# Red Bear OS — Deferred GPU Features Implementation Plan

**Created:** 2026-06-06 · **Cross-referenced:** Linux 7.1 i915, Redox kernel, relibc, redox-drm
**Status:** Analysis complete — concrete implementation plans for 7 deferred features

## Architecture Context

Red Bear OS is a microkernel OS. GPU drivers run as **userspace daemons** communicating via schemes:
- `scheme:memory/physical` — MMIO and DMA buffer allocation
- `scheme:irq` — interrupt delivery
- `scheme:pci` — PCI device enumeration and config
- `scheme:firmware` — firmware blob serving
- `scheme:event` — inter-process event queues + eventfd
- `scheme:drm` — DRM/KMS ioctl surface (our daemon)

All GPU memory is allocated from the daemon's address space via `DmaBuffer::allocate()` which
backs onto `scheme:memory/physical@wb?phys_contiguous`. There is **no kernel swap**, **no kswapd**,
and **no memory pressure notifier**. Cross-process buffer sharing is limited to in-process PRIME
token exchange.

---

## Feature 1: GEM Shrinker / Eviction

### Linux Pattern

Two trigger paths:
- **GTT allocation failure**: Walk `bound_list` in scan-order LRU, skip pinned/active/scanout, evict to make room
- **Memory pressure**: `I915_SHRINK_BOUND` on `purge_list` (MADV_DONTNEED), `I915_SHRINK_UNBOUND` for unbound objects, `I915_SHRINK_ACTIVE` for GPU idle + context eviction

### Redox Assessment

**Not applicable in its Linux form.** Redox has no kernel swap, no kswapd, no memory pressure callback, and no `/proc/meminfo`. GPU memory is direct physical allocation via `scheme:memory/physical` — there's no page reclaim mechanism.

### Redox Alternative

Instead of a shrinker, implement a **configurable hard cap with LRU eviction** within the DRM daemon:

```
redox-drm GemManager:
├── MAX_GEM_BYTES: u64 = 256 * 1024 * 1024  (configurable via recipe.toml)
├── eviction_queue: VecDeque<(GemHandle, Instant)>  // insertion-order LRU
├── on alloc when total > MAX_GEM_BYTES:
│   ├── Walk eviction_queue oldest-first
│   ├── Skip: pinned objects (fb-bound), active objects (GPU has fence)
│   ├── Drop DmaBuffer (frees physical pages)
│   └── Until total < watermark (75% of MAX_GEM_BYTES)
└── No kernel changes required
```

**Effort:** ~150 lines in `gem.rs`. Self-contained, no new schemes.
**Priority:** P1 — prevents OOM on memory-constrained systems.

---

## Feature 2: dma-resv / Cross-Driver Fences

### Linux Pattern

Four-function minimal API:
- `dma_resv_reserve_fences(obj, num)` — pre-allocate fence slots
- `dma_resv_add_fence(obj, fence, usage)` — add shared/exclusive fence
- `dma_resv_wait_timeout(obj, usage, intr, timeout)` — block until signaled
- `dma_resv_test_signaled(obj, usage)` — non-blocking check

Fence de-duplication: same context + later-or-same seqno → replace old fence.

### Redox Assessment

**Partially feasible now.** The `scheme:event` infrastructure provides the raw synchronization primitive. The `SyncobjManager` already has in-process fence tracking with FD interop. What's missing is cross-process sharing.

### Redox Implementation

**Phase 1 — In-process (feasible now, ~200 lines):**
```
redox-drm dma_fence crate:
├── FenceContext = u64 atomic counter (dma_fence_context_alloc)
├── FenceSeqno = u64 per-context monotonic
├── FenceState: UNSIGNALED | SIGNALED | ERROR
├── FenceOps trait: get_driver_name, get_timeline_name, enable_signaling, release
└── Fence::signal() → sets SIGNALED, wakes waiters
```

**Phase 2 — Cross-process (needs scheme:syncobj, ~500 lines):**
```
scheme:syncobj daemon:
├── Global syncobj registry (handle → state mapping)
├── Export: daemon calls scheme:syncobj/export → gets FD
├── Import: other daemon calls scheme:syncobj/import with FD → gets local handle
├── Wait: scheme:syncobj/{handle}/wait (blocks via scheme:event)
└── Signal: scheme:syncobj/{handle}/signal
```

**Priority:** P0 — everything else (PSR, FBC, GuC submission) depends on proper fence synchronization.
**Effort:** Phase 1 ~200 lines, Phase 2 ~500 lines + new daemon.

