[DSV4/SM120] Allow fused MHC opt-in with standalone TileLang pre disabled (#30954)

Signed-off-by: David Orman <ormandj@corenode.com>
Co-authored-by: Mohammad Miadh Angkad <176301910+mmangkad@users.noreply.github.com>

python/sglang/srt/models/deepseek_v4.py

@@ -153,6 +153,7 @@ from sglang.srt.utils import (
     log_info_on_rank0,
     make_layers,
 )
+from sglang.srt.utils.common import is_sm120_supported
 from sglang.srt.utils.custom_op import register_custom_op
 from sglang.srt.utils.hf_transformers_utils import get_rope_config
 
@@ -204,12 +205,13 @@ DEEPSEEK_V4_STACKED_PARAMS_MAPPING: List[Tuple[str, str, int]] = [
 
 
 def _is_fused_mhc_post_pre_enabled() -> bool:
-    # The fused path directly reuses TileLang mhc_post/mhc_pre kernels and their
-    # tensor layout assumptions, so keep it disabled when either dependency is off.
+    # SM120 disables the standalone TileLang pre path. mhc_fused_post_pre does
+    # not read that flag and dispatches independently for both small and large
+    # token batches, so the standalone pre flag must not veto the fused opt-in.
     return (
         envs.SGLANG_OPT_FUSE_MHC_POST_PRE.get()
-        and envs.SGLANG_OPT_USE_TILELANG_MHC_PRE.get()
         and envs.SGLANG_OPT_USE_TILELANG_MHC_POST.get()
+        and (envs.SGLANG_OPT_USE_TILELANG_MHC_PRE.get() or is_sm120_supported())
     )