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Device discovery (ACPI / device tree), the agnostic way

"Device discovery" is how the kernel learns what hardware exists and where — the MMIO addresses, IRQ numbers, CPU count, and interrupt controller it can't just assume. On x86 that description comes from ACPI tables; on ARM from a device tree (DTB). This note is a design plan, not built yet: when danos should tackle it, and how to keep it architecture-agnostic — the same discipline the memory map and arch split already follow.

What the kernel assumes today

Right now danos discovers almost nothing — it coasts on legacy PC fixtures that are guaranteed to exist under QEMU + UEFI:

arch/x86_64/apic.zig assumes the Local APIC at the default 0xFEE0_0000 and calibrates its timer against the PIT (the legacy 8254).
arch/x86_64/serial.zig hardcodes COM1 at I/O port 0x3F8.
The framebuffer and memory map come from UEFI — that is discovery, just done by the firmware and handed over, not read from ACPI.

This works only because PC-compatible hardware promises those legacy pieces exist at those addresses. It is a crutch, and it does not travel.

The forcing functions: when to build it

Two things drive the need, and they set the timing:

The second architecture makes it mandatory. ARM has no legacy fixtures — no PIT, no fixed serial port, no standard interrupt controller address. You can't find the UART to print a character without reading the device tree. So on x86 we can defer discovery a long time (until we want the IOAPIC, PCIe, or SMP), but on aarch64 it's required to boot at all. The aarch64 port is what forces the issue.
Isolated user-space drivers need it. In the microkernel vision, drivers live in user space — but something has to enumerate the hardware and hand each driver its MMIO regions and IRQs. That enumeration is device discovery. So discovery is a prerequisite for real drivers, not for user mode itself.

The conclusion on timing: don't build full discovery before user mode. User mode

address-space isolation needs none of it; the current assumptions are fine there. Build the agnostic discovery layer when the aarch64 port starts — because that's when a second, real implementation makes "agnostic" honest.

Why build it with the second arch, not before

The same lesson as the arch split: an abstraction with only one implementation quietly bends to that implementation. Build "agnostic discovery" x86-only first and you'll get an ACPI-shaped interface with device tree bolted on afterward. Build it when aarch64 lands and the small, clean DTB parser pulls the abstraction toward the right neutral shape, which the x86 side then fills. Design it against two backends or it isn't really agnostic.

The one cheap step to take sooner

Have the loader capture the description pointer into BootInfo — a neutral handle, no parsing:

pub const HardwareInfo = extern struct {
    kind: enum(u32) { none, acpi, device_tree },
    addr: u64, // ACPI RSDP, or the DTB blob
};

On x86-UEFI that's the ACPI RSDP, read from the UEFI configuration table before ExitBootServices — the same "grab it before exit" pattern as the framebuffer and memory map (already flagged in memory-map.md). On ARM it's the DTB pointer the firmware passes. This keeps the door open for near-zero cost without committing to the parser.

What "agnostic" looks like

The happy accident: the device-tree data model is already a good neutral representation. A DTB is a tree of nodes, each with:

a compatible string — what the device is,
reg — its MMIO base(s) and size(s),
interrupts — its IRQ number(s),
other properties.

Even OSes running on ACPI hardware normalize into a unified device model shaped like this. So the neutral layer is a device model — "here are the devices, each with a type, MMIO regions, and IRQs" — fed by two backends behind it:

a DTB parser (ARM) — a few hundred lines against a well-specified binary format;
static ACPI + PCI enumeration (x86).

The caveat that sets the effort: "ACPI" ≠ "AML interpreter"

A full ACPI namespace is AML (ACPI Machine Language) bytecode, and writing an AML interpreter is an enormous undertaking. We skip it. Interrupt routing and PCIe come entirely from the static tables:

MADT — the APICs and the IOAPIC (interrupt routing),
MCFG — PCIe configuration space (then walk the PCI bus to enumerate devices),
FADT, HPET — power/reset and a precise timer.

Walking PCI config space finds most devices without any AML. So the x86 static-ACPI backend is roughly comparable in scope to the DTB parser; it's full AML that's the monster, and it isn't on the path.

Where it sits in the seam

Discovery layers onto the existing loader↔kernel split cleanly:

Layer	Responsibility	x86	ARM
Capture (loader)	grab the description pointer	RSDP from UEFI config table	DTB pointer from firmware
Parse (kernel, per-mechanism)	pointer → neutral device model	static ACPI + PCI	DTB parser
Consume (generic)	use the device model	— same code —	— same code —

Boot-protocol-specific capture, mechanism-specific parse, generic consumption — exactly like the memory map, where the loader classifies and the kernel just sees neutral regions.

Where it lives in a microkernel

For isolated drivers, discovery is not one lump — it splits by who needs it and when:

Minimal, in-kernel: the interrupt controller and the timer. The IOAPIC/GIC and the timer are needed before user space exists (scheduling and preemption depend on them), so the kernel must parse at least these from ACPI/DTB itself. This is also the first real consumer of discovery — the first genuine reason to read MADT or the DTB is "where is the interrupt controller and how do I route an IRQ?" It's the point where x86 finally graduates from the PIT/legacy-LAPIC assumptions.
User-space enumeration: a device-manager server. Everything else — PCI devices, peripherals — is parsed (or queried from the kernel's parse) by a privileged user-space server that hands each driver process its MMIO regions and IRQ rights over IPC. Combined with interrupts-as-messages (an IRQ delivered to a driver as a message on a channel — a natural extension of the wait queues and channels already built), that's what makes drivers genuinely isolated.

So the device-manager server depends on user mode + IPC; only the interrupt-controller slice is unavoidably in-kernel.

A nice ARM contrast

On ARMv8 the generic timer exposes its frequency directly via the CNTFRQ register — no calibration needed. That's cleaner than the x86 side, where we measure the LAPIC and TSC against the PIT because nothing tells us their frequency (see device-interrupts.md). Discovery on ARM hands you more for free; discovery on x86 is partly about finding what ARM just tells you.

Suggested ordering

Now (cheap): plumb the neutral HardwareInfo pointer through the loader into BootInfo. No parser yet.
Next milestone unchanged: user mode + address-space isolation — needs no discovery.
With the aarch64 port: build the agnostic discovery layer, DTB first (the forcing function), then x86 static-ACPI to fill the same model — starting with the interrupt controller + timer.
Then: a user-space device-manager server + interrupts-as-messages → real isolated drivers (keyboard first).

acpi.md — the built x86 side of the "capture" step: how the loader grabs the RSDP and the platform follows it to the RSDT/XSDT and the SDTs.
arm.md — the aarch64 target that forces genuine discovery (DTB, GIC).
memory-map.md — the same loader-captures / kernel-consumes seam, and the note about grabbing the RSDP before ExitBootServices.
device-interrupts.md — the LAPIC/timer bring-up that discovery will eventually feed (IOAPIC, real IRQ routing).
ipc.md — the channels that interrupts-as-messages and the device manager will ride on.
vision.md — why drivers belong in isolated user space at all.

Update (M19.3, 2026-07-13): PCI enumeration left the kernel

The kernel now seeds only the pci_host_bridge node (ECAM window, MMIO apertures derived from the memory map's holes, bus range, and the 16-bit I/O window). The per-function walk moved to the ring-3 pci-bus driver (device-manager.md): it claims the bridge, repeats the ECAM scan through its mmio grant, and device_registers what it finds, which the device manager mirrors and matches. The ACPI namespace walk follows in M20; the static tables (MADT, HPET, MCFG, FADT + \\_S5) stay kernel-side.

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