[otbn] Fix multiple increment signals from decoder
When an instruction attempted to do multiple increments with indirect
register addressing (which produces an illegal instruction error),
multiple increment signals got set violating a unique constrait for the
case statement used to mux the possible increments.
Fixes #7884
Signed-off-by: Greg Chadwick <gac@lowrisc.org>
diff --git a/hw/ip/otbn/rtl/otbn_decoder.sv b/hw/ip/otbn/rtl/otbn_decoder.sv
index 8a85816..5c92027 100644
--- a/hw/ip/otbn/rtl/otbn_decoder.sv
+++ b/hw/ip/otbn/rtl/otbn_decoder.sv
@@ -562,13 +562,16 @@
end
if (insn[7]) begin
- d_inc_bignum = 1;
+ d_inc_bignum = 1'b1;
rf_we_base = 1'b1;
rf_wdata_sel_base = RfWdSelIncr;
end
if (insn[8] & insn[7]) begin
- illegal_insn = 1'b1;
+ // Avoid violating unique constraint for inc selection mux on an illegal instruction
+ a_wlen_word_inc_bignum = 1'b0;
+ d_inc_bignum = 1'b0;
+ illegal_insn = 1'b1;
end
end
3'b101: begin // BN.SID
@@ -591,7 +594,10 @@
end
if (insn[8] & insn[7]) begin
- illegal_insn = 1'b1;
+ // Avoid violating unique constraint for inc selection mux on an illegal instruction
+ a_wlen_word_inc_bignum = 1'b0;
+ b_inc_bignum = 1'b0;
+ illegal_insn = 1'b1;
end
end
3'b110: begin // BN.MOV/BN.MOVR
@@ -619,6 +625,9 @@
end
if (insn[9] & insn[7]) begin
+ // Avoid violating unique constraint for inc selection mux on an illegal instruction
+ a_inc_bignum = 1'b0;
+ d_inc_bignum = 1'b0;
illegal_insn = 1'b1;
end
end
@@ -947,9 +956,11 @@
`ASSERT(IbexRegImmAluOpBaseKnown, (opcode == InsnOpcodeBaseOpImm) |->
!$isunknown(insn[14:12]))
- // Can only do a single inc
+ // Can only do a single inc. Selection mux in controller doesn't factor in instruction valid (to
+ // ease timing), so these must always be one-hot to 0 to avoid violating unique constraint for mux
+ // case statement.
`ASSERT(BignumRegIncOnehot,
- insn_valid_o |-> $onehot0({a_inc_bignum, a_wlen_word_inc_bignum, b_inc_bignum, d_inc_bignum}))
+ $onehot0({a_inc_bignum, a_wlen_word_inc_bignum, b_inc_bignum, d_inc_bignum}))
// RfWdSelIncr requires active selection
`ASSERT(BignumRegIncReq,
