SBI Debug Console Extension Proposal (Draft v1)

[Heiko Stübner]

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.


Thanks
Heiko


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