hdl.{ast,cd,dsl,xfrm}: reject inappropriately used comb domain.

Fixes #125.

nmigen/hdl/ast.py

@@ -693,6 +693,8 @@ class ClockSignal(Value):
         super().__init__(src_loc_at=src_loc_at)
         if not isinstance(domain, str):
             raise TypeError("Clock domain name must be a string, not '{!r}'".format(domain))
+        if domain == "comb":
+            raise ValueError("Domain '{}' does not have a clock".format(domain))
         self.domain = domain
 
     def shape(self):
@@ -727,6 +729,8 @@ class ResetSignal(Value):
         super().__init__(src_loc_at=src_loc_at)
         if not isinstance(domain, str):
             raise TypeError("Clock domain name must be a string, not '{!r}'".format(domain))
+        if domain == "comb":
+            raise ValueError("Domain '{}' does not have a reset".format(domain))
         self.domain = domain
         self.allow_reset_less = allow_reset_less
 

nmigen/hdl/cd.py

@@ -50,6 +50,8 @@ class ClockDomain:
                 raise ValueError("Clock domain name must be specified explicitly")
         if name.startswith("cd_"):
             name = name[3:]
+        if name == "comb":
+            raise ValueError("Domain '{}' may not be clocked".format(name))
         self.name = name
 
         self.clk = Signal(name=self._name_for(name, "clk"), src_loc_at=1)

nmigen/hdl/dsl.py

@@ -261,6 +261,8 @@ class Module(_ModuleBuilderRoot, Elaboratable):
     @contextmanager
     def FSM(self, reset=None, domain="sync", name="fsm"):
         self._check_context("FSM", context=None)
+        if domain == "comb":
+            raise ValueError("FSM may not be driven by the '{}' domain".format(domain))
         fsm_data = self._set_ctrl("FSM", {
             "name":     name,
             "signal":   Signal(name="{}_state".format(name), src_loc_at=2),

nmigen/hdl/xfrm.py

@@ -326,6 +326,11 @@ class DomainRenamer(FragmentTransformer, ValueTransformer, StatementTransformer)
     def __init__(self, domain_map):
         if isinstance(domain_map, str):
             domain_map = {"sync": domain_map}
+        for src, dst in domain_map.items():
+            if src == "comb":
+                raise ValueError("Domain '{}' may not be renamed".format(src))
+            if dst == "comb":
+                raise ValueError("Domain '{}' may not be renamed to '{}'".format(src, dst))
         self.domain_map = OrderedDict(domain_map)
 
     def on_ClockSignal(self, value):