Re: SBI Debug Console Extension Proposal (Draft v1)


Anup Patel
 

On Thu, Jun 2, 2022 at 12:02 AM Heiko Stübner <heiko@...> 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).



Thanks
Heiko


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


Regards,
Anup

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