SchOS/docs/plan/roadmap.md

# Roadmap — everything else still open

[1.0.md](1.0.md) draws the finish line and holds the register of what is half-built.
This file holds what is left beyond it: work that is scoped, agreed and unstarted, with
nothing in it that is finished.

**Finished work is not here.** It moved to [../architecture/](../architecture/) and was
rewritten as a description of what it produced — see
[the rule](../README.md#the-rule-that-keeps-this-from-rotting).

---

## 1. Storage and the boot path

A USB disk, a SATA disk, an NVMe SSD and their partitions boot, and the root is chosen at boot from
what is present ([boot.md](../architecture/boot.md)).

- **An installer** that writes the layout `make disk` builds (a GPT disk with an ESP and a SchFS
  partition) to a machine's disk, so a root survives a reboot. See §5.
- **Interrupt-driven, queued I/O for AHCI and NVMe.** Both poll for completion, one command at a time
  per disk: enough to boot and run, not to be fast.
- **An EHCI driver.** USB 2.0 ports wired only to EHCI do nothing; xHCI ports work.

## 2. Graphics

- **A `/dev/dri` device with mode setting**, so a real panel's resolution stops being whatever the
  loader's menu chose and Mesa has something to open. This is the GPU plan, [gpu.md](gpu.md).
- **Double buffering and vblank**, which `display_ops` specifies and no driver implements; the same plan.
- **CFF/Type 2 charstrings**, so OTF fonts work. A separate `font_ops` provider and a bytecode
  interpreter, *not* an extension of the `glyf` path.
- **Text layout** — advance widths, kerning — as a library, so the terminal and the file manager are not
  each solving it.

## 3. Networking

- **A routing table** and `/proc/net/route`. With two interfaces up the first is the default route and the
  second is reachable only by an address on its own subnet; that is a rule and not routing.
- **Wi-Fi.** The laptop has no other way to be wireless. The route is the same as for the NIC: a Linux
  driver (`iwlwifi`, `mt76`…) through LDLS, which needs `nl80211`, a firmware loader that exists, and a
  supplicant (`wpa_supplicant` is an Alpine package).
- **IPv6.**
- **A netlink event stream**, for anything that wants to be told a link changed.

## 4. The hardware that makes a laptop a laptop

- **HDA audio.** AC'97 is what QEMU and old boards have; a laptop built this century has Intel HDA,
  which is codec enumeration, CORB/RIRB and stream descriptors. It arrives as a second `struct audio_ops`.
- **Embedded-controller events** (the ACPI SCI), so a lid, a power button on the EC and a battery change are seen.
- **CPU frequency control** (AMD CPPC; Intel HWP/EIST). Idle already reaches `_CST`'s C2; under load the cores run at
  the firmware's default clock.
- **Touchpad gestures** — two-finger scroll, palm rejection — on top of I2C-HID's mouse mode.
- **An IOMMU** (VT-d / AMD-Vi), which turns "a driver in a process is isolated against bugs" into "against malice" —
  the largest honest caveat on [ldls.md](../architecture/ldls.md).
- **udev's job** — a uevent socket and `/sys/devices` — so X and others are told about devices instead of being
  given a list.

## 5. The installer and `.sip`

Scoped in [`projects/schinst/README.md`](../../projects/schinst/README.md), which holds the detail. In short:

- [ ] `.sip` format specification, **written down before any code**
- [ ] `tools/mksip` — capture a directory tree into a package
- [ ] The apply path
- [ ] An installation environment that boots from the ISO, partitions, applies the `.sip`, and installs the loader

`.sip` is modelled on WIM, and the thing worth copying from WIM is that it is **file-based, not block-based**: one
file holds a whole filesystem, applies to a partition of any size, and several images can share their common files.

**Most of this already exists.** `tools/mkschfs.c` and `tools/mkfs.c` both do content dedup with an `(st_dev, st_ino)`
fast path, and SchFS's B-tree is content addressed. `mksip` should share that code rather than reimplement it. And
the live image is already a compressed block image of a SchFS root ([scff.md](../architecture/scff.md)); an installer
may need nothing more than to write that, decompressed, to a partition.

Open questions, all wanting a decision before code:

- **Compression.** Something with a small decompressor is worth more than something with a good ratio, because the
  decompressor runs in the installation environment. (zstd is already in the kernel for SCFF.)
- **Multiple images per `.sip`** — WIM's real trick, install/recovery/boot sharing one blob store. Worth the complexity
  now, or not?
- **Can the kernel mount a `.sip` read-only directly?** If it can, the installation environment's root can simply *be* the `.sip`.

The GUI for this is an X client, like everything else on the desktop.

## 6. SchFS beyond SchOS

In the order they should happen.

**An upgrade step.** The format version policy is done ([schfs.md](../architecture/schfs.md)); the upgrade step cannot be
written until there are two formats to convert between, and the first bump is when it becomes real work.

**A Linux implementation, FUSE first.** Three reasons in order of weight: it can be tested with `fsx` and `xfstests`, which
are far harsher than anything written alongside the code; it carries no licence entanglement, since libfuse is LGPL and used
across a process boundary; and it can reuse the read/write logic already in `tools/mkschfs` and `tools/schfs_fsck` rather than
being written twice and having the two disagree. A kernel module comes only after FUSE passes `xfstests`; SchOS is GPL-2.0-or-later
([../licensing.md](../licensing.md)), so a Linux module is a technical project now and not a licensing one.

**The missing features**, listed honestly before adding any: no journal replay across an unclean shutdown beyond what
copy-on-write gives for free, no snapshots, no sparse-file support, and no online `fsck`. **Snapshots are the one where the
existing design already pays for most of the cost** — the B-tree is copy-on-write, which is most of the machinery; nothing takes
or names a snapshot.

## 7. The kernel's own gaps

Not phases, and each one improves everything above it:

- **A page cache**, which is what a general writable `MAP_SHARED` of a file needs. `memfd` is the narrow answer and has been enough.
- **A working `mprotect`, and NX.** `mprotect` is accepted and ignored and every mapping is executable; RELRO, guard pages
  and a W^X JIT all want the real thing.
- **Per-CPU run queues** and work stealing, with a load balancer that is not worse than the lock it replaces.
- **A garbage collector for descriptors in flight.** An `AF_UNIX` socket passed over itself and never received leaks the pair;
  bounded at thirty-two, and nothing in the tree does it.
- **Kernel-time accounting**, so `ru_stime` is not zero.
- **A contiguous multi-page allocator**, so `struct proc` and `struct socket` can come from somewhere other than fixed tables.

---

## Kept out of scope on purpose

- TrueType hinting.
- DRM/KMS as a *kernel* subsystem; the GPU plan serves `/dev/dri` from a process.