# /etc/devices.csv — the device registry

**Status: built (2026-07-26).** The device manager reads `/etc/devices.csv` at
boot and binds every device a bus driver reports to the driver the registry
names. It replaces the three hand-written `switch` tables that used to live in
the manager (`pciDriverForIdentity`, `hidDriverFor`, `usbDriverForIdentity`) —
the "manifest" [device-manager.md](device-manager.md) anticipated once code
matching started to hurt. The parser and matcher are the pure, unit-tested
`device-registry` module (`library/device/registry/device-registry.zig`).

## Why a registry

The switch tables keyed PCI matches on the 24-bit class/subclass/prog-IF triple
alone. That is too coarse: a virtio-gpu is just "display / other" by class, so it
could only be *class-matched* and the driver had to re-confirm its real
`1AF4:1050` identity from config space **after** the manager had already spawned
it. The registry lets a rule bind on the full identity — down to vendor, device,
and subsystem — so the manager makes the precise decision itself, and the driver
comes up already knowing it is the right one.

It is also **data, not code**: teaching the system new hardware is a line in a
file, not an edit-and-recompile of the manager. And it is **greppable** — one
place to read "what binds what," the same idea as Linux's `modules.alias`.

## The file

One rule per line, nine comma-separated fields; `#` starts a comment (whole-line
or trailing); blank lines are ignored. Whitespace around a field is trimmed, so
columns may be padded for readability.

```
# bus   base class prog_if vendor device subsystem hid       driver
pci,    0C,  03,   30,     *,     *,     *,        *,        /system/drivers/usb-xhci-bus
pci,    03,  00,   00,     *,     *,     *,        *,        /system/drivers/display
pci,    03,  80,   *,      1AF4,  1050,  *,        *,        /system/drivers/virtio-gpu
usb,    03,  01,   01,     *,     *,     *,        *,        /system/drivers/usb-hid-keyboard
acpi,   *,   *,    *,      *,     *,     *,        PNP0303,  /system/drivers/ps2-bus
```

| Field | Meaning | Notes |
|---|---|---|
| `bus` | `pci` \| `usb` \| `acpi` | which bus reported the device; picks the namespace for the id columns |
| `base` | PCI base class / USB class | hex |
| `class` | PCI subclass / USB subclass | hex |
| `prog_if` | PCI prog-IF / USB protocol | hex |
| `vendor` | PCI vendor / USB idVendor | hex |
| `device` | PCI device / USB idProduct | hex |
| `subsystem` | PCI subsystem, `(ssvid<<16)\|ssid` | hex; blank for usb/acpi |
| `hid` | ACPI `_HID` (e.g. `PNP0303`) | blank for pci/usb |
| `driver` | full ramdisk path to spawn | e.g. `/system/drivers/virtio-gpu` |

`*` or an empty field is a **wildcard** — it matches anything and adds nothing to
a rule's specificity.

## Levels of detection: most-specific-wins

Several rows may match one device. The manager picks the **most specific** — the
one that pins the finest-grained fields. Specificity weights double from the
coarsest level so each outweighs all coarser levels combined:

```
base(1) < class(2) < prog_if(4) < vendor(8) < subsystem(16) < device(32) ≈ hid(32)
```

So the generic `pci, 03, 00, 00, …/display` rule and the precise
`pci, 03, 80, *, 1AF4, 1050, …/virtio-gpu` rule coexist: the virtio card
(vendor 1AF4, device 1050) takes the specific rule; a plain VGA adapter still
falls to the generic one. Two rules that match a device with the *same*
specificity are a registry authoring error — the manager logs it loudly and binds
the first, so the shadowed rule is visible rather than silently dropped.

## Authoritative — no code fallback

There is no compiled-in default table behind the registry. A device that no row
matches goes **unbound** and is logged; the manager never guesses. A missing or
empty `/etc/devices.csv` therefore means nothing matches — which is loud at boot,
not a silent half-working system.

## How the manager reads it

`/etc/devices.csv` is bundled into the initial ramdisk (`build.zig`'s `bundled`
list). The kernel serves the initrd's `/etc` tree directly — the `fat` service is
spawned *after* the device manager and is irrelevant to `/etc` — so the manager
reads the file with a plain `fs.open("/etc/devices.csv")` + `read`, with no
filesystem service running and no boot-ordering dependency. It parses the bytes
once in `initialise`, before any bus driver can report a device to match.

## Feeding the matcher: the widened report

Finer-grained matching needs identity the old ABI threw away. Two things carry it
now: `child_added` (and `DeviceDescriptor`) grew `vendor` / `device` /
`subsystem` fields, filled by the PCI bus driver from config space (offsets
0x00 and 0x2C); and each bus driver states its `bus` in the report (a `BusKind`),
so the manager reads a PCI class triple and a USB class triple — the same 24 bits
in different namespaces — against the right `bus` column.

## Adding a driver

(The step-by-step walkthrough with a worked example is
[new-driver-checklist.md](new-driver-checklist.md).)

1. Create `system/drivers/<name>/` with the driver source plus a ~15-line
   package `build.zig` + `build.zig.zon` (copy an existing driver package,
   e.g. `system/drivers/pci-bus/`; per-driver extras go through
   `build_support.programModule`). Then bundle it at `/system/drivers/<name>`:
   one dependency + one bundled entry in the root `build.zig`, one line in the
   root `build.zig.zon`.
2. Add a row to `etc/devices.csv` naming the identity it binds and its full path.

No device-manager change is required — the registry is the seam.
