sfifo_observe_gate_q is stuck at reset in entropy_src

[Nitsirks]

This register sfifo_observe_gate_q never gets out of reset because the set condition can't ever be true. The observe push is qualified by the observe full already, so we never detect when there was an attempt to push the fifo while it was full.

[moidx]

Hi @vogelpi, can we get your input here?

sfifo_observe_push is qualified by !sfifo_observe_full

caliptra-rtl/src/entropy_src/rtl/entropy_src_core.sv

Lines 2359 to 2361 in eb9e1d6

	// fifo controls
	assign sfifo_observe_push = fw_ov_mode && pfifo_postht_pop && !sfifo_observe_full &&
	(sfifo_observe_gate_q || !sfifo_observe_not_empty);

but then used in a condition with sfifo_observe_full, which triggers a linter error:

caliptra-rtl/src/entropy_src/rtl/entropy_src_core.sv

Lines 2338 to 2342 in eb9e1d6

	assign sfifo_observe_gate_d = (sfifo_observe_push && sfifo_observe_full) ? 1'b0 :
	!sfifo_observe_not_empty ? 1'b1 :
	sfifo_observe_gate_q;

This condition was updated in lowRISC/opentitan#21799, but the team is wondering if there is an incremental way to solve this problem without having to rebase the entire block.

@howardtr, it may be worth discussing the changes made to the entropy complex in the past six months. There have been many fixes introduced for issues found during DV and silicon testing. We can try to summarize the issues so that you can make a call on whether or not to cherry pick changes.

[howardtr]

@moidx - Thanks for summarizing the issue above

We'll certainly pick up the changes for the entropy block for caliptra 2.0 (next release), but for caliptra 1.1, we're trying to find an incremental set of changes as you mentioned

[vogelpi]

Hi,
21799 added another FIFO to deal with internal backpressure. Since this FIFO is inserted before the Observe FIFO, the gating condition needed to be updated. But it's a rather big change not easy to port over.

However, there was another PR before 21799 which fixed the gating condition issue. See lowRISC/opentitan#21640 . This one should be easy to cherry-pick over.

For your next release, I definitely recommend taking everything over including the big PR. As @moidx pointed out, there have been many changes improving the functionality and the verification of the module.