn;		/* Encoded device & function index */
+       u8              initial_bus_number;     /* Bus number when device was 
discovered */
+       u8              initial_secondary_bus_number;   /* Bus number behind 
this bridge */

Why do we need this in the pci_dev?  Is there something about
pci_scan_bridge_extend() that means it has to be in pci_dev as well as
pci_bus?  Multiple copies means possible inconsistencies.

Agreed that the initial bus number should have one canonical home. I'll 
remove the copy from pci_dev; the drivers now read it from pdev->bus.

The separate initial_secondary_bus_number is temporary bridge state: 
pci_scan_bridge_extend() captures the firmware value in pass 0, before 
clearing the bridge, then transfers it to the child bus in pass 1 and 
clears it.


        unsigned short  vendor;
        unsigned short  device;
        unsigned short  subsystem_vendor;
@@ -724,6 +726,7 @@ struct pci_bus {
        struct proc_dir_entry *procdir; /* Directory entry in /proc/bus/pci */
  
  	unsigned char	number;		/* Bus number */
+       unsigned char   initial_bus_number;     /* Bus number before 
reassignment */

I think it should be stylistically similar to the existing
"pci_bus.number"; adding "initial" before or after is ok, but adding
both "initial" and "bus" makes them less connected.

Users would check "vhdr->PCIBus == rdev->pdev->bus->number_initial" or
similar.

OK, will adjust.


        unsigned char   primary;        /* Number of primary bridge */
        unsigned char   max_bus_speed;  /* enum pci_bus_speed */
        unsigned char   cur_bus_speed;  /* enum pci_bus_speed */

--
2.43.0