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storage: AHCI, GPT/MBR partitions, and a disk a PC can boot

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q35 stopped with "no root filesystem" because its disk is SATA, like
every PC since 2008, and SchOS only had IDE. And SchFS could only sit
on a whole raw device, so nothing could boot from a disk that also
carried an ESP.

- include/blkdev.h, kernel/dev/blkdev.c: a registry of block devices
  found at probe time, numbered from 4. A disk's GPT (or MBR) is read
  when it registers, and each partition becomes a device whose block 0
  is the partition's first sector. SchFS needs no format change to live
  in a partition; a Linux SchFS driver will see the same bytes on
  /dev/sdX2. SchFS has its own GPT type:
  47138FA9-B79C-47A6-9F7D-AC1F7F572D5E.
- AHCI driver, written from the AHCI 1.3.1 spec: BIOS handoff,
  COMRESET when the link is down, IDENTIFY, 48-bit DMA read/write,
  FLUSH. Completion is polled, so the root disk does not depend on
  interrupt routing.
- USB sticks also register as usb0, so their partitions are found.
- The root probe tries registered devices between USB and IDE,
  SchFS-typed partitions first.
- make disk / tools/posix/mkdisk.sh: a GPT image with a 64 MB ESP and
  a SchFS partition, ready to dd to a stick or a disk.

Tested in QEMU q35: the old two-disk setup roots on sata1; disk.img as
the only SATA disk roots on sata0p2; disk.img as a USB stick (firmware
boots from it) roots on usb0p2. Writes survive sync in each case.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HH7xukt4aaiGj1nQEAoQf1
This commit is contained in:
Nexoniarz 2026-10-22 10:57:41 +02:00
commit d7352c0db0
10 changed files with 1027 additions and 17 deletions
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31
docs/architecture/boot.md
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@ -447,12 +447,28 @@ what makes a loop over candidates legal at all.
|---|---|---|
| 1 | `RAMDISK_DEV` | only in a `RAMDISK_ROOT` build, where it is an explicit choice |
| 2 | `USBDISK_DEV` | a stick that is plugged in was plugged in on purpose |
| 3 | IDE 1 | what the old constant named |
| 4 | IDE 0 | usually the ESP, so usually not SchFS |
| 3 | registered partitions typed SchFS | labelled that way on purpose |
| 4 | every other registered device | whole disks, untyped and MBR partitions |
| 5 | IDE 1 | what the old constant named |
| 6 | IDE 0 | usually the ESP, so usually not SchFS |
The **registered** devices are the ones found at probe time rather than known
at build time, numbered from 4 (`include/blkdev.h`): a SATA disk behind AHCI
(`sata0`), the USB stick seen a second time (`usb0`), and every partition on
either (`sata0p2`, `usb0p2`). When a disk registers, its GPT is read, or its MBR
if it has no GPT. Each partition then becomes a device whose block 0 is the
partition's first sector, so SchFS in a partition and SchFS on a whole disk are
the same bytes. The ESP is skipped because it is FAT.
SchFS's GPT partition type is `47138FA9-B79C-47A6-9F7D-AC1F7F572D5E`.
`make disk` builds `build/disk.img` (`tools/posix/mkdisk.sh`), a GPT disk with a
64 MB ESP and a SchFS partition of that type. `dd` it to a stick or a disk, or
give it to QEMU's q35 as the only disk. Both boot, the root comes up as
`sata0p2` or `usb0p2`, and the q35 run is the same path a PC takes.
`iderw()` *panics* on an absent drive rather than failing, so each candidate is
checked with `ide_disk_present()` / `usb_storage_present()` first. That is the
whole reason those predicates exist.
checked with `ide_disk_present()` / `usb_storage_present()` /
`blkdev_present()` first. That is the whole reason those predicates exist.
When nothing matches, the panic names what it looked at, because "no root
filesystem" with no list is the least useful thing a kernel can say at the one
@ -590,9 +606,10 @@ in the one process that runs first.
stall, which is the wrong order for finding out.
- **`/proc/bootmode` is read once, at start.** A program that wants to be
quieter or louder later has nowhere to say so, and nothing re-reads it.
- **The root device is compiled in.** Booting from a USB disk needs the root
device to be selectable, which it is not; the USB mass-storage driver works
and a superblock reads back through `bread()`, but nothing can mount it.
- **One USB disk, found at boot.** The mass-storage driver drives the first
stick and no other, and a stick plugged in later is not seen. See
[usb.md](usb.md).
- **No NVMe yet.** SATA (AHCI) and IDE are the only fixed disks.
- **`make ramroot` is the only way to boot with no data disk.** It links a
filesystem image into the kernel binary, which costs boot time and carries a
licensing constraint — see [../licensing.md](../licensing.md) §4.