ARM64 (aarch64) Linux build fails — char signedness + missing OpenVR prebuilt

[scottybo]

Summary

Building ALVR on ARM64 Linux (tested on NVIDIA DGX Spark / GB10, Ubuntu 24.04) fails at two points.

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Issue 1: char signedness in alvr_server_openvr

error[E0308]: mismatched types
   --> alvr/server_openvr/src/lib.rs:184:28
    |
184 |         m_captureFrameDir: capture_frame_dir,
    |                            ^^^^^^^^^^^^^^^^^ expected `[u8; 1024]`, found `[i8; 1024]`

On ARM64, C char is unsigned by default (u8). The array is declared as [0i8; 1024] but the FFI struct expects [u8; 1024].

Fix: [0i8; 1024] → [0u8; 1024]. The existing .cast() on the pointer copy already handles both types correctly.

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Issue 2: No ARM64 libopenvr_api.so prebuilt

OpenVR ships only linux64 (x86-64) prebuilts. On aarch64 the linker fails:

/usr/bin/ld: skipping incompatible openvr/lib/linux64/libopenvr_api.so
/usr/bin/ld: cannot find -lopenvr_api: No such file or directory

OpenVR builds cleanly for ARM64 from source in ~30 seconds. A separate PR has been filed on ValveSoftware/openvr to add the prebuilt: ValveSoftware/openvr#1924

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Environment

• Hardware: NVIDIA DGX Spark (GB10 / Blackwell), aarch64
• OS: Ubuntu 24.04 LTS
• Rust: 1.97.1
• Both issues fixed in: fix: ARM64 (aarch64) Linux build support #3345

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Why this matters

ARM64 Linux is increasingly common for GPU workloads (NVIDIA Grace Blackwell, Apple Silicon via Asahi, Ampere cloud instances). ALVR could support these platforms with minimal changes.