[Libre-soc-dev] [OpenPOWER-HDL-Cores] microwatt dcache potential bug (overlap r0 and r1)

lkcl luke.leighton at gmail.com
Fri Jan 14 22:25:07 GMT 2022

On January 14, 2022 9:46:41 PM UTC, Paul Mackerras <paulus at ozlabs.org> wrote:

>> * dcache is currently *still processing r1.state=RELOAD_WAIT_ACK*,
>>   r1.full is *ZERO*, and yet it gets asked to process the incoming
>>   "std" operation.
>In this situation, the incoming store request should get put into

i'm seeing that [in dcache.py]

> and then processed once the state machine gets back to IDLE

this is what's not happening: it looks like it's getting processed immediately even whilst r1.state != IDLE. i have no idea why... yet.

did a 10min video btw https://m.youtube.com/watch?v=_Q8J10nBGeo

> In the VHDL, the variable 'req' represents a multiplexer that
>selects between r1.req (the stored request) and a request constructed
>from req_op, ra, r0.req, etc.

yes, ditto, dcache.py is a literal line-for-line translation of dcache.vhdl.  mostly.

>  This enables the state machine to
>handle either a request that has just arrived, or one that came in
>previously while the state machine was busy.  It's like a 1-entry
>FIFO (but with 0 latency in the empty case).

got it.  i have a req as well, what with dcache.py being a direct translation.  nmigen does not have the concept of VHDL "variables", though: instead the rules for combinatorial assignment are followed [last assigner "wins" and *all* uses even ones in earlier HDL use that last combinatorially-assigned value]

[this made the translation of the PLRU to nmigen parteeecularly hairy :) i had to create multiple signals with new names each time inside the loop. works great: is just hard to understand]

>> now, whether this is a bug in the porting of dcache.vhdl to dcache.py
>> or whether it's a bug in the original i honestly couldn't say.
>I haven't looked at your dcache.py translation.  Is it accessible
>somewhere? (not that I'm much good at python...)

:) the names have [not] been changed, to protect the innocent...

comb is combinatorial, sync is DFFs.  should be fairly self explanatory, what with the names being the same. mentally ignore the word "with" before anything and it's readable

>The VHDL certainly is designed to be able to handle a store coming in
>while r1.full = 0 and the state machine is in RELOAD_WAIT_ACK state.

hmm then i need to look at that closely.


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