[GDN] Remove FlashInfer GDN decode + no_buffer guard and default to FlashInfer on SM100+ (#21861)

python/sglang/srt/server_args.py

@@ -2188,19 +2188,6 @@ class ServerArgs:
                 not self.enable_mamba_extra_buffer()
             ), f"mamba extra_buffer is not supported for {model_arch} model"
 
-        # FlashInfer GDN decode is incompatible with no_buffer scheduling.
-        # See https://github.com/sgl-project/sglang/issues/20791
-        if (
-            self.linear_attn_decode_backend == "flashinfer"
-            and self.mamba_scheduler_strategy == "no_buffer"
-        ):
-            raise ValueError(
-                "FlashInfer GDN decode (--linear-attn-decode-backend flashinfer) is not "
-                "compatible with --mamba-scheduler-strategy no_buffer. "
-                "Please use --mamba-scheduler-strategy extra_buffer instead. "
-                "See https://github.com/sgl-project/sglang/issues/20791"
-            )
-
         if self.enable_mamba_extra_buffer():  # extra_buffer
             if self.disable_radix_cache:
                 raise ValueError(
@@ -2627,9 +2614,27 @@ class ServerArgs:
                 )
 
     def _handle_linear_attn_backend(self):
-        # SM100+ FlashInfer GDN decode requires bf16 state; SM90 uses float32.
         import torch
 
+        # SM100+: default to FlashInfer GDN decode when the user hasn't
+        # explicitly chosen a decode backend and mamba-ssm-dtype is bf16
+        # (required by FlashInfer GDN on SM100+).
+        # Fixed in FlashInfer v0.6.7: flashinfer-ai/flashinfer#2810
+        # Excluded when MTP speculative decoding is enabled because
+        # FlashInfer GDN MTP verify is not yet supported on SM100+.
+        if (
+            self.linear_attn_decode_backend is None
+            and is_sm100_supported()
+            and self.mamba_ssm_dtype == "bfloat16"
+            and self.speculative_algorithm is None
+        ):
+            self.linear_attn_decode_backend = "flashinfer"
+            logger.info(
+                "SM100+ detected with mamba-ssm-dtype=bfloat16, "
+                "defaulting --linear-attn-decode-backend to flashinfer."
+            )
+
+        # SM100+ FlashInfer GDN decode requires bf16 state; SM90 uses float32.
         decode = self.linear_attn_decode_backend or self.linear_attn_backend
         if (
             decode == "flashinfer"