SBI Debug Console Extension Proposal (Draft v4)

Anup Patel

Hi All,

Based on feedback, below is the updated draft proposal of the
SBI Debug Console Extension ...

The motivations behind this proposal is as follows:
1) There is no new SBI extension replacing the legacy SBI v0.1
putchar() and getchar() calls. These legacy SBI v0.1 calls are
now disabled by default in Linux and will be eventually deprecated.
We need a replacement at least for the SBI v0.1 putchar() which
is widely used for early prints in various OSes and Bootloaders.
2) The OS-A Platforms (or SEE) specification need to mandate
a particular type of UART (8250, ns16550, or SiFive) as a standard
way for boot-time early prints in supervisor-mode software. Using
SBI debug console extension, the OS-A Platforms (or SEE)
specifications don't need to mandate any type of UART.
3) The legacy SBI v0.1 putchar() only supports printing one
character at a time. For some of the hypervisors (particularly KVM),
SBI based early prints (i.e. earlycon=sbi) is slow because each
SBI v0.1 putchar() trap will be forwarded to the KVM user-space

In other words, the SBI debug console extension defines a standard
way for boot-time early prints in supervisor-mode software.

We have completed the proof-of-concept implementation of this
SBI extension proposal. Refer, "riscv_sbi_dbcn_v1" branch in the
following repos:

Please review it and provide your feedback.


SBI 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 the legacy console putchar (EID #0x01)
extension with following improvements:
1) It follows the new calling convention defined by the SBI v1.0
(or higher) specification.
2) It allows supervisor software to print multiple characters
in a single SBI call.

Some of the functions defined by this SBI extension have physical
memory as parameters for input and/or output. This physical memory
parameter MUST satisfy following requirements:
1) The SBI implementation MUST check that the supervisor-mode
software is allowed to read and write the specified physical
memory on the calling HART.
2) The SBI implementation MUST access the specified physical
memory using the PMA attribute.
Note: If the supervisor-mode software access the same physical
memory using memory type different from PMA then a loss of
coherence or unexpected memory ordering may occur and the
invoking software should follow the rules and sequences defined
in the RISC-V Svpbmt specification to prevent the loss of
coherence and memory ordering.

If the underlying physical console has extra bits for error checking
(or correction) then these extra bits should be handled by the SBI

Function: Console Puts (FID #0)

struct sbiret sbi_debug_console_puts(unsigned long num_chars,
uint64_t base_addr)

Print a specified input string on the debug console.

The `num_chars` parameter specifies the number of characters (or
bytes) in the input string whereas `base_addr` parameter specifies
the physical base address of the input string.

This is a blocking SBI call and will only return after printing all
characters of the input string.

SBI_SUCCESS - Characters printed successfully.
SBI_ERR_INVALID_ADDRESS - The string pointed by `num_chars` and `base_addr`
parameters is not entirely accessible to
SBI_ERR_FAILED - Failed to print characters due to I/O errors.

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