serve: --device hrx decodes without decode-split flash attention by default (#140)

[bong-water-water-bong]

Mitigates #140 until the kernel is fixed.

flash_attention_decode_split_next_q8 gives nondeterministic, sometimes wrong attention on HRX0. It also faults the GPU intermittently on Qwen3-Coder-30B-A3B, at the current pin and at 5556bf2 alike, so it isn't a recent regression.

model (Q4_K_M, decode)	with decode-split	without (new default)
Qwen3-0.6B	169 ± 34 tok/s; identical requests differ by up to 3.66 nats	322 tok/s, deterministic after warm-up
Qwen3-Coder-30B-A3B	GPU fault in 2 of 3 runs (80–88 tok/s when it survives)	66–71 tok/s, 5 of 5 runs
ZAYA1-8B	23.5 tok/s	25.5 tok/s

Behaviour:

• 1bit serve --device hrx starts llama-server with GGML_HRX_DISABLE_DISPATCH=decode_split.
• A value the user sets for that variable wins.
• ONEBIT_HRX_DECODE_SPLIT=1 turns the kernel back on, for testing a fix.
• --prefill-device hrx is unchanged, since it decodes on Vulkan.

Checked on strixhalo:

• serve_e2e --device hrx passes 3 of 3 on Qwen3-Coder-30B-A3B.
• The child's environment carries the variable by default, and not with ONEBIT_HRX_DECODE_SPLIT=1.

🤖 Generated with Claude Code

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Commit c64f7f3

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PR Reviewer Guide 🔍

Here are some key observations to aid the review process:

🎫 Ticket compliance analysis 🔶 140 - Partially compliant Compliant requirements: Mitigates nondeterministic and wrong attention output from flash_attention_decode_split_next_q8 on HRX0 Makes decode deterministic by default on HRX0 Allows user to override the default behavior via GGML_HRX_DISABLE_DISPATCH environment variable Allows testing of the decode-split kernel via ONEBIT_HRX_DECODE_SPLIT=1 Documents the change in docs/hrx.md Non-compliant requirements: None Requires further human verification: None

⏱️ Estimated effort to review: 2 🔵🔵⚪⚪⚪

🧪 No relevant tests

🔒 No security concerns identified

⚡ Recommended focus areas for review Environment variable handling The code checks for ONEBIT_HRX_DECODE_SPLIT to determine whether to disable the decode-split kernel. However, it only checks if the variable is set to "1", which might lead to unexpected behavior if the variable is set to any other value (including empty string or non-numeric values). This could cause the intended behavior to be bypassed unintentionally. const char* keep_split = std::getenv("ONEBIT_HRX_DECODE_SPLIT"); if (device == "hrx" && !(keep_split && std::string(keep_split) == "1")) env.push_back("GGML_HRX_DISABLE_DISPATCH=decode_split");