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. Moving drivers out of the kernel entirely is ../plan/hybrid.md.

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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.

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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 ../plan/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.

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, "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, "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 says so at length.

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

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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 §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.

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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.