[lowrisc-dev] Porting TileLink2 to LowRISC

Sean Halle seanhalle at intensivate.com
Sun Jun 25 17:31:57 BST 2017


Hi Stefan,

Great discussion :-)

We are actually up against a decision at the moment.  We have to decide
within the next few days whether to put in the work of porting TL2 to
LowRISC.

To do that, I am hoping that we can get some kind of sense from the LowRISC
community about the likelihood that LowRISC will update to the new Berkeley
style.  What would you say, in your opinion, the probability is that
LowRISC will update to the new Berkeley Style?  Like 10% chance that
LowRISC will put in the effort to merge the new Berkley code and move to
Chisel3?   Or, is it a 90% chance that LowRISC is committed to staying in
sync with Berkeley, and so will do the work of updating to Chisel3, will
adopt the cake pattern, and will then do regular pulls from the Berkeley
code base?   To be clear, the question comes down to something specific --
will LowRISC at some point be doing frequent pulls from Berkeley code?  In
other words, I see three paths:

1) simply pull in pieces here and there -- mainly pull in the priviledged
spec when it updates -- so the bulk of the LowRISC code is unaffected.  The
bulk of LowRISC code continues as is, diverging from Berkeley.
2) bite the bullet and update all LowRISC code to the current Berkeley
version -- switch to Chisel3, adopt the cake pattern, and so forth -- but
then let the code diverge again..
3) the same as 2, do a full update of all the code, but then additionally
do frequent merges, to keep the code up to date with Berkeley.

Of those three paths, what percentage chance would you give to each?

Thanks Stefan,

Sean

P.S.  I'm thinking about reposting this under a new topic, to attract
everyone to weigh in  :-)



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On Sun, Jun 25, 2017 at 5:06 AM, Stefan Wallentowitz <stefan at wallentowitz.de
> wrote:

> On 25.06.2017 12:03, Sean Halle wrote:
> > I think the crux of this is whether the idea of merging Berkeley code
> > into LowRISC is realistic or not..  it seems like a very large amount of
> > work?  Do I have that right?  For example, might it even be easier be to
> > simply restart with the current Rocket repo and redo the integration
> > work, rather then trying to weed through all the merge conflicts..?  Or,
> > do I misunderstand the effort involved?
>
> I think Wei can best comment on this. The major addition of lowRISC is
> Tagged Memory and I agree the better approach is to rebase that work.
> Beside that it is actually TL2 that remains.
>
> > If that is the case, that the effort of a merge is prohibitive, and as a
> > result LowRISC never actually merge Berkeley code back in, then we would
> > be fine..  we port TL2 once, and then mainstream it into LowRISC, and
> > there's no more effort after that.  At least as long as that version is
> > good enough, then there is no strong reason to update it.
>
> For now its every privilege spec update that needs to be implemented,
> possibly further extensions if wanted. Then run-control debug and
> probably other stuff I am not even aware of.
>
> > Has there been any discussion about the likelihood and timing of
> > updating LowRISC to the new Berkeley style of things?  We have some
> > evidence that the new Berkeley style is less modular, and has a steeper
> > learning curve than the July 2016 version of the code.  We chose LowRISC
> > in part for this reason -- the older style is simpler, easier for us to
> > get up and running.
>
> Yeah, I agree on the learning curve issue, but am personally undecided
> if that is a good enough reason to permanently divert.
>
> Have a great weekend!
>
> Cheers,
> Stefan
>
>


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