build/platform: allow doing a loose lookup_request (return None instead of ConstraintError) and allow subname in lookup_request.

In the platforms, insead of doing:
self.lookup_request("eth_clocks").rx
we can now do:
self.lookup_request("eth_clocks:rx")

This allows some try/except simplifications on constraints.

litex/build/altera/platform.py

@@ -33,6 +33,7 @@ class AlteraPlatform(GenericPlatform):
         return self.toolchain.build(self, *args, **kwargs)
 
     def add_period_constraint(self, clk, period):
+        if clk is None: return
         if hasattr(clk, "p"):
             clk = clk.p
         self.toolchain.add_period_constraint(self, clk, period)

litex/build/generic_platform.py

@@ -205,12 +205,20 @@ class ConstraintManager:
         self.matched.append((resource, obj))
         return obj
 
-    def lookup_request(self, name, number=None):
+    def lookup_request(self, name, number=None, loose=False):
+        subname = None
+        if ":" in name: name, subname = name.split(":")
         for resource, obj in self.matched:
             if resource[0] == name and (number is None or
                                         resource[1] == number):
+                if subname is not None:
+                    return getattr(obj, subname)
+                else:
                     return obj
 
+        if loose:
+            return None
+        else:
             raise ConstraintError("Resource not found: {}:{}".format(name, number))
 
     def add_platform_command(self, command, **signals):

litex/build/lattice/platform.py

@@ -34,6 +34,7 @@ class LatticePlatform(GenericPlatform):
         return self.toolchain.build(self, *args, **kwargs)
 
     def add_period_constraint(self, clk, period):
+        if clk is None: return
         if hasattr(clk, "p"):
             clk = clk.p
         self.toolchain.add_period_constraint(self, clk, period)

litex/build/microsemi/platform.py

@@ -28,6 +28,7 @@ class MicrosemiPlatform(GenericPlatform):
         return self.toolchain.build(self, *args, **kwargs)
 
     def add_period_constraint(self, clk, period):
+        if clk is None: return
         clk.attr.add("keep")
         if hasattr(clk, "p"):
             clk = clk.p

litex/build/xilinx/platform.py

@@ -48,6 +48,7 @@ class XilinxPlatform(GenericPlatform):
         return self.toolchain.build(self, *args, **kwargs)
 
     def add_period_constraint(self, clk, period):
+        if clk is None: return
         if hasattr(clk, "p"):
             clk = clk.p
         self.toolchain.add_period_constraint(self, clk, period)