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

Makefile

@@ -38,17 +38,34 @@ KERN_DEV  = $(addprefix $(BUILD)/kernel/dev/, \
 # already eight files with four called usb_something, and each driver added made
 # the directory harder to read. The compile rule uses `mkdir -p $(dir $@)`, so
 # the depth costs nothing.
+# Which of them are compiled in is config/drivers.conf: y or n per driver, y by
+# default, overridable on the command line (make DRV_E1000=n). Only the drivers
+# that nothing else calls can be left out; the rest are always linked.
+-include config/drivers.conf
+DRV_LIST = VGA_PCI VIRTIO_GPU E1000 RTL8139 R8169 AC97 AHCI NVME
+DRV_DEFAULT_R8169 = n
+$(foreach d,$(DRV_LIST),$(eval DRV_$(d) ?= $(if $(DRV_DEFAULT_$(d)),$(DRV_DEFAULT_$(d)),y)))
+$(foreach d,$(DRV_LIST),$(if $(filter y n,$(DRV_$(d))),,$(error config/drivers.conf: DRV_$(d) must be y or n, not '$(DRV_$(d))')))
+DRV_DEFS = $(foreach d,$(DRV_LIST),$(if $(filter y,$(DRV_$(d))),-DCONFIG_DRV_$(d)=1))
+# Rewritten only when the choice changes, so registry.o is rebuilt then and not otherwise.
+DRV_STAMP = $(BUILD)/kernel/drivers/.config
+$(DRV_STAMP): FORCE
+	@mkdir -p $(dir $@)
+	@echo '$(DRV_DEFS)' | cmp -s - $@ || echo '$(DRV_DEFS)' > $@
+$(BUILD)/kernel/drivers/registry.o: $(DRV_STAMP)
+$(BUILD)/kernel/drivers/registry.o: CFLAGS += $(DRV_DEFS)
+
 KERN_DRIVERS  = $(BUILD)/kernel/drivers/registry.o
-KERN_DRIVERS += $(BUILD)/kernel/drivers/gpu/vga/vga_pci.o
 KERN_DRIVERS += $(BUILD)/kernel/drivers/gpu/vga/vga_fb.o
-KERN_DRIVERS += $(BUILD)/kernel/drivers/gpu/virtio_gpu/virtio_gpu.o
+KERN_DRIVERS += $(if $(filter y,$(DRV_VGA_PCI)),$(BUILD)/kernel/drivers/gpu/vga/vga_pci.o)
-KERN_DRIVERS += $(BUILD)/kernel/drivers/net/e1000/e1000.o
+KERN_DRIVERS += $(if $(filter y,$(DRV_VIRTIO_GPU)),$(BUILD)/kernel/drivers/gpu/virtio_gpu/virtio_gpu.o)
-KERN_DRIVERS += $(BUILD)/kernel/drivers/net/r8169/r8169.o
+KERN_DRIVERS += $(if $(filter y,$(DRV_E1000)),$(BUILD)/kernel/drivers/net/e1000/e1000.o)
-KERN_DRIVERS += $(BUILD)/kernel/drivers/net/rtl8139/rtl8139.o
+KERN_DRIVERS += $(if $(filter y,$(DRV_R8169)),$(BUILD)/kernel/drivers/net/r8169/r8169.o)
-KERN_DRIVERS += $(BUILD)/kernel/drivers/audio/ac97.o
+KERN_DRIVERS += $(if $(filter y,$(DRV_RTL8139)),$(BUILD)/kernel/drivers/net/rtl8139/rtl8139.o)
+KERN_DRIVERS += $(if $(filter y,$(DRV_AC97)),$(BUILD)/kernel/drivers/audio/ac97.o)
 KERN_DRIVERS += $(BUILD)/kernel/drivers/bus/xhci/xhci.o
-KERN_DRIVERS += $(BUILD)/kernel/drivers/storage/ahci/ahci.o
+KERN_DRIVERS += $(if $(filter y,$(DRV_AHCI)),$(BUILD)/kernel/drivers/storage/ahci/ahci.o)
-KERN_DRIVERS += $(BUILD)/kernel/drivers/storage/nvme/nvme.o
+KERN_DRIVERS += $(if $(filter y,$(DRV_NVME)),$(BUILD)/kernel/drivers/storage/nvme/nvme.o)
 KERN_DRIVERS += $(BUILD)/kernel/drivers/usb/core/usb.o
 KERN_DRIVERS += $(BUILD)/kernel/drivers/usb/hid/usb_hid.o
 KERN_DRIVERS += $(BUILD)/kernel/drivers/usb/storage/usb_storage.o
@@ -1825,3 +1842,6 @@ distclean: clean
 # build output: Alpine's usr/lib/sysctl.d is a directory, and including it
 # stopped every make with "Is a directory".
 -include $(shell find $(BUILD) \( -path $(BUILD)/aroot -o -path $(BUILD)/iso -o -path $(BUILD)/ports \) -prune -o -type f -name '*.d' -print 2>/dev/null)
+
+FORCE:
+.PHONY: FORCE

