[Libre-soc-dev] [OpenPOWER-HDL-Cores] bug in libre-soc "modsd" and possibly in microwatt as well

[Luke Kenneth Casson Leighton]

On Sun, Aug 23, 2020 at 2:55 PM Luke Kenneth Casson Leighton
<lkcl at lkcl.net> wrote:

> which now leaves me *really* confused because in that debug int
> regfile dump you can clearly see r0 == 0xff not 0x7fff_ffff yet the
> answer being returned in r17 is "0x1".
>
> this leads me to suspect that there's something wrong with the DMI FSM
> i created.

found it, sigh.  the FSM that issues the DMI "GPR ADDR set" was being
corrupted by a prior command, leaving the core_debug.vhdl address at
its previous value, which happened to be 31 (from the end of the loop
of getting int regs 0-31).  thus the value reported for int reg 0 was
in fact reg 31.

sigh.

fixed it and also added a better FSM, one that now waits for "core stopping".

paul, mikey: i am however noticing that _some_ operations - div, mul,
shift in particular - simply do not report their values correctly even
after "stabilisation".  only after the *next* "STEP" does the regfile
update and the correct values can be enumerated.

this is... slightly annoying because a comparative diff
(cycle-accurate) is impossible.  can this be fixed in microwatt, such
that any outstanding pipeline operations are not paused and are
allowed to complete and update the regfile?

l.