console putchar/getchar ecalls and sbi v02

[Anup Patel]

On Wed, Jun 1, 2022 at 8:25 PM Heiko Stübner <heiko at sntech.de> wrote:
>
> Hi Anup,
>
> Am Mittwoch, 1. Juni 2022, 16:46:17 CEST schrieb Anup Patel:
> > Hi Heiko,
> >
> > On Wed, Jun 1, 2022 at 8:00 PM Heiko Stübner <heiko at sntech.de> wrote:
> > >
> > > Hi,
> > >
> > > I just stumbled across the issue that the console_putchar and _getchar
> > > ecalls are only part of the v0.1 specification and the spec states that
> > > they "are expected to be deprecated; they have no replacement."
> > >
> > > As I was using that for their earlycon functionality for a bit and haven't
> > > found the reasoning behind the deprecation so far, I guess I'm curious
> > > why it is going away :-) .
> > >
> > > The commit adding the spec-note [0] also gives no explanation for it
> > > and Google also wasn't helpful.
> > >
> > > Any hints?
> >
> > Almost all SBI v0.1 calls have been replaced by SBI v0.2 extensions,
> > except console_putchar() and _getchar() calls.
> >
> > The console_putchar() was certainly quite useful for early prints but
> > since it is going to be deprecated, we need something equivalent or
> > better in the SBI spec.
> >
> > I do have a draft "SBI debug console" proposal which is based on
> > shared memory and replaces the SBI v0.1 console_putchar(). I will
> > send-out this proposal soon.
>
> Hmm, that sounds great for the earlycon-portion, but could you maybe
> also think about a really simple putchar option like the current one?
>
> Background is that the current putchar works in all conditions, so even
> in the earliest kernel startup code I could manually put in a
> sbi_console_putchar() call to track down where a hang was.
>
> That even worked in the early startup assembly when doing the ecall
> manually.
>
> This is extremely helpful when you are on "consumer-grade"
> hardware were other debug options are somewhat limited.

Setting up shared memory is really simple. In fact, it is just one
additional call for assembly level debugging.

Once the shared memory is set up, users can print any number
of characters in one SBI call from assembly sources.

Regards,
Anup

>
>
> Thanks
> Heiko
>
>
>
> > >
> > >
> > > Thanks
> > > Heiko
> > >
> > >
> > >
> > > [0] https://github.com/riscv-non-isa/riscv-sbi-doc/commit/705e9556d25dc070926b99f8ccbcdc5073407447
> > >
> > >
> >
>
>
>
>