d0f7acf9eb
An x86-64 operating system built from xv6, now largely rewritten: SMP, a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF rasteriser, and a Linux-numbered syscall ABI. No history before this point -- the tree was never under version control. This commit exists so the rename that follows has something to undo to.
130 lines
5.9 KiB
Markdown
130 lines
5.9 KiB
Markdown
# Drivers
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How hardware gets found, claimed and used, and where drivers that are not part
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of the build will attach.
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---
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## 1. The device model
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There is one list of everything attached, whatever bus found it:
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`include/device.h` and `kernel/dev/device.c`, readable at `/proc/devices`.
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```
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name bus class driver device
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kbd0 platform input ps2kbd PS/2 keyboard
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mouse0 platform input ps2mouse PS/2 mouse
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uart0 platform other uart8250 16550 serial port
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ide0 platform storage ide ATA disk controller
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vga0 pci display vga display controller
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```
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Before this there were two registries in `kernel/drivers/registry.c` — an array
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of PCI drivers and an array of USB drivers — with nothing that knew about both.
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That is sufficient for *binding* and useless for the three things wanted next.
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**Enumeration.** "What is in this machine, and what claimed it" had no answer
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covering a USB disk and a PCI display at once. A device manager is exactly that
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question.
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**Classes.** A compositor should ask for *the display* and not care whether that
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is the VBE framebuffer or virtio-gpu. Without a class every consumer hardcodes a
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driver, and adding the second driver means editing the consumers — which is how
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a framebuffer address ended up hardcoded in three places here once already.
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**A boundary for drivers that are not compiled in.** They have to attach
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somewhere that is not an array fixed at link time.
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Two vocabularies are kept deliberately separate: a device's **bus** is
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provenance (`platform`, `pci`, `usb`), its **class** is capability
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(`display`, `input`, `storage`, `net`, `bus`). A USB disk and a SATA disk are
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both `storage` despite sharing no numbering at all. Where a driver claims a
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device, the driver's opinion wins over the descriptor's: `struct usb_driver` has
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a `devclass` field separate from the USB interface class, because the descriptor
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says what the device *claims* to be and the driver knows what it *made* of it.
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It is a flat list with a bus tag, **not** a device tree. Nothing yet needs to
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walk a hierarchy, and a tree nobody traverses is just a harder list. Hot-unplug
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would need more; the `in_use` flag is the hook.
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Platform devices are registered in one place
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(`platform_register_devices()` in `registry.c`) rather than inside each driver's
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init, for the same reason the driver arrays are written out by hand: a list you
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can read beats a list assembled by side effects.
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---
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## 2. Where this is going: a hybrid model
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The intended shape is the NeXTSTEP/macOS one rather than Linux's — a device is
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an object with a class and an ops vtable, and consumers talk to the class.
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Two things often get bundled together as "loadable drivers" and they are
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separable:
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|---|---|
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| **Loadable modules** | A stable driver ABI, drivers as files loaded at runtime (`.sys`, `.kext`). A linker/loader job on top of the model above. |
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| **Drivers out of kernel** | Drivers as user-space processes (macOS DriverKit, which is why kexts are being retired). An IPC and memory-mapping job; the `AF_UNIX` and shared-memory work it needs already exists. |
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The second is more in the spirit of the SchAPI direction and is slower and
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larger. Neither is built yet. What exists is the attachment point they both
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need.
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---
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## 3. Linux drivers
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**Decided 2026-08-08.** Linux drivers are not shipped with Schunix and never
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will be. The user may install one on their own machine, through a device
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manager or equivalent, and Schunix will load it.
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The licensing reasoning matters and is worth stating exactly, because it is
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easy to get wrong in both directions:
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- **Distribution is what triggers the GPL.** Shipping a GPL-2.0-only driver in
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an Apache-2.0 image is what cannot be done — see [licensing.md](licensing.md)
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§2. A user obtaining a driver themselves and loading it on their own machine
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is their act, not ours, and is not a distribution by us.
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- **Implementing a loader is not copying.** Writing support for a module format
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and a driver ABI from published documentation produces our own work. It is a
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clean-room implementation, and it stays one only if it is written from
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documentation rather than transcribed from Linux source. Reading Linux to
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understand *hardware* is fine; translating its code is not, whatever the
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identifiers say.
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So the rule for this subsystem is the same as everywhere else in the tree: the
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loader is ours and Apache-2.0, what it loads is the user's business, and nothing
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GPL ever enters an image we build. `RAMDISK_EXCLUDE` and `NOTPORTS` are the
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existing mechanisms for keeping that true mechanically rather than by intention.
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---
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## 4. Adding a driver today
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1. Write `kernel/drivers/<class>/<name>/<name>.c` exporting a
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`struct pci_driver` or `struct usb_driver`. One directory per driver, so a
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driver that grows a register header, a second source file or a firmware blob
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has somewhere to put it:
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kernel/drivers/
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registry.c
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gpu/vga/ gpu/virtio_gpu/
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net/e1000/
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bus/xhci/
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usb/core/ usb/hid/ usb/storage/ usb/hub/
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2. Declare it in `kernel/drivers/registry.c` and add it to the array.
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3. Add the object to `KERN_DRIVERS` in the `Makefile`.
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Registration in the device model is automatic — `pci_bind()` and
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`usb_bind_all()` publish whatever they claim, so a driver cannot forget to. Set
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`devclass` on a USB driver, or the device lands in `other`.
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The best source for a new driver is the **specification**, not another kernel:
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xHCI, AHCI, virtio, NVMe and the Intel datasheets are public documents with no
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licence on what you write from them. `kernel/drivers/bus/xhci/xhci.c` came from the spec, and so did
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`net/e1000/` and `gpu/virtio_gpu/`. Where a *table* of constants is genuinely
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faster to transcribe than to retype from a PDF -- device IDs, register offsets --
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taking it from a BSD tree is fine after the licence check, and goes in `NOTICE`.
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That is data; the logic stays ours.
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