blitterd

CopperlineOS 2D blitter service.
blitterd performs fast 2D buffer operations—rect fill, rect copy, and format convert—on shared GPU buffers (DMABUFs). In Phase‑0 it uses Vulkan transfer/compute queues to move pixels with minimal CPU involvement. The service exposes a tiny JSON protocol over a Unix domain socket.

TL;DR: move pixels fast and predictably, without dragging a full graphics stack into the hot path.

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Why it exists

The compositor (compositord) wants clean, ready‑to‑draw surfaces. Apps shouldn’t have to carry bespoke GPU code just to clear/copy/convert textures. blitterd centralises those operations so they’re consistent, low‑latency, and zero‑copy across the system.

• Zero‑copy pipelines via DMABUF import/export.
• Deterministic execution with optional fences and completion events.
• Simple JSON API (binary ring in later phases).
• Future‑proof path to FPGA offload for “Blitter‑2.0”.

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Status

• Phase‑0 JSON protocol: stable‑ish (v0)
• Backends: Vulkan transfer/compute on Linux
• Language: Rust
• License: MIT OR Apache‑2.0

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Architecture (Phase‑0)

flowchart LR
  subgraph Apps
    A1[Producer app] -- DMABUF --> B[blitterd]
    A2[Tools/CLI] -- JSON over UDS --> B
  end
  B -- writes DMABUF --> C[compositord]
  subgraph Kernel/Drivers
    DRM[KMS/DRM] --> C
    VK[Vulkan driver] --> B
  end
  COP[copperd] -. orchestrates timing .-> C

Loading

• Apps or compositord hand blitterd DMABUF FDs (or, in debug mode, file paths).
• blitterd runs the operation (fill/copy/convert) on the GPU’s transfer/compute queue.
• Results are visible to compositord on the next latch/commit.

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Concepts

• Surface: a DMABUF (preferred) or debug file-backed buffer with w, h, format, stride, modifier.
• Job: a one‑shot operation that may batch multiple ops and signal a fence/event on completion.
• Region: {x, y, w, h} rectangle in pixels; coordinates are top‑left origin.
• Format: e.g., RGBA8, BGRA8 (Phase‑0); YUV/NV12 in later revisions.

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Protocol (Phase‑0 JSON, v0)

Socket (default): /run/copperline/blitterd.sock
Configurable via BLITTERD_SOCKET.

Client → blitterd

• {"cmd":"ping"} → {"ok":true,"name":"blitterd","version":"0.1.0"}
• {"cmd":"rect_fill","dst":<surface>,"rect":{"x":0,"y":0,"w":128,"h":128},"color":{"r":255,"g":0,"b":255,"a":255}}
• {"cmd":"rect_copy","src":<surface>,"dst":<surface>,"src_rect":{"x":0,"y":0,"w":128,"h":128},"dst_xy":{"x":200,"y":100}}
• {"cmd":"convert","src":<surface>,"dst":<surface>} (format/stride may differ; sizes must match)
• {"cmd":"batch","ops":[ ... ]} (run multiple ops as one job)
• {"cmd":"subscribe","events":["complete","error"]}

Surface objects (DMABUF recommended):

{
  "fd": 17,
  "w": 1920,
  "h": 1080,
  "format": "RGBA8",
  "stride": 7680,
  "modifier": "LINEAR"
}

Debug file surfaces (for quick demos; slower, copies via staging):

{
  "path": "/tmp/sprite.rgba",
  "w": 128,
  "h": 128,
  "format": "RGBA8",
  "stride": 512
}

Events (server → client)

• {"event":"complete","job":123,"usec":4213}
• {"event":"error","job":123,"message":"description"}

Notes

• Rects are clipped to destination bounds.
• Overlapping src/dst with rect_copy is implementation‑defined in v0 (avoid or use blit when available).
• Color components are 0–255 in v0; HDR/linear color is planned for later.

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Quick start

Option A: Debug file‑backed buffer (fastest to see something)

# 1) Start compositord (in another terminal) and create a layer bound to /tmp/sprite.rgba
portctl /run/copperline/compositord.sock '{"cmd":"create_layer"}'
portctl /run/copperline/compositord.sock   '{"cmd":"bind_image","id":1,"path":"/tmp/sprite.rgba","w":256,"h":256,"format":"RGBA8"}'
portctl /run/copperline/compositord.sock   '{"cmd":"set","id":1,"x":100,"y":100,"alpha":1.0,"z":5,"visible":true}'

# 2) Fill a magenta square into the file via blitterd
portctl /run/copperline/blitterd.sock   '{"cmd":"rect_fill","dst":{"path":"/tmp/sprite.rgba","w":256,"h":256,"format":"RGBA8","stride":1024}, "rect":{"x":0,"y":0,"w":256,"h":256}, "color":{"r":255,"g":0,"b":255,"a":255}}'

You should see a 256×256 magenta square appear at (100,100).

Option B: Proper zero‑copy DMABUFs

1. Allocate a DMABUF in your app (or request one from compositord—API to be finalised).
2. Bind it to a layer in compositord.
3. Send its FD as the dst surface to blitterd and issue rect_fill / rect_copy.
4. The layer updates on the next vsync commit without staging copies.

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Building from source

# Prereqs: Rust stable, Vulkan loader/headers, libdrm for DMABUF helpers
git clone https://github.com/CopperlineOS/blitterd
cd blitterd
cargo build --release

RUST_LOG=info ./target/release/blitterd

Environment variables:

• BLITTERD_SOCKET=/run/copperline/blitterd.sock
• BLITTERD_DEVICE=auto|/dev/dri/renderD128
• BLITTERD_DEBUG_FILES=0|1 (enable file‑backed surfaces)

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Development guide

• Crates: blitterd-core (ops & scheduler), blitterd-ipc (JSON protocol), blitterd-backend (Vulkan), blitterd-bin (daemon).
• Ops: implemented as GPU pipelines (transfer for copy/fill; compute for format convert).
• Sync: implicit within a job; explicit external fences planned (shared with copperd timelines).
• Validation: surface dims/format/stride/modifier are checked; out‑of‑bounds rects are clipped.
• Performance: batches coalesce ops to minimize barriers; device selection prefers a render node.

Supported formats (v0): RGBA8, BGRA8 (more via feature flags).
Planned: NV12, RGB10A2, sRGB awareness, color‑managed convert.

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Roadmap

• v0.1: rect_fill, rect_copy, convert (RGBA/BGRA), debug file surfaces.
• v0.2: blit (scale/nearest and linear), per‑op alpha modulate, tiled ops.
• v0.3: YUV/NV12 ↔ RGBA convert, partial damage guidance to compositord.
• v0.4: FPGA Blitter‑2.0 backend with DMA command ring.
• v0.5: Security caps (FD quotas, rate limits), binary protocol.

RFCs will appear in CopperlineOS/rfcs.

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Contributing

We welcome issues and PRs!

1. Read CONTRIBUTING.md.
2. Check good first issue / help wanted labels.
3. Protocol/format additions → open an RFC and link it in your PR.

Code of Conduct: CODE_OF_CONDUCT.md.

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Security

blitterd handles FDs received from untrusted clients—validate everything.
Prefer running as an unprivileged user with access to a render node (/dev/dri/renderD*), not as root.
Report vulnerabilities to [email protected] (placeholder) or via GitHub advisories. See SECURITY.md.

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License

Dual‑licensed under Apache‑2.0 OR MIT.

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See also

• compositord: Vulkan/KMS compositor + layer registers
• copperd: timeline engine (drives per‑frame changes)
• ports: message‑port protocol & client libs
• examples: DMABUF demos and integration tests