pci-bus: support I/O port BARs and per-function Subsystem IDs

[SolAstrius]

Two related extensions needed by Linux drivers that match on strict subsystem IDs and/or probe via inb/outb on the BAR. Both are purely additive — defaults preserve the existing behaviour for every device already in the tree.

Per-function Subsystem Vendor / Subsystem Device ID

The PCI configuration Subsystem IDs at config-space offset 0x2C (§6.2.4) were hardcoded to 0x510010DC (CERN/ECP/EDU) for every emulated function. Most Linux drivers ignore subsys IDs and match on vendor:device, so this was invisible — until parport_pc, whose PCI table for the NetMos 9900 entry strictly requires 0xA000:0x2000:

{ PCI_VENDOR_ID_NETMOS, PCI_DEVICE_ID_NETMOS_9900,
  0xA000, 0x2000, 0, 0, netmos_9900 },

Without the match, parport_pc skips the device, no parport0 / lp0 nodes appear, and userspace can't reach the emulated port even though lspci correctly shows the MosChip MCS9900 ID.

Add subsys_vendor_id / subsys_device_id to pci_func_desc_t and pci_func_t. Default zero — the read handler falls back to the original 0x510010DC placeholder so existing devices that don't set these (sound-hda, nvme, rtl8169, usb-xhci, ata, bochs-display, pci-vfio) round-trip the same value they always have.

I/O port BARs

The bus only emitted Memory-type BARs — every BAR allocation went through pci_assign_mmio_addr (0x40000000+ range), and the BAR config read never set PCI_BAR_IO_SPACE. This worked for every existing device because all of them use memory-mapped registers.

Fails for any Linux driver whose PCI probe path does:

int io = pci_resource_start(dev, n);
inb(io); outb(val, io);

On RISC-V (and other non-x86 platforms where Linux maps PCI I/O space to MMIO) inb(addr) resolves to a memory access at PCI_IOBASE + addr. With a Memory BAR, pci_resource_start returns e.g. 0x40009000 — inb() then reads at PCI_IOBASE + 0x40009000, way outside the legal I/O window, returns 0xFF, the driver sees no data and skips the device.

parport_pc is the canonical example. Linux's __parport_pc_probe_port takes raw I/O addresses and feeds them through inb/outb. There is no memory-mapped probe path. The same applies to many vintage / legacy PCI drivers (some serial chips, ISA-style PCI cards, GPIB controllers).

Add I/O BAR support:

• pci_func_desc_t.bar_io_mask: bitmap of BAR indices that should be declared I/O type. Default 0 = all Memory.
• pci_assign_io_addr: allocates from bus->io_addr range (PCI_IO_ADDR_DEFAULT = 0x03000000, length 0x10000) instead of the memory pool.
• pci_attach_bar takes is_io, threads to the right allocator.
• BAR config-space read: I/O BARs return the I/O port number (offset within bus->io_addr) ORed with PCI_BAR_IO_SPACE (bit 0).
• BAR config-space write: I/O BARs use 4-byte alignment per PCI spec §6.2.5.1, preserving type bits and relocating to bus->io_addr + (val & ~0x3).
• MSI-X BAR allocation always uses the memory pool — those are architecturally Memory BARs per spec §6.8.