Re: Dovetail on risc-v: Testing strategy, managing efforts for forward porting

Florian Bezdeka <[email protected]> writes:

> Hi Philippe,
>
> I'm currently waiting for a v8 of Tobias' risc-v implementation. A
> couple of minor things still need fixing, but overall I would say that
> we are looking good - ready for merging - assuming that currently
> running tests are successful.
>

Really nice stuff, thanks to anyone involved.

> From my point of view there is one issue left, that needs a discussion:
> Testing and forward porting in the future.
>
> Up to now I was able to test all architectures using cobalt / x3. That
> will change now, as we don't have risc-v support in x3.
>
> I see the following options:
>
> 1: Additionally forward port linux-evl in the future, so that we can run
> tests against risc-v using x4. Additional effort for me, especially
> because we don't have an in-house user for that. Can you estimate how
> much time it takes you to forward port linux-evl in comparison to linux-
> dovetail?
>

Porting evl to a new arch kernel-wise is fundamentally trivial, the real
work is done by Dovetail. For every currently supported arch, the amount
of arch-specific evl code is 60 lines of C, merely wrappers in headers
(arch/*/include/asm/evl/*.h). Tobias already contributed the few bits
libevl needs to support risc-v.

> 2: Add risc-v support to x3 as well. To my understanding there were no
> plans yet, but it might be worth it to limit forward porting efforts. I
> could focus on x3 for testing. This might also help to compare risc-v
> against the other architectures.
>

Although this may require a little more effort compared to x4, such port
would still be tractable since Dovetail does most of the work here as
well. Adding risc-v support to x3 is Jan's call.

> 3: Ignore risc-v during forward porting. Yes, this option exists, but I
> don't consider it as a real / valid option.
>

Neither do I. Merged implies maintained.

> 4: Merge the risc-v support into linux-evl. That would make risc-v
> support exclusive to evl, where we could already test it. Would that be
> an valid option for the evl side?
>

No, I believe that we can deal with one arch more such as risc-v in the
Dovetail maintenance process. Besides, we really want the generic
Dovetail code to be shared by all architectures, otherwise this would
become a maintenance nightmare for the risc-v port.

> Any additional option available, that I missed?
>

I'd say, the simpler the better. Today, enabling risc-v for x4 would be
straightforward while keeping the current maintenance model for Dovetail
mostly unaffected.  Granted, forward porting to the latest kernel
release may require a bit more work in order to maintain the risc-v part
too, but my understanding is that Tobias is willing to help with this in
the long term. AFAICS, the arch-specific part in risc-v for Dovetail is
fairly reasonable in size as well.

So, first step would be to merge the Dovetail risc-v support into
linux-dovetail like other supported archs, enabling it for x4/evl in the
same move. It could be enabled for x3 next, when time allows on your end
if desired.

-- 
Philippe.