Drive the compositor from a separate process, proving the pipeline end to end.
- runtime.display: a Layer handle (fill / blitTile / configure / damage /
destroy), createLayer, and a color(r,g,b) helper that caches the mode and
packs via protocol.pack. Coordinates are signed over the u32 wire fields
(@bitCast both ways), so a layer may sit or move partly off-screen.
- system/services/display-demo: the input-source analog for the compositor — a
full-screen wallpaper, a rectangle it slides back and forth (moved by
configure each frame, so the damage-driven present repaints old + new), and a
cursor. Presents in a loop paced by runtime.time. Wired into build + initrd.
Fix this surfaced: protocol.message_maximum was 4096, but the kernel caps every
IPC message at MESSAGE_MAXIMUM = 256, so replyWait rejected the oversized receive
buffer with -E2BIG and the serve loop had been spinning since D2 (invisibly, as
those gates matched init-time heartbeats). Set it to 256; blit_tile is now
explicitly a small-tile path (larger bitmaps are the deferred shm surface).
Gate: `python3 test/qemu_test.py display-demo` — the demo drives frames of
motion through the layer client API and logs `display-demo: ok`. Regression:
zig build test, display (D1), display-service (D2/D3), and default zig build.
Stand up /system/services/display: a ring-3 process that claims the framebuffer
D1 seeded, maps it write-combining as the front buffer, allocates a cacheable
back buffer, and presents composed frames. The GUI track's compositor, reached
by name over ServiceId.display (= 9).
- protocol.zig: the display wire protocol (info/create_layer/configure_layer/
destroy_layer/fill_rect/blit_tile/damage/present). `info` and a whole-screen
`present` are live; the layer ops fail-stub until D3.
- display.zig: enumerate -> claim -> mmio_map(WC) the LFB, mmap a cacheable back
buffer, clear it and present it (proving the double-buffer path), then serve.
- runtime.display + barrel exports (display, display_protocol): a cached
`.display` client with info()/present(), the runtime.block shape.
- init spawns "display" in boot_services; build.zig wires the protocol onto the
runtime, builds the exe, packs it into the initial-ramdisk, installs it.
mmap fix the back buffer forced: systemMmap was capped at 256 pages (1 MiB) by a
fixed kernel-stack scratch array. Rewrote it to map page-by-page with rollback
(no array) and raised the cap to 8192 pages (32 MiB) — enough for a 4K back
buffer. A real limitation met.
Gate: `python3 test/qemu_test.py display-service` matches the service's own
serial heartbeats (display: online WxH / presented frame 0), printed only after
the full claim -> WC-map -> back-buffer -> present chain. Regression-checked
usermem, heap, init, and D1's display.
Daniel Samson