Introduce _is_host_cpu utility predicate used in __dlpack__

_is_host_cpu(dl_device) checks if user request export for
host CPU device. Recognized inputs are

(1, 0)
(_usmarray.DLDeviceType.kDLCPU, 0)
("kDLCPU", 0)

Add test to exercise __dlpack__ with non-default dl_device
keyword arguments

Author: oleksandr-pavlyk

dpctl/tensor/_usmarray.pyx

@@ -87,10 +87,34 @@ cdef object _as_zero_dim_ndarray(object usm_ary):
     view.shape = tuple()
     return view
 
+
 cdef int _copy_writable(int lhs_flags, int rhs_flags):
     "Copy the WRITABLE flag to lhs_flags from rhs_flags"
     return (lhs_flags & ~USM_ARRAY_WRITABLE) | (rhs_flags & USM_ARRAY_WRITABLE)
 
+
+cdef bint _is_host_cpu(object dl_device):
+    "Check if dl_device denotes (kDLCPU, 0)"
+    cdef object dl_type
+    cdef object dl_id
+    cdef Py_ssize_t n_elems = -1
+
+    try:
+        n_elems = len(dl_device)
+    except TypeError:
+        pass
+
+    if n_elems != 2:
+        return False
+
+    dl_type = dl_device[0]
+    dl_id = dl_device[1]
+    if isinstance(dl_type, str):
+        return (dl_type == "kDLCPU" and dl_id == 0)
+
+    return (dl_type == DLDeviceType.kDLCPU) and (dl_id == 0)
+
+
 cdef class usm_ndarray:
     """ usm_ndarray(shape, dtype=None, strides=None, buffer="device", \
            offset=0, order="C", buffer_ctor_kwargs=dict(), \
@@ -1148,8 +1172,7 @@ cdef class usm_ndarray:
                             raise BufferError(
                                 "array cannot be placed on the requested device without a copy"
                             )
-                        if dl_device[0] == (DLDeviceType.kDLCPU):
-                            assert dl_device[1] == 0
+                        if _is_host_cpu(dl_device):
                             if stream is not None:
                                 raise ValueError(
                                     "`stream` must be `None` when `dl_device` is of type `kDLCPU`"

dpctl/tests/test_usm_ndarray_dlpack.py

@@ -24,6 +24,8 @@
 import dpctl.tensor as dpt
 import dpctl.tensor._dlpack as _dlp
 
+import dpctl.tensor._usmarray as dpt_arr
+
 device_oneAPI = 14  # DLDeviceType.kDLOneAPI
 
 _usm_types_list = ["shared", "device", "host"]
@@ -470,3 +472,21 @@ def test_dlpack_kwargs():
     assert y._pointer != x2._pointer
     del x2, y
     del cap
+
+
+def test_dlpack_dl_device():
+    try:
+        x = dpt.arange(100, dtype="i4")
+    except dpctl.SyclDeviceCreationError:
+        pytest.skip("No SYCL devices available")
+    max_supported_ver = _dlp.get_build_dlpack_version()
+    cap1 = x.__dlpack__(dl_device=x.__dlpack_device__(), max_version=max_supported_ver)
+    cap2 = x.__dlpack__(dl_device=(1, 0), max_version=max_supported_ver)
+    cap3 = x.__dlpack__(dl_device=(dpt_arr.DLDeviceType.kDLCPU, 0), max_version=max_supported_ver)
+    cap4 = x.__dlpack__(dl_device=("kDLCPU", 0), max_version=max_supported_ver)
+    with pytest.raises(NotImplementedError):
+        # pass method instead of return of its __call__ invocation
+        cap5 = x.__dlpack__(dl_device=x.__dlpack_device__, max_version=max_supported_ver)
+    with pytest.raises(NotImplementedError):
+        # exercise check for length
+        cap6 = x.__dlpack__(dl_device=(3,), max_version=max_supported_ver)