SchOS/docs/architecture/devices.md

# The device model

How hardware gets found, claimed and used, and what the rest of the kernel sees
of it.

Writing a driver is [../guides/adding-a-driver.md](../guides/adding-a-driver.md).
Moving drivers out of the kernel entirely is [../plan/hybrid.md](../plan/hybrid.md).

---

## One list of everything attached

There is one list of everything attached, whatever bus found it:
`include/device.h` and `kernel/dev/device.c`, readable at `/proc/devices`.

```
name      bus       class     driver    device
kbd0      platform  input     ps2kbd    PS/2 keyboard
mouse0    platform  input     ps2mouse  PS/2 mouse
uart0     platform  other     uart8250  16550 serial port
ide0      platform  storage   ide       ATA disk controller
vga0      pci       display   vga       display controller
```

Before this there were two registries in `kernel/drivers/registry.c` — an array
of PCI drivers and an array of USB drivers — with nothing that knew about both.
That is sufficient for *binding* and useless for the three things wanted next.

**Enumeration.** "What is in this machine, and what claimed it" had no answer
covering a USB disk and a PCI display at once. A device manager is exactly that
question.

**Classes.** A compositor should ask for *the display* and not care whether that
is the VBE framebuffer or virtio-gpu. Without a class every consumer hardcodes a
driver, and adding the second driver means editing the consumers — which is how
a framebuffer address ended up hardcoded in three places here once already.

**A boundary for drivers that are not compiled in.** They have to attach
somewhere that is not an array fixed at link time.

Two vocabularies are kept deliberately separate: a device's **bus** is
provenance (`platform`, `pci`, `usb`), its **class** is capability
(`display`, `input`, `storage`, `net`, `bus`). A USB disk and a SATA disk are
both `storage` despite sharing no numbering at all. Where a driver claims a
device, the driver's opinion wins over the descriptor's: `struct usb_driver` has
a `devclass` field separate from the USB interface class, because the descriptor
says what the device *claims* to be and the driver knows what it *made* of it.

It is a flat list with a bus tag, **not** a device tree. Nothing yet needs to
walk a hierarchy, and a tree nobody traverses is just a harder list. Hot-unplug
would need more; the `in_use` flag is the hook.

Platform devices are registered in one place
(`platform_register_devices()` in `registry.c`) rather than inside each driver's
init, for the same reason the driver arrays are written out by hand: a list you
can read beats a list assembled by side effects.

---

## Drivers that are not in the kernel

`/dev/pci` (major 9) hands a PCI device to a process. It is deliberately about
*PCI devices* and not about any driver model, and its first users are SchOS's
own drivers, evicted one at a time — which is the ordering that keeps it a
general mechanism rather than a hole shaped like something foreign.

What a driver needs from a kernel, and all it needs:

| | |
|---|---|
| find its device, and read what the bus says | `PCIIOC_INFO`, `PCIIOC_CFG` |
| be given it, exclusively | `PCIIOC_CLAIM` |
| reach its registers | `mmap`, where the offset is a BAR's physical address |
| memory it can hand to the device | `PCIIOC_DMA` |
| be told when the device wants attention | `PCIIOC_IRQ`, then `read()` |

Three of those are worth expanding on, because each encodes a decision:

**`mmap` is checked against the caller's claims.** An offset that is not inside
a BAR of a device this process holds is refused. That is the whole difference
between this and `/dev/mem` with extra steps.

The check is against the *pages* a BAR lies in rather than the BAR itself,
because a page is the only granularity an MMU has. A BAR smaller than a page
therefore hands the process the whole page it sits in — forced by the hardware
rather than chosen. Comparing against the BAR's own size instead meant a
sub-page BAR could not be mapped at all, which nothing noticed until an
rtl8139 with 256 bytes of registers turned up; see
[ldls.md](ldls.md), "The bug a foreign driver found in its
first minute".

**DMA is a call, not a mapping.** The process is told the *physical* address,
which is precisely what it cannot work out for itself, and the memory comes from
an arena reserved at boot before the general allocator gets anything.

**Interrupts arrive by `read()`**, the way Linux's uio does it and for the same
reason: a descriptor composes with `poll`, and a driver that stops reading falls
behind rather than losing events. The kernel masks the line when the interrupt
arrives and leaves it masked until the driver `write()`s back. **That write is
not optional.** A PCI interrupt is level-triggered — it stays asserted until the
cause is cleared *at the device*, which is a different register on every chip and
therefore something only the driver can do. Signal EOI on a line that is still
asserted and it re-fires immediately, for ever. That is not hypothetical: it
happened here at 72,000 interrupts a second, and what it looked like from
userspace was a clock running seven times slow. See §13 of
[../history/kernel.md](../history/kernel.md).

### Handing a device over

`struct pci_driver` has a `release` hook. A driver that does not define one keeps
its device — always safe, and for anything on the path to a shell also correct.
A driver that does define one has to give back everything, not just the device:
the first eviction found this the hard way, because releasing the GPU while
keeping its 4 MB framebuffer meant the incoming driver was told there was no DMA
memory left.

`cmd/vgpud` is the virtio-gpu driver as an ordinary process. It does the whole
virtio 1.0 bring-up from outside — reset, feature negotiation, a virtqueue in DMA
memory, `SET_SCANOUT`, transfer and flush — and while it owns the GPU the kernel
console falls back to the VBE framebuffer.

