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[BugFix] Skip marlin backend for fp4 on SM100 (Blackwell) - fixes #43988 #44091

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[BugFix] Skip marlin backend for fp4 on SM100 (Blackwell) - fixes #43988 #44091

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765a3f7
python/sglang/srt/layers/quantization/fp4_utils.py: Skip marlin backe…
yancy Oct 13, 2026
061e040
style: apply pre-commit lint fixes
yancy Oct 13, 2026
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15 changes: 15 additions & 0 deletions python/sglang/srt/layers/quantization/fp4_utils.py
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Original file line number Diff line number Diff line change
Expand Up @@ -159,6 +159,21 @@ def initialize_fp4_gemm_config() -> None:
else:
backend = "flashinfer_cutlass"

# FP4 Marlin is weight-only (W4A16) and does not support W4A4 GEMM.
# FlashInfer's mm_fp4 rejects backend='marlin' on SM100+, so force a
# fallback whenever the user (or auto-selection) picks marlin on
# Blackwell GPUs. See issue #43988.
if backend == "marlin" and get_platform().is_blackwell:
fallback = (
"flashinfer_cutedsl" if get_platform().is_sm100 else "flashinfer_cutlass"
)
logger.warning(
"FP4 Marlin does not support SM100+ (Blackwell); falling back to %s. "
"FlashInfer's mm_fp4 has no 'marlin' backend for capability 100.",
fallback,
)
backend = fallback

FP4_GEMM_RUNNER_BACKEND = Fp4GemmRunnerBackend(backend)


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