[otbn,dv,doc] Describe some extra coverage points for loop handling
When implementing the coverage points for LOOP and LOOPI, I realised
that they missed some important corners of loop handling. In
particular, the LOOP* instructions can only track the *start* of
loops, and can't track their end. Here's a list of other things that
we should probably see happen.
Signed-off-by: Rupert Swarbrick <rswarbrick@lowrisc.org>
diff --git a/hw/ip/otbn/doc/dv/index.md b/hw/ip/otbn/doc/dv/index.md
index 811e2f7..8060bec 100644
--- a/hw/ip/otbn/doc/dv/index.md
+++ b/hw/ip/otbn/doc/dv/index.md
@@ -127,7 +127,18 @@
#### Loop stack
The [loop stack]({{< relref ".#loop-stack" >}}) is accessed by executing `LOOP` and `LOOPI` instructions.
-Important events for it are tracked at those instructions, rather than separately.
+Events concerning the start of loops are tracked at those instructions, but we can't track things like loop completion there.
+
+We expect to:
+- Complete a loop.
+- Complete the last loop on the stack (emptying it) after the stack has been full.
+- Complete a loop with one instruction and an iteration count of one.
+- Complete a loop with the maximal number of iterations.
+ Obviously, this isn't a reasonable thing to do for real in a test (there's a 32-bit iteration counter!).
+ The testbench will have to force a signal to skip past some iterations.
+- Run through a "badly nested" loop, where the body contains the final instruction from an outer loop (checking that we don't wrongly skip back).
+- Jump into a loop body from outside.
+- Jump/branch to the final instruction of a loop
#### Flags
