[util] Document default resval for non-hwext registers
Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/util/reggen/field.py b/util/reggen/field.py
index a85fe7f..56fe9a7 100644
--- a/util/reggen/field.py
+++ b/util/reggen/field.py
@@ -141,11 +141,16 @@
if raw_resval is None:
# The field doesn't define a reset value. Use bits from reg_resval
- # if it's defined, otherwise None (which means "x").
- if reg_resval is None:
+ # if it's defined. If not, we assume that a hwext register comes up
+ # as "x" (because the reggen code doesn't have any way to predict
+ # it). That's represented as None. A non-hwext register is reset to
+ # zero.
+ if reg_resval is not None:
+ resval = bits.extract_field(reg_resval) # type: Optional[int]
+ elif hwext:
resval = None
else:
- resval = bits.extract_field(reg_resval)
+ resval = 0
else:
# The field does define a reset value. It should be an integer or
# 'x'. In the latter case, we set resval to None (as above).
