Re: [PATCH] Semihost SYS_READC implementation (v4)

[Peter Maydell]

On Fri, 25 Oct 2019 at 20:15, Keith Packard <address@hidden> wrote:
>
> Peter Maydell <address@hidden> writes:
>
> > Is there a specification for RISC-V semihosting? This is
> > likely to be my first question when the support comes
> > round for review, so you can have it early :-)  We'd
> > prefer to implement specified interfaces, not random
> > ad-hoc "this seems to be what newlib wants to see,
> > which is turn got hacked together by copying some other
> > architecture's code".
>
> There seems to be convergence on a pretty simple interface which uses
> ebreak surrounded by a couple of specific no-ops:
>
>       slli x0, x0, 0x1f
>       ebreak
>       srai x0, x0, 0x7
>
> There are implementations in rust and openocd, and I've got one for
> picolibc.

I'm going to push for somebody actually writing out a
document and putting it somewhere that we can point to
and say "that's the authoritative spec", please...
it doesn't have to be a big formal thing, but I do
think you want it written down, because the whole point
is for multiple implementations and users to interoperate.


> > Isn't the answer to this "don't use a command line that tries
> > to connect stdio to multiple things" ?
>
> Uh, we do that all the time? The mux device is designed to handle this
> so that you can use stdio for both monitor commands and application
> I/O. It's very convenient, the only issue is that the last device that
> hooks to the mux ends up getting input first (you use ^Ac to rotate
> among the selected devices).

Yeah, the mux works fine for this kind of thing. There's
no inherent reason why semihosting ought to "win" as
the initially selected thing on the mux, though --
typically that would be expected to be the UART/serial
console.

thanks
-- PMM