dcp: don't fail boot when B12X DCP collectives don't support the world size

tests/distributed/test_dcp_a2a.py

@@ -438,6 +438,32 @@ def test_b12x_query_gather_requires_env(monkeypatch: pytest.MonkeyPatch):
     assert actual is expected
 
 
+def test_warmup_skips_unsupported_world_size(monkeypatch: pytest.MonkeyPatch):
+    from vllm.v1.attention.ops import dcp_alltoall
+
+    monkeypatch.setenv("VLLM_USE_B12X_DCP_A2A", "1")
+    monkeypatch.setattr(
+        dcp_alltoall,
+        "_try_b12x_dcp_all_gather_heads",
+        lambda *args, **kwargs: pytest.fail(
+            "warmup must not touch the B12X channel for world size 6"
+        ),
+    )
+    group = _FakeCPGroup(6, None)  # type: ignore[arg-type]
+
+    # Must log-and-return instead of raising: the runtime dispatchers fall
+    # back to NCCL for DCP world sizes without a B12X channel (e.g. TP6).
+    dcp_alltoall.warmup_b12x_dcp_a2a(
+        group,  # type: ignore[arg-type]
+        device=torch.device("cpu"),
+        dtype=torch.bfloat16,
+        max_batch_size=8192,
+        total_heads=66,
+        head_dim=512,
+        query_head_dim=576,
+    )
+
+
 class TestPackedA2AKernels:
     @pytest.mark.skipif(
         torch.accelerator.device_count() < 1, reason="CUDA is required."

vllm/model_executor/layers/attention/mla_attention.py

@@ -556,6 +556,11 @@ def __init__(
             self.dcp_a2a
             and envs.VLLM_USE_B12X_DCP_A2A
             and self.attn_backend.get_name() == "B12X_MLA_SPARSE"
+            # The B12X PCIe DCP channel only exists for world sizes 2/4/8;
+            # other DCP sizes (e.g. TP6 with DCP3/DCP6) use NCCL collectives.
+            and _vllm_config is not None
+            and _vllm_config.parallel_config.decode_context_parallel_size
+            in (2, 4, 8)
         )
         self.dcp_max_batch_size = (
             int(_vllm_config.scheduler_config.max_num_batched_tokens)

vllm/v1/attention/ops/dcp_alltoall.py

@@ -280,6 +280,16 @@ def warmup_b12x_dcp_a2a(
     """Create and exercise the B12X DCP channel before CUDA graph capture."""
     if not envs.VLLM_USE_B12X_DCP_A2A:
         return
+    if cp_group.world_size not in (2, 4, 8):
+        # The PCIe channel only exists for these world sizes. The runtime
+        # dispatchers already fall back to NCCL collectives per call, so an
+        # unsupported DCP size (e.g. TP6 with DCP3/DCP6) must not fail boot.
+        logger.warning_once(
+            "B12X PCIe DCP collectives support world sizes 2/4/8; "
+            "DCP world size %d uses NCCL collectives instead.",
+            cp_group.world_size,
+        )
+        return
     if query_head_dim is None:
         query_head_dim = head_dim
     local_query = torch.empty(