SBI Debug Console Extension Proposal (Draft v1)

[Heiko Stübner]

Am Donnerstag, 2. Juni 2022, 10:50:56 CEST schrieb Heiko Stübner:
> Am Donnerstag, 2. Juni 2022, 10:47:58 CEST schrieb Anup Patel:
> > On Thu, Jun 2, 2022 at 12:02 AM Heiko Stübner <heiko at sntech.de> wrote:
> > >
> > > Hi Anup,
> > >
> > > Am Mittwoch, 1. Juni 2022, 18:17:32 CEST schrieb Anup Patel:
> > > > Hi All,
> > > >
> > > > Below is the draft proposal for SBI Debug Console Extension.
> > > >
> > > > Please review it and provide feedback.
> > > >
> > > > Thanks,
> > > > Anup
> > > >
> > > > Debug Console Extension (EID #0x4442434E "DBCN")
> > > > ================================================
> > > >
> > > > The debug console extension defines a generic mechanism for boot-time
> > > > early prints from supervisor-mode software which allows users to catch
> > > > boot-time issues in supervisor-mode software.
> > > >
> > > > This extension replaces legacy console putchar (EID #0x01) extension
> > > > and it is better in following ways:
> > > > 1) It follows the new calling convention defined for SBI v1.0
> > > >    (or higher).
> > > > 2) It is based on a shared memory area between SBI implementation
> > > >    and supervisor-mode software so multiple characters can be
> > > >    printed using a single SBI call.
> > > >
> > > > The supervisor-mode software must set the shared memory area before
> > > > printing characters on the debug console. Also, all HARTs share the
> > > > same shared memory area so only one HART needs to set it at boot-time.
> > > >
> > > > Function: Set Console Area (FID #0)
> > > > -----------------------------------
> > > >
> > > > struct sbiret sbi_debug_console_set_area(unsigned long addr_div_by_4,
> > > >                                          unsigned long size)
> > > >
> > > > Set the shared memory area specified by `addr_div_by_2` and `size`
> > >
> > > typo in the "div_by_2" (not 4 like below and in the function itself) ?
> > >
> > >
> > > > parameters. The `addr_div_by_4` parameter is base address of the
> > > > shared memory area right shifted by 2 whereas `size` parameter is
> > > > the size of shared memory area in bytes.
> > > >
> > > > The shared memory area should be normal cacheable memory for the
> > > > supervisor-mode software. Also, the shared memory area is global
> > > > across all HARTs so SBI implementation must ensure atomicity in
> > > > setting the shared memory area.
> > > >
> > > > Errors:
> > > > SBI_SUCCESS                - Shared memory area set successfully.
> > > > SBI_ERR_INVALID_ADDRESS - The shared memory area pointed by
> > > >                           `addr_div_by_2` and `size` parameters
> > > >                           is not normal cacheable memory or not
> > > >                           accessible to supervisor-mode software.
> > > >
> > > > Function: Console Puts (FID #1)
> > > > -------------------------------
> > > >
> > > > struct sbiret sbi_debug_console_puts(unsigned long area_offset,
> > > >                                      unsigned long num_chars)
> > > >
> > > > Print the string specified by `area_offset` and `num_chars` on
> > > > the debug console. The `area_offset` parameter is the start of
> > > > string in the shard memory area whereas `num_chars` parameter
> > > > is the number of characters (or bytes) in the string.
> > > >
> > > > This is a blocking SBI call and will only return after printing
> > > > all characters of the string.
> > > >
> > > > Errors:
> > > > SBI_SUCCESS                - Characters printed successfully.
> > > > SBI_ERR_INVALID_ADDRESS    - The start of the string (i.e.
> > > >                           `area_offset`) or end of the string
> > > >                           (i.e. `area_offset + num_chars`) is
> > > >                           outside shared memory area.
> > >
> > > This will vastly reduce the number of needed ecalls when outputting
> > > characters, so this will probably improve performance quite a bit :-)
> > >
> > >
> > > I guess I still would like to have an _additional_ single-character
> > > putc call. As mentioned in the other thread [0], especially on consumer
> > > hardware [where there is no elaborate debug infrastructure] this can
> > > be a very handy debugging tool even in the earliest stages of a
> > > booting kernel (both before relocation and even inside the startup
> > > assembly).
> > >
> > > I.e. just doing a
> > >         li a7, 1
> > >         li a6, 0
> > >         li a0, 36
> > >         ecall
> > >
> > > in any kernel assembly will just output a "$" character right now, without
> > > needing any preparation at all - same with using the current
> > > sbi_console_putchar() directly in c-code.
> > >
> > > This _can_ be very helpful in some cases, so I guess it would be nice
> > > to keep such a functionality around also in the new spec.
> > 
> > You can easily create multiple pre-populated strings using ".asciiz" in
> > assembly sources. Just set the base address of pre-populated strings
> > once on boot hart and print from anywhere using usual 4-5 instruction
> > (similar to what posted above).
> 
> ok, sounds like a plan as well :-)

though, how does that relate to the time before MMU setup?

I.e. in response to Heinrich's mail you talk about svpbmt, so I guess you
expect virtual memory there, so what is the expected value-type before
the mmu is setup in S-mode ?


> > > [0] http://lists.infradead.org/pipermail/opensbi/2022-June/002796.html
> > >
> > >
> > >
> > 
> > Regards,
> > Anup
> > 
> 
>