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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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.
• Drivers are fixed at link time. There is no module loader, and nothing can be installed after the build — the class vtable is the attachment point a loader would use, and it is the only part that exists.
• 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.