---
name: schunix-device-model
description: "The bus-agnostic device model behind /proc/devices, and the settled decision on how Linux drivers may be used."
metadata: 
  node_type: memory
  type: project
  originSessionId: 9d8a2aef-bcf0-47a0-96c6-f706efab4a95
  modified: 2026-08-08T18:05:13.551Z
---

Built 2026-08-08. `include/device.h` + `kernel/dev/device.c`, readable at
`/proc/devices`. Written up in `docs/architecture/devices.md`. Supersedes the two separate
arrays in `kernel/drivers/registry.c` as the answer to "what is attached".

**Bus is provenance, class is capability** — keep them separate. `bus` is
`platform`/`pci`/`usb`; `class` is `display`/`input`/`storage`/`net`/`bus`. A USB
disk and a SATA disk are both `storage`. `device_find_class()` is how a consumer
asks for "the display" without naming a driver, which is what stops the next
display driver requiring edits to every consumer.

Where a driver claims a device, **the driver's opinion beats the descriptor's**:
`struct usb_driver` gained a `devclass` field distinct from the USB interface
class, because the descriptor says what the device claims to be and the driver
knows what it made of it. A USB driver that forgets to set it lands in `other`.

Registration is automatic in `pci_bind()` and `usb_bind_all()` so a driver
cannot forget; platform devices are registered by hand in
`platform_register_devices()` in registry.c, in one readable block, matching
that file's existing "a list you can read beats a list assembled by side
effects" reasoning.

Flat list with a bus tag, **not a device tree** — nothing walks a hierarchy yet.
No lock: everything is added during single-threaded boot. **USB hotplug would
break both assumptions** and needs a spinlock plus a reader lock in procfs.

## The display class

`include/display.h` + ops in `kernel/dev/vga.c`, published on the device by
`vga_pci_driver.ops`. `struct pci_driver` gained an `ops` field for this —
**it has to live on the driver, not be set inside probe()**, because
`pci_bind()` registers the device *after* probe returns.

Ops: `mode`, `acquire`, `present(x,y,w,h)`, `wait_vblank`. The wrappers in
device.c null-check every op so consumers are not each written around a
maybe-absent vtable.

- **Back buffer comes from `dma_alloc()`, which is the wrong pool** and is used
  because it is the only *physically contiguous* allocator in the kernel —
  kalloc hands out single pages. 1.9 MB of a 4 MB arena meant for device rings.
  Allocated lazily on first `acquire()`, never at boot. The real fix travels
  with the E820 work ([[schunix-smp-and-memory-bugs]]).
- **vblank is the 1987 VGA port 0x3DA bit 3**, not VBE — VBE's protected-mode
  interface for this is optional and usually absent. Two loops (wait for the
  current retrace to *end*, then for the next to begin), or you return
  immediately having just missed one. Both loops are bounded so hardware that
  never reports retrace cannot hang a caller holding the console lock.

`echo d > /proc/sysrq` runs `display_selftest()`; `tools/dispshot.ps1`
photographs it to `build/dispshot.ppm` — it was derived from crashshot.ps1 and
originally kept its output path, so the two silently overwrote each other and
`crashqr.py` then failed on a capture that was not a crash screen. The pattern is designed to fail visibly: quadrants slant on a
stride error, change colour on a bpp error, and the 1px inset frame clips on an
off-by-one in the present rectangle. Verified 2026-08-08.

The console still writes *directly* to the framebuffer; the two paths coexist
deliberately. Routing text through the back buffer would mean presenting per
character.

## Linux drivers — settled 2026-08-08

Nexoniarz's decision, and the reasoning is correct: **not shipped, ever**; the
user installs one themselves through a device manager and Schunix loads it.

- *Distribution* is what triggers the GPL. A user obtaining and loading a driver
  on their own machine is their act, not ours.
- Implementing a *loader* from published documentation is clean-room and fine —
  it stays clean-room only if written from docs, not transcribed from Linux
  source. See [[schunix-licensing]].

Also worth keeping separate when this resumes: "loadable modules" (a stable ABI
plus a linker/loader, on top of this model) and "drivers out of kernel"
(user-space processes, DriverKit-style — needs IPC and mmap, both of which
already exist). They get bundled together in conversation and are different
projects.

## Adding /proc files

Three places, all needed: a `PROC_*` id and a case in `proc_gen_global()` in
`kernel/dev/procfs.c`, and a row in the `globals[]` table in
`kernel/fs/procfs.c`. Miss the third and the file exists but cannot be opened by
name. `sb_pad()` exists there now for column layout — cprintf has no width
specifier.