display: stop calling the fenced present 'vsync' — it isn't
The virtio-gpu present fence completes when the device has consumed the frame: real completion feedback, and tear-freedom by snapshot semantics. It is not a vblank — base virtio-gpu 2D has no display-refresh event at all (Linux fakes one with a timer), so nothing paces presents to the monitor. The code and docs claimed vsync anyway; now they don't. - backend.hasVsync -> hasFencedPresent, with an honest doc comment - marker 'display: vsync present ok' -> 'display: fenced present ok' (display-modeset test expectation updated, passes) - display-v2.md gains a 'Fenced is not vsync' note; the vsync claims in both v2 docs are corrected - true vsync arrives with a native driver's vblank IRQ, or approximated by a compositor frame clock
This commit is contained in:
Daniel Samson
@@ -6,7 +6,7 @@ lands on its own and ends in a **verifiable gate** — shaped for a `/loop` run,
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## Locked decisions (do not relitigate)
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- **First native backend = virtio-gpu** (VM standard: mode-set + present/flush + vsync).
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- **First native backend = virtio-gpu** (VM standard: mode-set + fenced present/flush).
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- **Dynamic hot-attach**: boot on GOP, upgrade to native when the driver **announces**
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(push, not polling); re-attach across driver restarts; GOP is the floor for "no driver
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ever," not a live fall-back after a reprogram.
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@@ -46,7 +46,7 @@ Extract scanout from the compositor so today's path becomes one backend among fu
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- [x] `system/services/display/backend.zig`: a `Backend` tagged union with `info()`,
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`surface()` (the cacheable compose target), `present(damage)`, and capability flags
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(`canModeSet`/`hasVsync`, both false for GOP).
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(`canModeSet`/`hasFencedPresent`, both false for GOP).
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- [x] The v1 GOP path is now `backend.Gop` (claims the `display` node, WC-maps the LFB,
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keeps the cacheable back buffer, `present` = the damage-rect WC copy). display.zig
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composes into `backend.surface()` and calls `backend.present(damage)` — no LFB or
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@@ -123,7 +123,7 @@ confirm the composited frame landed (`display: native present verified`), while
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ok` still fires — checked order-independently. Without `-device virtio-gpu-pci` nothing is
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announced and it stays on GOP: the v1 `display-service`/`display-demo` gates pass unchanged.
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## V5 — Mode-setting, EDID, and vsync ✅
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## V5 — Mode-setting, EDID, and fenced presents ✅
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- [x] The driver negotiates `VIRTIO_GPU_F_EDID` (when offered) and reads the monitor's EDID,
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logging its preferred mode; it offers a small mode list over `.scanout` `get_modes`. The
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@@ -131,14 +131,16 @@ announced and it stays on GOP: the v1 `display-service`/`display-demo` gates pas
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scanout rectangle (no resource/surface churn) — a runtime resolution change. `runtime.display`
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gains `modes()` / `setMode()` (display-protocol `get_modes`/`set_mode`, forwarded to the backend).
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- [x] Every `resource_flush` is issued fenced (`VIRTIO_GPU_FLAG_FENCE`); the device signals the
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fence when the frame is on screen, which the used-ring ack the synchronous present waits on
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already gates — a tear-free present.
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- [x] `backend.VirtioGpu` reports `canModeSet` / `hasVsync` = true.
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fence when it has consumed the frame, which the used-ring ack the synchronous present waits
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on already gates — a tear-free present. (Completion feedback, **not vblank**: base
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virtio-gpu 2D has no display-refresh event, so nothing paces presents to the monitor —
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see the "Fenced is not vsync" note in [display-v2.md](display-v2.md).)
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- [x] `backend.VirtioGpu` reports `canModeSet` / `hasFencedPresent` = true.
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**Gate (met):** the `display-modeset` case (reusing the display-native boot) upgrades to
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virtio-gpu, queries the driver's modes, `setMode`s to a different resolution, and confirms the
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change by reading the backend's geometry back (`display: mode set to {w}x{h}, verified`); the
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fenced present path is exercised and confirmed (`display: vsync present ok`) — both from serial,
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fenced present path is exercised and confirmed (`display: fenced present ok`) — both from serial,
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passing 3/3. The driver also logs the EDID preferred mode (`virtio-gpu: EDID preferred mode …`).