---

## Feature 3: GuC/HuC Firmware Loading

### Linux Pattern

DMA engine upload sequence:
1. Write `DMA_ADDR_0` (source GGTT address) + `DMA_ADDR_1` (WOPCM dest with `DMA_ADDRESS_SPACE_WOPCM`)
2. Write `DMA_COPY_SIZE` (CSS header + uCode size)
3. Write `DMA_CTRL` = `START_DMA` | flags
4. Poll `DMA_CTRL` for `START_DMA` clear (timeout 100ms)
5. Write `SOFT_SCRATCH(n)` with H2G action + params
6. Write `GUC_SEND_INTERRUPT` (Gen9) or `GEN11_GUC_HOST_INTERRUPT` (Gen11+)
7. Poll `GUC_STATUS[16]` for `GS_MIA_CORE_STATE`

Key registers: `GUC_STATUS` (0xc000), `SOFT_SCRATCH(n)` (0xc180+), `DMA_ADDR_0/1` (0xc300-0xc30c), `DMA_COPY_SIZE` (0xc310), `DMA_CTRL` (0xc314), `DMA_GUC_WOPCM_OFFSET` (0xc340), `GUC_WOPCM_SIZE` (0xc050).

### Redox Assessment

**Fully feasible now.** All prerequisites met:
- `scheme:firmware` daemon already serves GPU firmware blobs
- DMC firmware already loaded via same path
- DMA engine registers known and accessible via MMIO
- GGTT mapping infrastructure exists in our driver

### Redox Implementation (~300 lines)

```
redox-drm intel/guc.rs:
├── GucFirmware struct with mmio: Arc<MmioRegion>
├── upload(firmware: &[u8]) → parse CSS header → DMA transfer → poll GUC_STATUS
├── Wire into IntelDriver::new() after DMC upload
└── Add guc_fw_key field to info.rs device table per platform

Prerequisites:
├── Firmware blobs in /lib/firmware/i915/ (add to fetch-firmware.sh)
├── GGTT mapping of firmware blob (alloc 2MB below GUC_GGTT_TOP = 0xFEE00000)
└── WOPCM size register programmed before upload
```

**Priority:** P2 — needed for Gen9+ GPU scheduling. Not required for display-only.
**Effort:** ~300 lines + firmware blob packaging.

---

## Feature 4: PSR (Panel Self Refresh)

### Linux Pattern

Dual-side enable:
- **Sink** (via DP AUX): Write `DP_PSR_EN_CFG` = `DP_PSR_ENABLE` | link_standby | CRC verify
- **Source** (MMIO): Write `EDP_PSR_CTL` = `EDP_PSR_ENABLE` | idle_frames | TP times | max_sleep

PSR2 adds `EDP_PSR2_CTL` with selective update tracking (`SU_TRACK_ENABLE`, `Y_COORDINATE`).

Frontbuffer tracking origins: `ORIGIN_CS` (GPU write → exit PSR), `ORIGIN_DIRTYFB` (CPU dirty).

Key registers: `EDP_PSR_CTL` (0x60800 + transcoder*0x100), `EDP_PSR_STATUS` (0x60840), `TRANS_EXITLINE` (0x70034).

### Redox Assessment

**Feasible now.** Prerequisites:
- eDP panel with `DP_PSR_EN_CFG` support in DPCD
- DMC firmware loaded (required for PSR)
- VBT timing data (`tp1_wakeup_time_us`, `tp2_tp3_wakeup_time_us`, `idle_frames`)
- No interlaced mode, no per-pixel alpha

### Redox Implementation (~200 lines)

```
redox-drm intel/display_psr.rs:
├── PsrState struct with mmio, enabled: bool, psr2: bool
├── enable() → AUX write DP_PSR_EN_CFG → MMIO write EDP_PSR_CTL
├── disable() → AUX write DP_PSR_EN_CFG=0 → MMIO disable
├── flush() → AUX exit → wait vblank → re-enable
└── Wire into set_crtc (enable after modeset on eDP) and page_flip (flush)

Prerequisites:
├── eDP connector detection (already working)
├── DP AUX channel (already working)
└── DMC firmware loaded (already working)
```

**Priority:** P2 — power savings for laptop/embedded use. Not needed for desktop.
**Effort:** ~200 lines.

---

## Feature 5: FBC (FrameBuffer Compression)

### Linux Pattern

Compression trigger on primary plane commit:
1. Clear FBC tags
2. Program `DPFC_CB_BASE` = stolen memory offset (4k aligned)
3. Write `DPFC_CONTROL` = `DPFC_CTL_EN` | `DPFC_CTL_LIMIT_1X` | `DPFC_CTL_PLANE(pipe)` | fence
4. Poll `DPFC_STATUS` for `FBC_STAT_COMPRESSING` clear

Nuke on frontbuffer modify: rewrite `DSPADDR` to trigger re-compression.

Key registers: `DPFC_CB_BASE` (0x3200), `DPFC_CONTROL` (0x3208), `DPFC_STATUS` (0x3210), `DPFC_FENCE_YOFF` (0x3218). ILK+: `ILK_DPFC_CONTROL(fbc_id)` (0x43208).