**No syscall on that path knows it is talking about a GPU.** The kernel hands
over a PCI device; the process decides what it is.

It also **changes the display mode**, on a control socket, with `schmode` as the
client — and that is worth a sentence here because it is the thing the device
model makes possible rather than the thing it does. A modeset is five commands
to the device. In the kernel those five commands are also a reason to be
nervous, because a wrong one takes the machine; in a process the worst case is
that `schinit` starts another one. See
[../plan/hybrid.md](../plan/hybrid.md), "A modeset, from a process".

### Taking it back

A driver that dies is the case this whole arrangement exists for, so release is a
symmetric operation rather than a courtesy. Three things happen that are not
obvious from "the process exited":

**Bus mastering is cleared before the DMA is freed.** A device whose driver has
just died is still writing into memory the kernel is about to hand to somebody
else. The kernel cannot quiesce a device it does not understand — that is a
different register on every chip — but it can clear `PCI_CMD_BUS_MASTER`, which
stops the writes whatever the device is, and it is the only halt available to a
kernel that knows nothing but the header. So `PCIIOC_CLAIM` sets the bit and
release clears it, in that order relative to freeing the memory. `pcitest` prints
the bit in a `bm` column, with a separate mark for "could not read it", because a
failed read rendered as "off" is worse than no column at all.

**DMA memory a driver mapped is not `freevm()`'s to release.** `freevm()` frees
every mapped page below `PHYSTOP` and the DMA arena is the top 16 MB of exactly
that range, so a DMA block left for it went onto the page allocator's free list
while `pciuser_release_all()` handed the same pages back to the arena — two
allocators, one set of pages. `PCIIOC_DMA` records a VMA with `no_free` set, the
same way shared memory does, and `proc_release_shared()` unmaps the range before
the page tables are torn down. The same rule covers every future case: **anything
mapped into a process that the process does not own the pages of needs a VMA, or
exit will free it twice.**

**A device that is the only display will not be released.** `vgpu_release()` asks
`vga_fb_restore()` whether there is a framebuffer to put the console back on, and
declines if the answer is no. The rule that the kernel keeps a minimal driver for
everything on the path to a shell is only worth writing down if something
enforces it.

Coming back is a job, not a kernel feature. `etc/schinit/vgpud.job` carries
`KeepAlive = yes`, so killing the driver is the entire test: `schinitd` starts
another one, it claims the device, and the pid of whichever process drew the
frame is legible in the frame itself. See
[../plan/hybrid.md](../plan/hybrid.md), "The driver came back".

### What this is not

**It is not a security boundary.** There is no IOMMU, so a process holding a DMA
buffer can point the device at any physical address. This isolates a driver
against its own *bugs* and not against its own *intent* — a crashed driver that
restarts instead of taking the machine down is the entire benefit, and it is
worth having on those terms. [../plan/hybrid.md](../plan/hybrid.md) says so at
length.

**One device per process.** A driver drives one thing; if you need two, run two.

---

## Linux drivers

**Decided 2026-08-08.** Linux drivers are not shipped with SchOS and never
will be. The user may install one on their own machine, through a device
manager or equivalent, and SchOS will load it.

The licensing reasoning matters and is worth stating exactly, because it is
easy to get wrong in both directions:

- **Distribution is what triggers the GPL.** Shipping a GPL-2.0-only driver in
  an Apache-2.0 image is what cannot be done — see [../licensing.md](../licensing.md)
  §2. A user obtaining a driver themselves and loading it on their own machine
  is their act, not ours, and is not a distribution by us.
- **Implementing a loader is not copying.** Writing support for a module format
  and a driver ABI from published documentation produces our own work. It is a
  clean-room implementation, and it stays one only if it is written from
  documentation rather than transcribed from Linux source. Reading Linux to
  understand *hardware* is fine; translating its code is not, whatever the
  identifiers say.

So the rule for this subsystem is the same as everywhere else in the tree: the
loader is ours and Apache-2.0, what it loads is the user's business, and nothing
GPL ever enters an image we build. `RAMDISK_EXCLUDE` and `NOTPORTS` are the
existing mechanisms for keeping that true mechanically rather than by intention.

---


---

## What it does not do

- **It is a flat list, not a device tree.** There is no parent/child nesting,
  only a bus tag. Nothing yet needs to walk a hierarchy, and a tree nobody
  traverses is just a harder list.
- **No hot-plug.** Devices are registered at boot and never removed; the
  `in_use` flag is the hook a hot-unplug path would need, and nothing sets it
  for that purpose.
- **`NDEVICE` is 32**, compile-time, like every other table in the kernel.
- **In-kernel drivers are fixed at link time.** There is no module loader, and
  nothing can be added to the kernel after the build. A driver *outside* the
  kernel is an ordinary program and can be started whenever — but it reaches its
  device through `/dev/pci` and publishes nothing to the class vtables, so the
  console cannot be handed to one yet.
- **The kernel does not rebind its own driver.** `PCIIOC_RELEASE`, or the process
  exiting, leaves the interrupt line masked and the in-kernel driver unbound —
  the console stays wherever the release put it. A *supervisor* can start a
  replacement driver, and `schinitd` does; the kernel itself watches for
  nothing.
- **Only two class vtables are defined**: `display_ops` and `netdev_ops`.
  Storage and input devices appear in the list with `ops = 0` and are reached
  through their own subsystems instead.