[J-core] Is a small Linux distribution available for J2 on Numato Mimas?

[Rob Landley]

On 08/07/2016 09:06 PM, Rich Felker wrote:
> On Sun, Aug 07, 2016 at 03:16:45PM +0200, txt.file wrote:
>> It's probably easier to get involved in debian instead of starting a
>> complete new port on another distribution. At least debian already has
>> an SH4 port and there is already the plan to support J-Core.
>> Running Ubuntu on a 50 MHz single core cpu sounds awful to me.
>>
>> Gentoo also has an SH4 port.
> 
> To be able to use Debian SH4 directly on J3/J4, several things would
> have to happen:
> 
> - Either we'd need to add a little-endian build option for J-Core (I
>   think this would be a good idea to work on soon if it's not too
>   hard) or everything would have to be rebuilt for a big-endian
>   variant.

There's debian for ppc, so big endian support is already there, and
rebuilding for j-core isn't a big deal. (MMU and nommu have never run
the same binaries. I know you think they should, but you're alone in this.)

> - To support SMP, glibc would need to add runtime atomics selection
>   with support for cas.l rather than hard-coding gusa, which is
>   fundamentally UP-only. Having the kernel provide a vdso function for
>   atomics might be a good idea since SH has so many variants.

No, the logical thing to do is have a musl version of debian. Long ago
there was a uClibc version of debian, it can be done.

(I note that my post about doing so has been open half-finished for a
week, and was sent before I saw  this. :)

> Also, while Debian is a great resource in terms of its extensive
> package coverage, it also tends to have very large install-size and
> runtime memory needs, and so it may not be ideal for much of the early
> hardware people will be using for J3/J4.

Indeed. My main worry would be the 64 megabyte memory limit for lpddr.
(A 128 meg part exists, but I don't believe even turtle is using it. was
that a cost thing or compatibility with the dram controller?)

The other immediately interesting j2 target is buildroot,  which in
theory can already use an external toolchain ala musl-cross-make.

Rob