[Toybox] FYI musl's support horizon.

[Rob Landley]

re: https://github.com/landley/toybox/pull/292#issuecomment-899895909

I asked Rich Felker on IRC and he said he/musl plans to _never_ drop support for
any old kernels, and 3.x or newer is plenty. (Didn't ask about compilers or
build environments because you're shipping binaries...)

Rob

<landley> dalias: it means support horizon, ala
https://landley.net/toybox/faq.html#support_horizon
<dalias> i think i gathered it's about support for old kernels. musl has no
moving target for that, or if it's moving it only moved further back
<dalias> so yeah, there is no 'support horizon' for musl. kernel versions
>=2.6.0 that work now will continue to work to the extent they do now
<dalias> you need mid to late 2.6.x for full posix functionality; there's a slim
possibility we may emulate more of that on older kernels in the future, but we
won't drop any compatibility that's already there
<dalias> making binaries to run on really old devices whose kernels can't be
upgraded because nobody has the source or there's hacked up vendor spaghetti
that's impossible to modernize is a major use case for musl
<dalias> we actually have someone who just got musl arm-eabi binaries running on
a pre-eabi kernel with a few minor hacks to the syscall entry point and similar
...
<landley> So you're targeting "2.6 kernel or newer"?
<landley> I dunno how to digest the above into a policy statement.
<landley> Other than cut and paste the whole thing...
<landley> Anyway, if you wanted to reply to the github thread directly, elliott
would get the ping.
<dalias> ok
<dalias> it's documented as such. 2.6.0 is min version but subject to
functionality limits of the kernel
<dalias> >= 2.6.39 is recommended for full functionality (but actually .29 or
something is all you need