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PCI Bus Subsystem

Overview

Scarlet implements PCI/PCIe bus support using ECAM (Enhanced Configuration Access Mechanism), the standard configuration-space access method for PCI Express on RISC-V and ARM platforms.

Module Structure

kernel/src/device/pci/
├── mod.rs       – PciAddress, PciBus (core types)
├── config.rs    – PciConfig (ECAM-based config-space access)
├── device.rs    – PciDeviceInfo (device properties, DeviceInfo impl)
├── driver.rs    – PciDeviceDriver, PciDeviceId (driver matching)
└── scan.rs      – Bus enumeration and device scanning

Architecture

FDT / ACPI
    │ ecam_base, ecam_size
    ▼
 PciBus ──────────────────────────────────────────
    │                                               │
    │ scan()                                        │
    ▼                                               │
 PciConfig ── ECAM MMIO read/write ──► Config Space │
    │                                               │
    ▼                                               │
 PciDeviceInfo ──► DeviceManager ──► PciDeviceDriver │
 (vendor/device ID, class, BARs)     (probe/remove)  │
 ────────────────────────────────────────────────────┘

PciAddress

Represents a PCI device location: { segment, bus, device, function }.

ECAM offset: (bus << 20) | (device << 15) | (function << 12) — each function receives 4 KB of config space.

PciBus

Central manager initialized with the ECAM base physical address and size from the device tree:

let pci_bus = PciBus::new(ecam_base, ecam_size);
pci_bus.scan_and_register(); // enumerate → register with DeviceManager

ECAM is lazily mapped via vm::ioremap on first access.

PciConfig

Provides 8/16/32-bit read/write to configuration-space registers through the mapped ECAM region.

PciDeviceInfo

Holds device properties discovered during enumeration:

  • Vendor ID, Device ID
  • Class code (base class, subclass, interface)
  • Subsystem IDs
  • BAR addresses
  • Interrupt pin/line

Implements the DeviceInfo trait so that DeviceManager can match devices to drivers generically.

PciDeviceDriver

Wraps an ID table (PciDeviceId) and probe/remove callbacks. Supports:

  • Exact match: vendor + device ID
  • Class match: class code with mask (e.g., match all network controllers)
let driver = PciDeviceDriver::new(
    "my-driver",
    vec![PciDeviceId::new(0x8086, 0x1234)],
    |device| { /* probe */ Ok(()) },
    |device| { /* remove */ Ok(()) },
);
DeviceManager::get_mut_manager()
    .register_driver(Box::new(driver), DriverPriority::Standard);

Boot Flow

  1. Platform init reads ECAM base/size from FDT.
  2. PciBus::scan_and_register() walks bus/dev/func.
  3. Each valid device becomes a PciDeviceInfo, registered with DeviceManager.
  4. Registered PciDeviceDrivers are matched and probed automatically.

Supported Devices

  • VirtIO over PCI (network, block, GPU, input, RNG, etc.)
  • xHCI USB host controllers
  • Any PCI device with a registered driver