add check with sysctl for bf16 support

Author: ignaciosica

vllm/platforms/cpu.py

@@ -75,11 +75,13 @@ class CpuPlatform(Platform):
     def supported_dtypes(self) -> list[torch.dtype]:
         if self.get_cpu_architecture() == CpuArchEnum.POWERPC:
             return [torch.bfloat16, torch.float32]
-        elif sys.platform.startswith(
-                "darwin") and self.get_cpu_architecture() == CpuArchEnum.ARM:
-            # TODO: change this condition to check if the platform support bf16
-            # instead of checking the OS.
-            return [torch.bfloat16, torch.float16, torch.float32]
+        elif (self.get_cpu_architecture() == CpuArchEnum.ARM
+            and sys.platform.startswith("darwin")):
+            if (subprocess.check_output(
+                    ["sysctl", "-n", "-e", "hw.optional.arm.FEAT_BF16"]
+                ).strip() == b"1"):
+                return [torch.bfloat16, torch.float16, torch.float32]
+            return [torch.float16, torch.float32]
         # x86/aarch64 CPU has supported both bf16 and fp16 natively.
         return [torch.bfloat16, torch.float16, torch.float32]