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## V6 — Resilience (restart + re-attach) + tests + docs ✅
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+18
-10
@@ -2,7 +2,7 @@
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**Status: complete (V1–V6).** The compositor boots on the GOP framebuffer and, when a
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virtio-gpu driver announces itself, hot-attaches a native backend over the shared `shm`
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scanout surface — with runtime mode-setting, EDID, and fenced (vsync) presents, and it
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scanout surface — with runtime mode-setting, EDID, and fenced presents, and it
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re-attaches across driver restarts. All serial-gated (see [display-v2-plan.md](display-v2-plan.md)).
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v1 ([display.md](display.md)) is a compositor that owns the **GOP framebuffer** — it
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@@ -25,10 +25,10 @@ The compositor itself (layers, back buffer, damage) does not change. Only the la
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scanout backend (selected at runtime — GOP by default, native when it appears)
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│
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├─ GopBackend the v1 path: WC copy back→front to the firmware LFB.
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│ Always available. No mode-set, no vsync. THE FLOOR.
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│ Always available. No mode-set, no present fence. THE FLOOR.
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│
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└─ VirtioGpuBackend talks to a virtio-gpu driver process over a `scanout`
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service: present via a shared resource + flush (real vsync),
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service: present via a shared resource + fenced flush,
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EDID mode list, runtime mode-set.
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```
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@@ -37,8 +37,8 @@ A **backend** is a small interface the compositor calls:
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- `surface()` → the pixels to compose into and their geometry `{ptr, pitch, format, w, h}`
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(the LFB for GOP; a shared scanout resource for virtio-gpu),
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- `present(damage: Rect)` → make the damaged region visible (a no-op-ish WC copy for GOP;
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a virtio flush, optionally vsync-fenced, for the native path),
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- capability queries — `canModeSet`, `hasVsync` — and, when supported, `modes()` /
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a fenced virtio flush for the native path),
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- capability queries — `canModeSet`, `hasFencedPresent` — and, when supported, `modes()` /
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`setMode(m)`.
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The compositor composes into `surface()` and calls `present(damage)` exactly as it does
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@@ -98,23 +98,31 @@ compositor when a second backend arrives"). It claims the virtio-gpu PCI functio
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at a chosen mode for **runtime mode-setting**,
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- registers a `scanout` service and announces to the display service.
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Its `resource_flush` is the real **present** — and gives a genuine **vsync/tear-free**
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path a dumb GOP framebuffer can't.
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Its `resource_flush` is the real **present** — and gives a **fenced, tear-free** path a
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dumb GOP framebuffer can't.
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**Fenced is not vsync.** The fence completes when the device has *consumed* the frame:
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real completion feedback, and tear-freedom by snapshot semantics (the host displays
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discrete transferred frames, never a half-written surface). It is **not** a vblank —
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base virtio-gpu 2D has no display-refresh event at all (Linux's driver for this device
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fakes one with a software timer), so nothing paces presents to the monitor's refresh.
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Refresh-paced presents need either a native driver's vblank interrupt (delivered over
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the existing IRQ-as-IPC path) or the compositor's own frame clock.
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## What v2 unlocks — and its honest scope
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Behind the abstraction, a native backend gives runtime **mode-setting** (resolution /
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refresh / bpp), **EDID** enumeration, and **vsync**. But only on devices we have a driver
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refresh / bpp), **EDID** enumeration, and **fenced presents**. But only on devices we have a driver
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for — realistically **VMs** (virtio-gpu, and later maybe Bochs DISPI). Real discrete GPUs
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need per-vendor KMS-class drivers that aren't getting written, so they **stay on GOP** —
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which is genuinely fine (v1 on the NVIDIA box is smooth). So v2's real value is twofold:
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the **pluggable architecture** (a driver slots in when one exists) and a **rich, vsync'd
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the **pluggable architecture** (a driver slots in when one exists) and a **rich, fenced
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path in VMs**, where danos development happens. The framebuffer floor never goes away.
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## Locked decisions
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- **First native backend: virtio-gpu** — the VM standard; gives mode-set + a real
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present/flush (and vsync), and exercises the whole pluggable design. Tested with QEMU
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present/flush (fenced), and exercises the whole pluggable design. Tested with QEMU
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`-device virtio-gpu`.
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- **Dynamic hot-attach** — boot on GOP, upgrade to native on the driver's announce,
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re-attach across driver restarts; GOP is the floor for "no driver ever," not a live
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