26bca020b6
Nexoniarz is the sole author and SchOS was never published under Apache-2.0, so this is a straight relicence. LICENSE is the FSF GPLv2 text, every first-party SPDX header moves to GPL-2.0-or-later, and NOTICE, README.md, docs/licensing.md and the site say so. projects/schfs-linux follows the rest of the tree. The GPL gives copyleft: a fork has to ship its source. It also lifts the old "Linux drivers, never" rule, since GPL-2.0-only code combines with GPL-2.0-or-later. "Or later" leaves room for GPLv3, but one binary still has to pick one version, so docs/licensing.md now keeps Apache-2.0 and GPLv3 code out of the kernel to keep Linux code possible there. BSD 4-clause code goes from "avoid" to "no", because the advertising clause is GPL-incompatible. LDLS still ships no .ko. That boundary is now project policy rather than a legal requirement, and ldls.md, devices.md and licensing.md section 6 say so. refmod declares license=GPL, and ldls-check passes (27 checks). Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
110 lines
5.5 KiB
Markdown
110 lines
5.5 KiB
Markdown
# Schcom — the SchOS Compositor
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Owns the display. Every pixel that reaches the screen goes through it.
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Clients do not draw to the framebuffer; they draw into their own buffers and
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hand them over. Schcom decides what is on top of what, composites the result,
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and presents it. It also owns input: it reads the raw event devices and sends
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each event to whichever client should have it.
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That is the whole job. Window *placement* is not part of it — that is SchWM's,
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and Schcom exposes a protocol for it rather than implementing policy itself.
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## Status
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**v0 runs.** A client connects, is given a shared buffer, draws into it and
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commits; the result is composited and presented. Verified end to end by
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`make schcom-test`, which boots SchOS, runs the compositor and a client, then
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screendumps the framebuffer and asserts the pixels **from the host** — the
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compositor's own report that it composited something proves nothing. It runs
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two overlapping clients and checks that the newer one wins in the overlap: a
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compositor that cannot order two surfaces cannot host a window manager.
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What exists:
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- framebuffer output through `/dev/fb0` and the fbdev ioctls, double-buffered
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- a versioned protocol over a Unix socket, with a handshake that must come first
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- shared-memory buffers, one per surface, owned by the compositor
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- surface / attach / **commit** semantics: nothing is shown mid-draw
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- input from `/dev/input/eventN`, routed to the topmost surface
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- clients that die are cleaned up, including their shm segments
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- **multiple clients**, composited bottom-to-top in creation order
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What does not exist yet: damage tracking beyond "repaint everything", multiple
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outputs, subsurfaces, and any focus model beyond "topmost". Those are additions
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rather than rewrites, which is the reason for committing to the surface/commit
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model now instead of growing into it later.
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## Building
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Not part of the OS build. Needs `make ports` first, for the sysroot's libc:
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```sh
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make projects # builds and stages into sysroot/bin
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make # rebuilds the image so the binaries are on it
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make schcom-test # boots it and checks the pixels
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```
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## What it needs from the kernel
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| Need | State |
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|---|---|
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| Shared memory (`shmget`/`shmat`/`shmdt`) | done |
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| `mmap` of `/dev/fb0` | done |
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| Framebuffer geometry via the fbdev ioctls | done |
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| Unix domain sockets | done |
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| Raw input events — key down/up with modifiers | done |
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| `SCM_RIGHTS` | done — `sendmsg`/`recvmsg`, see [`docs/architecture/networking.md`](../../docs/architecture/networking.md#descriptor-passing) |
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| A shareable memory object | **missing.** A Wayland buffer is a descriptor for memory; a System V segment id is not one. Item 2 of [`docs/plan/foundations.md`](../../docs/plan/foundations.md), and the one thing now standing between this directory and a Wayland compositor. |
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| Mode setting and double buffering | partial — virtio-gpu binds, no modeset API yet |
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> **The v0 protocol below is superseded and the code is frozen.** It runs, and
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> it is kept because it is the only thing in the tree that has ever composited
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> anything — but nothing more will be added to it. SchWM and SchDE are referred
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> to throughout and both have been deleted; read those mentions as "the policy
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> module" and "a layer-shell client". See
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> [`docs/plan/wayland.md`](../../docs/plan/wayland.md).
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## Design notes
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- **Buffers are shared memory, not pipes.** A pipe means copying every frame; at
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800×600×32 that is 1.9 MB per frame per client.
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- **The compositor owns the segment, not the client.** It has to survive a
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client crashing mid-frame, and it must not be something a client can resize
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underneath the compositor.
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- **One protocol, versioned from the start.** SchWM and SchDE are separate
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programs, so this is a real compatibility surface on day one. Every message
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carries its own length, so an unknown message is skipped rather than
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desynchronising the stream — that is what lets a v0 compositor survive a v1
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client.
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- **Policy belongs to SchWM.** If Schcom starts deciding where windows go, the
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split has already failed. `CREATE_SURFACE` carries x/y today only because
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there is no SchWM yet to ask.
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## On Wayland
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> **Decided: Schcom moves onto Wayland**, replacing the protocol below while
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> keeping the architecture. See [`docs/plan/wayland.md`](../../docs/plan/wayland.md).
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The protocol is deliberately Wayland-shaped — surfaces rather than windows,
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buffers attached and made visible only on commit, and the compositor telling
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clients when to draw. Wayland is **MIT-licensed**, so it can be linked against
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without any conflict with GPL-2.0-or-later (see `docs/licensing.md`), and
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supporting it later is intended.
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The one place v0 knowingly diverges: a buffer handle is a System V shared memory
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id passed as an integer, where Wayland passes a file descriptor over
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`SCM_RIGHTS`. `struct schcom_surface` carries a `handle_kind` field precisely so
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that switching would be a new value rather than a protocol break.
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**That field could not have saved us.** Wayland's `wl_shm_create_pool` passes a
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descriptor with no alternative form, so *our* protocol being the flexible one
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was never the flexibility that was needed — and for a long time this README
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claimed `SCM_RIGHTS` was done when there was no `sendmsg`/`recvmsg` in the
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kernel at all. An entire GUI plan was drawn on top of that line.
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It is true now. Item 1 of
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[`docs/plan/foundations.md`](../../docs/plan/foundations.md) is built and
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tested; what a buffer still needs is item 2, a shareable memory object, because
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a descriptor for a System V segment is not a thing that exists.
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