danos/docs/discovery.md

# 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](memory-map.md) and [arch split](arch.md) 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:

1. **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](arm.md) is what
   forces the issue.

2. **Isolated user-space drivers need it.** In the [microkernel vision](vision.md),
   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:

```zig
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](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](ipc.md). 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](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

1. **Now (cheap):** plumb the neutral `HardwareInfo` pointer through the loader into
   `BootInfo`. No parser yet.
2. **Next milestone unchanged:** user mode + address-space isolation — needs no
   discovery.
3. **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**.
4. **Then:** a user-space device-manager server + interrupts-as-messages → real
   isolated drivers (keyboard first).

## Related

- [acpi.md](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](arm.md) — the aarch64 target that forces genuine discovery (DTB, GIC).
- [memory-map.md](memory-map.md) — the same loader-captures / kernel-consumes seam,
  and the note about grabbing the RSDP before `ExitBootServices`.
- [device-interrupts.md](device-interrupts.md) — the LAPIC/timer bring-up that
  discovery will eventually feed (IOAPIC, real IRQ routing).
- [ipc.md](ipc.md) — the channels that interrupts-as-messages and the device manager
  will ride on.
- [vision.md](vision.md) — why drivers belong in isolated user space at all.