Difference between PCIe host bridge and PCI-PCI bridge

[Tan]

Hi,

I would appreciate if someone explains the difference between PCIe host bridge and PCI-PCI bridge on Intel platforms.

On Intel systems, I see that there is only one Host Bridge and multiple PCI-PCI bridges. When I dump the config spaces, I found that PCI-PCI bridges have Header Type '1' while Host Bridge have header type '0'. As per my understanding, only non Bridge devices support header type '0'.

Thanks,
Tan

[Clemens Ladisch]

Tan wrote:
> I would appreciate if someone explains the difference between PCIe host
> bridge and PCI-PCI bridge on Intel platforms.

PCI/PCI bridges are interfaces between two PCI buses.
The host bridge (or root complex) is the interface between the top-level
PCI bus and the CPU (the host).

> I found that PCI-PCI bridges have Header Type '1' while Host Bridge
> have header type '0'. As per my understanding, only non Bridge devices
> support header type '0'.

From the point of view of the PCI bus, the host is not visible as a PCI
device, so the device called "host bridge" is not actually a bridge.


Regards,
Clemens

[Zihan]

Nice explanation, but I wonder what is the situation when I have multiple PCI segment(domain)?

Suppose I setup 3 PCI domains, do I have 3 Root Bridges in a single Host Bridge, or do I have 3 Host Bridges? In either case, do theses bridges share a common south bridge, or they have their own ones?

Please correct me if I'm wrong as I just start to learn PCI(e).

Thanks.