Make ~LONGESTPERIOD return 100 ns at minimum

Sidesteps #2455

Author: martijnbastiaan

changelog/2023-04-25T08_32_54+02_00_minimum_longest_period

@@ -0,0 +1 @@
+CHANGED: Clock generators now wait at least 100 ns before producing their first tick. This change has been implemented to account for Xilinx's GSR in clock synchronization primitives. This change does not affect Clash simulation. See [#2455](https://github.com/clash-lang/clash-compiler/issues/2455).

clash-lib/src/Clash/Netlist/BlackBox/Types.hs

@@ -179,7 +179,8 @@ data Element
   -- ^ Period of a domain. Errors if not applied to a @KnownDomain@ or
   -- @KnownConfiguration@.
   | LongestPeriod
-  -- ^ Longest period of all known domains
+  -- ^ Longest period of all known domains. The minimum duration returned is
+  -- 100 ns, see https://github.com/clash-lang/clash-compiler/issues/2455.
   | ActiveEdge !Signal.ActiveEdge !Int
   -- ^ Test active edge of memory elements in a certain domain. Errors if not
   -- applied to a @KnownDomain@ or @KnownConfiguration@.

clash-lib/src/Clash/Netlist/BlackBox/Util.hs

@@ -14,6 +14,7 @@
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE NumericUnderscores #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE QuasiQuotes #-}
 {-# LANGUAGE RankNTypes #-}
@@ -463,7 +464,9 @@ renderElem b (Period n) = do
 
 renderElem _ LongestPeriod = do
   doms <- domainConfigurations
-  let longestPeriod = maximum [vPeriod v | v <- HashMap.elems doms]
+  -- Longest period with a minimum of 100 ns, see:
+  -- https://github.com/clash-lang/clash-compiler/issues/2455
+  let longestPeriod = maximum (100_000 : [vPeriod v | v <- HashMap.elems doms])
   return (const (Text.pack (show longestPeriod)))
 
 renderElem b (Tag n) = do