Neophyte questions about PCIe

[Bjorn Helgaas]

On Tue, Mar 14, 2017 at 04:54:00PM +0100, Mason wrote:
> On 14/03/2017 15:00, Mason wrote:
> 
> > On 10/03/2017 18:49, Mason wrote:
> > 
> >> /* Root complex reports incorrect device class */
> >> static void tango_pcie_fixup_class(struct pci_dev *dev)
> >> {
> >> 	dev->class = PCI_CLASS_BRIDGE_PCI << 8;
> >> }
> > 
> > Gen1 controller reports class/rev = 0x04800001
> > Gen2 controller reports class/rev = 0x06000001
> > 
> > #define PCI_CLASS_BRIDGE_HOST		0x0600
> > #define PCI_CLASS_BRIDGE_ISA		0x0601
> > #define PCI_CLASS_BRIDGE_EISA		0x0602
> > #define PCI_CLASS_BRIDGE_MC		0x0603
> > #define PCI_CLASS_BRIDGE_PCI		0x0604
> > #define PCI_CLASS_BRIDGE_PCMCIA		0x0605
> > #define PCI_CLASS_BRIDGE_NUBUS		0x0606
> > #define PCI_CLASS_BRIDGE_CARDBUS	0x0607
> > #define PCI_CLASS_BRIDGE_RACEWAY	0x0608
> > #define PCI_CLASS_BRIDGE_OTHER		0x0680
> > 
> > My fixup replaces 0x048000 with 0x060400.
> > 
> > 	0x060400 != 0x060000
> > 
> > Which is correct:
> > PCI_CLASS_BRIDGE_HOST or PCI_CLASS_BRIDGE_PCI?
> > 
> > Naively, I would expect Host/PCI bridge to be more correct
> > for a root complex.
> 
> But that's very likely wrong, since the code in Linux does:
> 
> 	switch (dev->hdr_type) {		    /* header type */
> 	case PCI_HEADER_TYPE_BRIDGE:		    /* bridge header */
> 		if (class != PCI_CLASS_BRIDGE_PCI)
> 			goto bad;
> 		/* The PCI-to-PCI bridge spec requires that subtractive
> 		   decoding (i.e. transparent) bridge must have programming
> 		   interface code of 0x01. */
> 
> So a class of PCI_CLASS_BRIDGE_HOST would error out, I think.
> Does this mean I need to fixup Gen2 as well?
> (Since it reports 0x060000)

Header Type 1 (PCI_HEADER_TYPE_BRIDGE) identifies PCI-to-PCI bridges
(see PCI spec r3.0, sec 6.1).  These devices use the type 1
configuration header (PCI-to-PCI Bridge spec r1.2, sec 3.2) and they
exist on a primary PCI bus and forward transactions between that bus
and a secondary PCI bus.  The PCI core controls the memory, I/O, and
prefetchable memory windows between the primary and secondary buses
using the window registers in the type 1 header.

So I guess if this device has a type 1 header, it should use
PCI_CLASS_BRIDGE_PCI; otherwise use PCI_CLASS_BRIDGE_HOST.

I don't know the topology of your hardware, but normally a Root
Complex itself is not a PCI-to-PCI bridge.  It usually *contains*
several Root Ports, which appear to be on the root PCI bus and act as
bridges to a secondary bus.  The RC might appear as a type 0 device
but the programming model is not specified.  The Root Ports would be
type 1 devices like standard PCI-to-PCI bridges (PCIe spec r3.0, sec
1.3.1 and 1.4).

Bjorn