[J-core] j3/mmu and first silicon status.

[Rob Landley]

Part of the reason we've been so quiet recently is that J2 development
is now in feature freeze preparing for first silicon. We're happy enough
with what we've got that we want to do a small initial proving run of
actual ASICs. There's still some development on peripherals and bus size
tweaks going on, but the SOC core you'd run on Numato or Turtle hasn't
changed in a while and isn't expected to start changing again for a few
months.

That said, Jeff recently looked at the mmu design Hitachi did for the
sh3, and it's very heavyweight. We'd rather do a simpler implementation
for our first pass, and need to study what Linux actually wants out of
an MMU first. And if we're doing a new mmu design, we'd rather not open
that can of worms until after the ASIC tapes out. (Which is mostly
testing work, not development work.)

The problem with the existing sh3 MMU is there's no WAY you can fit it
in an lx9, and probably not in an lx25. The problem is their MMU walks
the page tables in software, which completely flushes a simple L1 cache
like j-core has. To work around that they added L2 cache and made the L1
4-way associative, and which drives the FPGA routing nuts. A simple
implementation of what they did would be several times larger than the
existing j2 SOC implementation _combined_. (We talked about making our
L1 2-way associative someday, but what the hitachi mmu needs is not a
"sweet spot".)

What we really want, to go along with our tiny processor implementation,
is an mmu that walks the page tables in _hardware_, at least for simple
faults where the TLB refill is just looking up the translation of an
existing physical page address. This would avoid the need to run page
fault handling code through the cache for "soft faults" (or run the same
code uncached, which would suck about as badly).

But what we really want is an mmu that's a good fit for Linux. Note that
Linus has strong opinions about this sort of thing:

http://yarchive.net/comp/powerpc_page_tables.html
http://yarchive.net/comp/linux/page_tables.html

And apparently wrote his master's thesis on it:

ftp://ftp.polsl.pl/pub/linux/kernel/people/torvalds/thesis/torvalds97.pdf

And so did Mel Gorman a few years later:

https://www.kernel.org/doc/gorman/

That said stuff's changed a lot since then (we're up to what, 5 levels
now?) so we'd like to take the time and get the design right.

We'd also want to have a patch ready to push into qemu to support the
new design if it's not the existing one...

So that's the current state of that work.

Rob