[csrng/lint] Waive VAR_INDEX_RANGE warning shown in new tool versions

The corresponding RTL is okay, the computed index is always valid.
Otherwise, we end up in a different branch and the index is not used.
It's okay to waive this warning.

Signed-off-by: Pirmin Vogel <vogelpi@lowrisc.org>

Author: vogelpi

hw/ip/csrng/lint/csrng.waiver

@@ -8,3 +8,5 @@ waive -rules {ONE_BIT_MEM_WIDTH} -location {prim_arbiter_ppc.sv} -regexp {.*has
       -comment "Usage case specific to CSRNG and how the arbiter is used."
 waive -rules {LHS_TOO_SHORT} -location {aes_cipher_control_fsm.sv aes_cipher_core.sv aes_key_expand.sv aes_sbox.sv} -regexp {Bitlength mismatch between 'unused_assert_static_lint_error' length 1 and.*} \
       -comment "CSRNG intentionally uses an unmasked AES implementation."
+waive -rules {VAR_INDEX_RANGE} -location {csrng_ctr_drbg.sv} -regexp {'\(NumBlkPerUpd - 1 - concat_ctr_q\) \* BlkLen \+: BlkLen' minimum value -128 is too small for 'concat_key_v_d' range.*} \
+      -comment "concat_ctr_q must be lower than NumBlkPerUpd, otherwise concat_ctr_done is set and we end up in the other if/else branch, the index into concat_key_v_d is always valid."

hw/ip/csrng/rtl/csrng_ctr_drbg.sv

@@ -285,7 +285,9 @@ module csrng_ctr_drbg import csrng_pkg::*; (
       concat_key_v_d = '0;
     end else if (concat_ctr_inc) begin
       concat_ctr_d = concat_ctr_q + 1;
-      // Steer the v-response from block encrypt to the correct lane, MSB down
+      // Steer the v-response from block encrypt to the correct lane, MSB down.
+      // concat_ctr_q must be lower than NumBlkPerUpd, otherwise concat_ctr_done is set and we end
+      // up in the if branch above, i.e., the index into concat_key_v_d is always valid.
       concat_key_v_d[(NumBlkPerUpd - 1 - concat_ctr_q) * BlkLen +: BlkLen]
         = block_encrypt_rsp_data_i;
     end