Understanding segment group, bus, device and function numbers from SMBIOS

Question

Objective

I'm trying to programmatically find out on which physical slot a particular PCIe device is connected. The premise is that I have the PCI-ID of a card that is surely occupying a slot, and the solution would be finding out which one.

Note that I'm only interested in working on the PCI Express bus, although the device identification/enumeration process I think is the same as the old PCI.

Attempt

The SMBIOS contains information about the motherboard slots available. It is possible to examine this information from the linux command line:

$ sudo dmidecode -t slot
# dmidecode 2.11
SMBIOS 2.7 present.

Handle 0x003A, DMI type 9, 17 bytes
System Slot Information
    Designation: PCIEX16_1
    Type: x16 PCI Express x16
    Current Usage: In Use
    Length: Short
    ID: 1
    Characteristics:
        3.3 V is provided
        Opening is shared
        PME signal is supported
    Bus Address: 0000:03:02.0

Handle 0x003B, DMI type 9, 17 bytes
System Slot Information
    Designation: PCIEX16_2
    Type: x8 PCI Express x8
    Current Usage: In Use
    Length: Short
    ID: 2
    Characteristics:
        3.3 V is provided
        Opening is shared
        PME signal is supported
    Bus Address: 0000:04:02.2

Handle 0x003C, DMI type 9, 17 bytes
System Slot Information
    Designation: PCIEX16_3
    Type: x16 PCI Express x16
    Current Usage: In Use
    Length: Short
    ID: 3
    Characteristics:
        3.3 V is provided
        Opening is shared
        PME signal is supported
    Bus Address: 0000:05:03.0

However, I can't find any device in the PCI bus (lspci) that has a PCI ID of 0000:03:02.0, 0000:04:02.2 or 0000:05:03.0. What I know by manual inspection is the following:

1. Card 0000:03:00.0 is in the first slot. The PCI bridge appears to be 0000:00:02.0
2. Card 0000:04:00.0 is in the second slot. The PCI bridge appears to be 0000:00:02.2
3. Card 0000:05:00.0 is in the third slot. The PCI bridge appears to be 0000:00:03.0

So a pattern emerges here. From the SMBIOS structure, the bus number is the one given to the card that is plugged in but the device/function numbers are the same as the corresponding PCI bridge.

Is this pattern just a coincidence in my motherboard or there is a rationale behind? It is an Asus motherboard, with an AMI BIOS. I've read that some BIOSes do not provide accurate information about their slots so I would like to know how much generality I can achieve.

Any hint or pointer to reference documentation is also greatly appreciated.

Answer 1

You may find the lspci command helpful. Here's a sample output:

# lspci -v -t
-[0000:00]-+-00.0  Intel Corporation 5500 I/O Hub to ESI Port
           +-01.0-[01]--+-00.0  Broadcom Corporation NetXtreme II BCM5709 Gigabit Ethernet
           |            \-00.1  Broadcom Corporation NetXtreme II BCM5709 Gigabit Ethernet
           +-03.0-[02]--+-00.0  Broadcom Corporation NetXtreme II BCM5709 Gigabit Ethernet
           |            \-00.1  Broadcom Corporation NetXtreme II BCM5709 Gigabit Ethernet
           +-07.0-[04]----00.0  LSI Logic / Symbios Logic MegaRAID SAS 2108 [Liberator]
           +-09.0-[05]----00.0  LSI Logic / Symbios Logic MegaRAID SAS 2108 [Liberator]
           +-14.0  Intel Corporation 7500/5520/5500/X58 I/O Hub System Management Registers
           +-14.1  Intel Corporation 7500/5520/5500/X58 I/O Hub GPIO and Scratch Pad Registers
           +-14.2  Intel Corporation 7500/5520/5500/X58 I/O Hub Control Status and RAS Registers
           +-1a.0  Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #4
           +-1a.1  Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #5
           +-1a.7  Intel Corporation 82801I (ICH9 Family) USB2 EHCI Controller #2
           +-1c.0-[03]----00.0  LSI Logic / Symbios Logic MegaRAID SAS 2108 [Liberator]
           +-1d.0  Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #1
           +-1d.1  Intel Corporation 82801I (ICH9 Family) USB UHCI Controller #2
           +-1d.7  Intel Corporation 82801I (ICH9 Family) USB2 EHCI Controller #1
           +-1e.0-[06]----03.0  Matrox Electronics Systems Ltd. MGA G200eW WPCM450
           +-1f.0  Intel Corporation 82801IB (ICH9) LPC Interface Controller
           \-1f.2  Intel Corporation 82801IB (ICH9) 2 port SATA Controller [IDE mode]