[kmac] Add entropy_mode restriction
As discussed in #6831, entropy_mode cannot be changed after
entropy_ready to prevent the KMAC from using LFSR-only entropies. This
commit adds the description to `entropy_mode` field.
Signed-off-by: Eunchan Kim <eunchan@opentitan.org>
diff --git a/hw/ip/kmac/data/kmac.hjson b/hw/ip/kmac/data/kmac.hjson
index 406caee..ee20541 100644
--- a/hw/ip/kmac/data/kmac.hjson
+++ b/hw/ip/kmac/data/kmac.hjson
@@ -318,6 +318,11 @@
In SwMode, the software should update the internal LFSR seed
through !!ENTROPY_SEED_LOWER and !!ENTROPY_SEED_UPPER.
+
+ Out of the reset, the entropy module inside KMAC IP generates LFSR to
+ support the ROM_CTRL operation. The logic does not go back to the
+ reset state to prevent KMAC from using LFSR with seeds. SW must
+ configure correct entropy_mode prior to setting the entropy_ready.
'''
enum: [