### Redox Assessment

**Feasible now.** Prerequisites:
- Stolen memory reservation (~2048KB at 4k alignment)
- Primary plane with linear or X-tiled buffer
- Stride 512-byte aligned (SKL+), no rotation, no interlaced
- Fence register for nuke-on-dirty

### Redox Implementation (~200 lines)

```
redox-drm intel/display_fbc.rs:
├── FbcState struct with mmio, enabled: bool, cfb_base: u64
├── enable(fb_info) → check constraints → program DPFC_CB_BASE → DPFC_CONTROL
├── disable() → clear DPFC_CTL_EN
├── nuke() → rewrite DSPADDR → poll DPFC_STATUS
└── Wire into page_flip (enable on new FB, nuke on modify)

Prerequisites:
├── Stolen memory reservation in GGTT
├── Fence register setup (already have GGTT infrastructure)
└── Plane format/stride constraint checking
```

**Priority:** P3 — memory bandwidth savings. Optimization, not required for enablement.
**Effort:** ~200 lines.

---

## Feature 6: DP MST (Multi-Stream Transport)

### Linux Pattern

DP AUX sideband messaging for topology discovery and stream allocation:
- `PATH_REPLY` messages for topology enumeration
- `CONNECTION_STATUS_NOTIFY` for hotplug
- `ALLOCATE_PAYLOAD` for virtual channel allocation
- `REMOTE_DPCD_READ/WRITE` for remote sink access

### Redox Assessment

**Feasible but protocol-heavy.** Prerequisites:
- DP AUX channel (already working)
- Sideband message parsing (new protocol layer)
- Topology manager (new state machine)

### Redox Implementation (~500 lines)

```
redox-drm intel/dp_mst.rs:
├── MstTopology struct: Vec<MstPort> tree
├── MstPort: port_number, peer_device_type, dpcd_rev, mst_cap
├── enumerate() → sideband PATH_REPLY messages → build topology tree
├── allocate_stream(port, bw) → ALLOCATE_PAYLOAD message → virtual channel
└── Wire into connector detection for DP sinks with MST_CAP

Prerequisites:
├── DP AUX channel (already working)
└── Sideband message handler (new ~300 lines)
```

**Priority:** P4 — multi-monitor support. Important but not urgent.
**Effort:** ~500 lines.

---

## Feature 7: HDMI/DP Audio

### Linux Pattern

Three-layer audio stack:
- **HDA controller**: CORB/RIRB command rings, stream descriptors, DMA engine
- **ELD** (EDID-Like Data): retrieved from display sink, programs audio infoframe
- **Audio infoframe**: HDMI/DP specific, carries channel count, sample rate, speaker allocation

### Redox Assessment

**Partially feasible.** Prerequisites partially met:
- Intel HDA driver (`ihdad`) exists in `local/sources/base/drivers/audio/ihdad/`
- `audiod` mixer daemon exists with `scheme:audio` and `scheme:audiohw`
- USB Audio daemon (`redbear-usbaudiod`) is a **stub** — must be replaced with real UAC driver
- No ALSA compatibility layer

### Redox Implementation

**Not recommended for immediate implementation.** The audio stack needs:
1. Real USB Audio Class driver (replace `redbear-usbaudiod` stub) — ~500 lines
2. Audio infoframe programming in HDMI/DP output path — ~200 lines
3. ELD retrieval from display sink via DP AUX — ~100 lines
4. Integration with existing `audiod` mixer

**Priority:** P5 — blocked on USB audio driver completion.
**Effort:** ~800 lines across multiple daemons.

---

## Implementation Priority Matrix

| Priority | Feature | Lines | New Schemes | Prerequisites Met? | Impact |
|----------|---------|-------|-------------|-------------------|--------|
| **P0** | dma-fence (in-process) | 200 | None | ✅ All met | Everything depends on fences |
| **P1** | GEM LRU eviction | 150 | None | ✅ All met | Prevents OOM |
| **P2** | GuC firmware | 300 | None | ✅ All met | Enables Gen9+ GPU scheduling |
| **P2** | PSR | 200 | None | ✅ DMC + eDP + AUX | Laptop power savings |
| **P3** | FBC | 200 | None | ✅ Stolen mem + fence | Memory bandwidth savings |
| **P4** | DP MST | 500 | None | ✅ DP AUX | Multi-monitor support |
| **P5** | dma-fence (cross-proc) | 500 | scheme:syncobj | ❌ No cross-proc fd passing | Cross-driver sync |
| **P5** | HDMI/DP audio | 800 | None (uses existing) | ❌ USB audio is stub | Audio output |

**Total P0-P3 effort: ~850 lines across 4 new modules. All feasible now with zero new scheme infrastructure.**