config/drivers.conf: which drivers are compiled in

y or n per optional driver (virtio-gpu, stdvga, e1000, rtl8139, native r8169, AC'97, AHCI, NVMe), all y except the native
r8169, overridable with make DRV_X=n. Only drivers nothing else calls can be optional; the registry #ifdefs them and the
Makefile leaves their objects out. Built with every one off (links, none of their symbols present), with r8169 on, and
by default.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EVRhxi9zpdW2uDUN4FVcao

docs/architecture/devices.md

@@ -9,6 +9,17 @@ Drivers that live in a process rather than in the kernel are described below
 
 ---
 
+## Which drivers are compiled in
+
+`config/drivers.conf` lists the drivers that can be left out of a kernel, `y` (compiled in and offered every device it can
+drive) or `n` (not linked at all), all `y` unless a line says otherwise — the one exception is the native `r8169`, `n`
+because Linux's runs in its place under LDLS. Override one for a build with `make DRV_E1000=n DRV_NVME=n`. A changed
+choice rebuilds only `registry.o` and the link. The list is short on purpose: a driver is optional only if nothing else in
+the kernel calls it (display: `vga_pci`, `virtio_gpu`; network: `e1000`, `rtl8139`, `r8169`; storage: `ahci`, `nvme`;
+audio: `ac97`). `vga_fb`, the USB stack and the legacy PS/2, IDE and UART devices are always there, because the console, the
+keyboard and the root disk can depend on them. A kernel with every optional driver off builds and links; whether such a
+kernel is useful on any particular machine is the person who turned them off to say.
+
 ## What stays in the kernel, and what may leave
 
 "Hybrid" is used to mean everything from "a monolith with a nice device model" to

docs/guides/adding-a-driver.md

@@ -61,6 +61,10 @@ ours.
 
 2. Declare it in `kernel/drivers/registry.c` and add it to the array.
 3. Add the object to `KERN_DRIVERS` in the `Makefile`.
+3½. If nothing else in the kernel calls it, make it optional: a `DRV_<NAME>` line in
+   `config/drivers.conf`, the name in `DRV_LIST` in the Makefile, the object behind
+   `$(filter y,$(DRV_<NAME>))`, and an `#ifdef CONFIG_DRV_<NAME>` around its line in `registry.c`.
+   A driver other code calls (`vga_fb`, the USB stack) cannot be optional.
 4. Fill in the **class vtable** for what it is, and point `.ops` at it. That is
    the whole of what the rest of the kernel sees:
 
@@ -113,7 +117,7 @@ That is data; the logic stays ours.
 - [ ] One directory under `kernel/drivers/<class>/<name>/`
 - [ ] Exports a `struct pci_driver` or `struct usb_driver`
 - [ ] Declared in `kernel/drivers/registry.c` **and** added to its array
-- [ ] Object added to `KERN_DRIVERS` in the `Makefile`
+- [ ] Object added to `KERN_DRIVERS` in the `Makefile`, and an option in `config/drivers.conf` if it can be left out
 - [ ] Class vtable filled in and `.ops` pointed at it
 - [ ] `.attached` implemented if it can drive more than one card
 - [ ] `devclass` set, if it is a USB driver

docs/guides/building.md

@@ -17,6 +17,9 @@ hundred megabytes, kept in `dist/`) and compiles Xorg's dependencies not at all:
 packages are Alpine's binaries, pinned in `ports/alpine/world.lock`. See
 [packages](../architecture/packages.md).
 
+Which drivers go into the kernel is `config/drivers.conf` (all of the optional ones by default);
+`make DRV_E1000=n` leaves one out for a build. See [devices](../architecture/devices.md).
+
 ## Prerequisites
 
 | Need | Why | Windows | Linux | macOS | BSD |