Check for supported tool versions

Define supported tool versions in tool_requirements.py, and check them
in a fusesoc run. If an unsupported tool version is found, fusesoc
outputs an error like this:

```
$ fusesoc --cores-root . run --target=lint lowrisc:ibex:ibex_core
INFO: Preparing lowrisc:ibex:check_tool_requirements:0.1
INFO: Preparing lowrisc:prim:assert:0.1
INFO: Preparing lowrisc:ibex:sim_shared:0
INFO: Preparing lowrisc:ibex:ibex_core:0.1
INFO: Setting up project

INFO: Running pre_build script check_tool_requirements
ERROR: verilator is too old: found version 4.010, need at least 4.028
ERROR: Tool requirements not fulfilled. Please update the tools and retry.
ERROR: Failed to build lowrisc:ibex:ibex_core:0.1 : pre_build script 'check_tool_requirements' exited with error code 1
```

The only version checked at this point is Verilator, which is set
somewhat arbitrarily to the version used by me (and I know it works). CI
uses a slightly newer version. As we are about to merge changes soon
which require a newer Verilator version, there's not much point in
finding the oldest supported version right now.

check_tool_requirements.core

@@ -0,0 +1,31 @@
+CAPI=2:
+# Copyright lowRISC contributors.
+# Licensed under the Apache License, Version 2.0, see LICENSE for details.
+# SPDX-License-Identifier: Apache-2.0
+name: "lowrisc:ibex:check_tool_requirements:0.1"
+description: "Check tool requirements"
+
+filesets:
+  files_check_tool_requirements:
+    files:
+      - ./util/check_tool_requirements.py : { copyto: util/check_tool_requirements.py }
+      - ./tool_requirements.py : { copyto: tool_requirements.py }
+
+scripts:
+  check_tool_requirements:
+    cmd:
+      - python3
+      - util/check_tool_requirements.py
+    # TODO: Use this syntax once https://github.com/olofk/fusesoc/issues/353 is
+    # fixed. Remove the filesets from the default target, and also remove the
+    # copyto.
+    #filesets:
+    #  - files_check_tool_requirements
+
+targets:
+  default:
+    filesets:
+      - files_check_tool_requirements
+    hooks:
+      pre_build:
+        - tool_verilator ? (check_tool_requirements)

ibex_core.core

@@ -4,6 +4,7 @@ CAPI=2:
 # SPDX-License-Identifier: Apache-2.0
 name: "lowrisc:ibex:ibex_core:0.1"
 description: "CPU core with 2 stage pipeline implementing the RV32IMC_Zicsr ISA"
+
 filesets:
   files_rtl:
     depend:
@@ -40,6 +41,10 @@ filesets:
     files:
       - lint/verilator_waiver.vlt: {file_type: vlt}
 
+  files_check_tool_requirements:
+    depend:
+     - lowrisc:ibex:check_tool_requirements
+
 parameters:
   RVFI:
     datatype: bool
@@ -73,12 +78,13 @@ targets:
   default:
     filesets:
       - files_rtl
-
+      - files_check_tool_requirements
   lint:
     filesets:
       - tool_verilator ? (files_lint_verilator)
       - files_rtl
       - files_lint
+      - files_check_tool_requirements
     parameters:
       - SYNTHESIS=true
       - RVFI=true

tool_requirements.py

@@ -0,0 +1,9 @@
+# Copyright lowRISC contributors.
+# Licensed under the Apache License, Version 2.0, see LICENSE for details.
+# SPDX-License-Identifier: Apache-2.0
+
+# Version requirements for various tools. Checked by tooling (e.g. fusesoc),
+# and inserted into the Sphinx-generated documentation.
+__TOOL_REQUIREMENTS__ = {
+    'verilator': '4.010',
+}

util/check_tool_requirements.py

@@ -0,0 +1,63 @@
+#!/usr/bin/python3
+# Copyright lowRISC contributors.
+# Licensed under the Apache License, Version 2.0, see LICENSE for details.
+# SPDX-License-Identifier: Apache-2.0
+
+from distutils.version import StrictVersion
+import logging as log
+import os
+import subprocess
+import sys
+
+# Display INFO log messages and up.
+log.basicConfig(level=log.INFO, format="%(levelname)s: %(message)s")
+
+# Populate __TOOL_REQUIREMENTS__
+topsrcdir = os.path.join(os.path.dirname(__file__), '..')
+exec(open(os.path.join(topsrcdir, 'tool_requirements.py')).read())
+
+def get_verilator_version():
+    try:
+        # Note: "verilator" needs to be called through a shell and with all
+        # arguments in a string, as it doesn't have a shebang, but instead
+        # relies on perl magic to parse command line arguments.
+        version_str = subprocess.run('verilator --version', shell=True,
+                                     check=True, stdout=subprocess.PIPE,
+                                     stderr=subprocess.STDOUT,
+                                     universal_newlines=True)
+        return version_str.stdout.split(' ')[1].strip()
+
+    except subprocess.CalledProcessError as e:
+        log.error("Unable to call Verilator to check version: " + str(e))
+        log.error(e.stdout)
+        return None
+
+def check_version(tool_name, required_version, actual_version):
+    if required_version is None or actual_version is None:
+        return False
+
+    if StrictVersion(actual_version) < StrictVersion(required_version):
+        log.error("%s is too old: found version %s, need at least %s",
+                  tool_name, actual_version, required_version)
+        return False
+    else:
+        log.info("Found sufficiently recent version of %s (found %s, need %s)",
+                 tool_name, actual_version, required_version)
+        return True
+
+
+def main():
+    any_failed = False
+
+    if not check_version('verilator', __TOOL_REQUIREMENTS__['verilator'],
+                         get_verilator_version()):
+        any_failed = True
+
+    if any_failed:
+        log.error("Tool requirements not fulfilled. "
+                  "Please update the tools and retry.")
+        return 1
+    return 0
+
+if __name__ == "__main__":
+    sys.exit(main())