config/drivers.conf

@@ -0,0 +1,45 @@
+# Which drivers are compiled into the kernel.
+#
+#   y   compiled in, and enabled: it is offered every device it can drive
+#   n   not compiled in: the code is not linked and the kernel never knows it
+#
+# Everything is y unless it says otherwise. Change a line, or override one for
+# a single build:
+#
+#     make DRV_E1000=n DRV_NVME=n
+#
+# A changed value rebuilds only kernel/drivers/registry.o and the link.
+#
+# These are the drivers that can be left out cleanly: nothing else in the kernel
+# calls them, they are only offered a device by the registry
+# (kernel/drivers/registry.c). The others cannot be, because the rest of the
+# kernel calls into them directly, and are not listed:
+#
+#     vga_fb      the boot framebuffer: the console and the crash screen draw on it
+#     xhci, usb core, usb hid / storage / hub
+#                 the keyboard and the root disk can arrive over USB, bio.c and
+#                 the hotplug thread call in, and so does the I2C-HID touchpad
+#     PS/2, IDE, the 16550 UART
+#                 fixed-address legacy devices (kernel/dev/)
+#
+# Linux's own drivers, run in a process by ldlsd, are not here: they are files in
+# the image (make ldls-r8169), not code in the kernel.
+
+# --- display --------------------------------------------------------------
+DRV_VGA_PCI    = y     # the stdvga/Bochs card (the framebuffer driver stays regardless)
+DRV_VIRTIO_GPU = y     # QEMU's virtio-gpu: mode setting and an explicit flush
+
+# --- network ----------------------------------------------------------------
+DRV_E1000      = y     # Intel 8254x and 8257x
+DRV_RTL8139    = y     # Realtek 8139 (the one Realtek part QEMU emulates)
+# The native RTL8168/8111 driver. Off by default because the laptop's NIC is
+# driven by Linux's r8169 under LDLS (etc/schinit/ldls-r8169.job), and a driver
+# here would claim the device first. Turn it on for a machine without that job.
+DRV_R8169      = n
+
+# --- storage ----------------------------------------------------------------
+DRV_AHCI       = y     # SATA
+DRV_NVME       = y     # NVMe SSDs
+
+# --- audio ------------------------------------------------------------------
+DRV_AC97       = y     # Intel AC'97 (QEMU's and old boards')

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 |

kernel/drivers/registry.c

@@ -12,9 +12,10 @@
 // a driver silently dropped by the linker looks exactly like one that was
 // never written. At this size, being able to read the list wins.
 //
-// To add a driver: write kernel/drivers/<name>.c exporting a struct
+// To add a driver: write kernel/drivers/<class>/<name>/<name>.c exporting a
-// pci_driver, declare it here, and add it to the array and to the Makefile's
+// struct pci_driver, declare it here, add it to the array and to the Makefile's
-// KERN_DRIVERS.
+// KERN_DRIVERS -- and, if nothing else in the kernel calls it, give it a
+// DRV_<NAME> option in config/drivers.conf so it can be left out.
 
 #include "core/types.h"
 #include "core/defs.h"
@@ -28,14 +29,34 @@
 #include "dev/usb.h"
 #include "dev/device.h"
 
+// Which of these exist in this kernel is config/drivers.conf: the Makefile
+// leaves a disabled driver's object out and defines CONFIG_DRV_<NAME> for the
+// ones it keeps. The ones with no option (xHCI) are always here.
+#ifdef CONFIG_DRV_VGA_PCI
 extern const struct pci_driver vga_pci_driver;
+#endif
 extern const struct pci_driver xhci_driver;
+#ifdef CONFIG_DRV_VIRTIO_GPU
 extern const struct pci_driver virtio_gpu_driver;
+#endif
+#ifdef CONFIG_DRV_E1000
 extern const struct pci_driver e1000_driver;
+#endif
+#ifdef CONFIG_DRV_AC97
 extern const struct pci_driver ac97_driver;
+#endif
+#ifdef CONFIG_DRV_RTL8139
 extern const struct pci_driver rtl8139_driver;
+#endif
+#ifdef CONFIG_DRV_R8169
+extern const struct pci_driver r8169_driver;
+#endif
+#ifdef CONFIG_DRV_AHCI
 extern const struct pci_driver ahci_driver;
+#endif
+#ifdef CONFIG_DRV_NVME
 extern const struct pci_driver nvme_driver;
+#endif
 
 // Order matters here in one respect: virtio-gpu is offered the bus before the
 // VBE framebuffer driver, so that on a machine with both, the device with real
@@ -43,23 +64,42 @@ extern const struct pci_driver nvme_driver;
 // claims the stdvga card underneath, which is what the boot splash and the
 // text console are drawn on.
 static const struct pci_driver *drivers[] = {
+#ifdef CONFIG_DRV_VIRTIO_GPU
     &virtio_gpu_driver,
+#endif
+#ifdef CONFIG_DRV_VGA_PCI
     &vga_pci_driver,
+#endif
+#ifdef CONFIG_DRV_E1000
     &e1000_driver,
-    // &r8169_driver -- the RTL8168 is driven by Linux's own r8169 through
+#endif
-    // LDLS (etc/schinit/ldls-r8169.job), which a kernel driver here would
+    // The RTL8168 is driven by Linux's own r8169 through LDLS
-    // claim first. The native driver stays in the tree as the fallback.
+    // (etc/schinit/ldls-r8169.job), which a kernel driver here would claim
+    // first -- so DRV_R8169 is off by default. The native driver stays in the
+    // tree as the fallback for a machine without that job.
+#ifdef CONFIG_DRV_R8169
+    &r8169_driver,
+#endif
+#ifdef CONFIG_DRV_RTL8139
     &rtl8139_driver,
+#endif
     &xhci_driver,
+#ifdef CONFIG_DRV_AHCI
     &ahci_driver,
+#endif
+#ifdef CONFIG_DRV_NVME
     &nvme_driver,
+#endif
+#ifdef CONFIG_DRV_AC97
     &ac97_driver,
+#endif
+    0,      // keeps the array non-empty when everything optional is off
 };
 
 void
 pci_bind_all(void)
 {
-  for (unsigned i = 0; i < sizeof(drivers) / sizeof(drivers[0]); i++)
+  for (unsigned i = 0; drivers[i]; i++)
     pci_bind(drivers[i]);
 }