python: limit tooltip string length to 16384

Pydot limits the maximum line length to this value

JIRA:https://gem5.atlassian.net/browse/GEM5-1200

Change-Id: I0e6423b79f014695496dad279322304ae10a3978
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/61009
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

src/python/m5/util/dot_writer.py

@@ -149,7 +149,13 @@ def dot_create_cluster(simNode, full_path, label):
             ini_strings.append(str(param) + "=" +
                                simNode._values[param].ini_str())
     # join all the parameters with an HTML newline
+    # Pydot limit line length to 16384.
+    # Account for the quotes added later around the tooltip string
     tooltip = "
\\".join(ini_strings)
+    max_tooltip_length = 16384 - 2
+    if len(tooltip) > max_tooltip_length:
+        truncated = '... (truncated)'
+        tooltip = tooltip[:max_tooltip_length-len(truncated)] + truncated
 
     return pydot.Cluster( \
                          full_path, \