build.dsl: clean up inversion logic.

* Add invert= argument to DiffPairs() constructor, like in Pins().
  * Make PinsN() and DiffPairsN() pass invert= to the corresponding
    construtor instead of mutating.

nmigen/build/dsl.py

@@ -55,13 +55,11 @@ class Pins:
 
 
 def PinsN(*args, **kwargs):
-    pins = Pins(*args, **kwargs)
-    pins.invert = True
-    return pins
+    return Pins(*args, invert=True, **kwargs)
 
 
 class DiffPairs:
-    def __init__(self, p, n, *, dir="io", conn=None, assert_width=None):
+    def __init__(self, p, n, *, dir="io", invert=False, conn=None, assert_width=None):
         self.p = Pins(p, dir=dir, conn=conn, assert_width=assert_width)
         self.n = Pins(n, dir=dir, conn=conn, assert_width=assert_width)
 
@@ -71,7 +69,7 @@ class DiffPairs:
                             .format(self.p, self.n))
 
         self.dir    = dir
-        self.invert = False
+        self.invert = bool(invert)
 
     def __len__(self):
         return len(self.p.names)
@@ -85,9 +83,7 @@ class DiffPairs:
 
 
 def DiffPairsN(*args, **kwargs):
-    diff_pairs = DiffPairs(*args, **kwargs)
-    diff_pairs.invert = True
-    return diff_pairs
+    return DiffPairs(*args, invert=True, **kwargs)
 
 
 class Attrs(OrderedDict):

tests/test_build_dsl.py

@@ -94,6 +94,10 @@ class DiffPairsTestCase(FHDLTestCase):
         self.assertEqual(dp.n.names, ["B0"])
         self.assertEqual(dp.invert, True)
 
+    def test_invert_arg(self):
+        dp = DiffPairs(p="A0", n="B0", invert=True)
+        self.assertEqual(dp.invert, True)
+
     def test_conn(self):
         dp = DiffPairs(p="0 1 2", n="3 4 5", conn=("pmod", 0))
         self.assertEqual(list(dp), [