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[riscv] Add support for writing prefix debug messages direct to a UART

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Some platforms (such as the Sipeed Lichee Pi 4A) do not provide a
functional SBI debug console.  We can obtain early debug messages on
these systems by writing directly to the UART used by the vendor
firmware.

There is no viable way to parse the UART address from the device tree,
since the prefix debug messages occur extremely early, before the C
runtime environment is available and therefore before any information
has been parsed from the device tree.  The early UART model and
register addresses must be configured by editing config/serial.h if
needed.  (This is an acceptable limitation, since prefix debugging is
an extremely specialised use case.)

Signed-off-by: Michael Brown <mcb30@ipxe.org>
This commit is contained in:
Michael Brown
2025-05-27 14:49:06 +01:00
parent 2e8d45aeef
commit 98fdfdd255
2 changed files with 84 additions and 0 deletions
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79
src/arch/riscv/prefix/libprefix.S
View File

@@ -29,6 +29,8 @@
*
*/
#include <config/serial.h>
.section ".note.GNU-stack", "", @progbits
.text
@@ -110,6 +112,81 @@ prefix_virt:
ecall
.endm
/*****************************************************************************
*
* Print character via early UART
*
*****************************************************************************
*
* Print a single character via a UART.
*
* For devices without a functional SBI console, a UART at a hardcoded
* address can be used as a last resort mechanism for obtaining debug
* output from the prefix.
*
* Parameters:
*
* a0 - Character to print
*
* Returns:
*
* a0 - Preserved
* a1 - May be overwritten
* a6 - May be overwritten
* a7 - May be overwritten
*
*/
/* Default to no UART, if not specified */
#ifndef EARLY_UART_MODEL
#define EARLY_UART_MODEL none
#endif
/* Default to a register shift of zero, if not specified */
#ifndef EARLY_UART_REG_SHIFT
#define EARLY_UART_REG_SHIFT 0
#endif
#define print_char_uart _C2 ( print_char_uart_, EARLY_UART_MODEL )
/* Print character via nonexistent UART */
.macro print_char_uart_none
.endm
/*****************************************************************************
*
* Print character via 8250-compatible early UART
*
*****************************************************************************
*
* Print a single character via an 8250- or 16550-compatible UART.
*
* Parameters:
*
* a0 - Character to print
*
* Returns:
*
* a0 - Preserved
* a1 - Overwritten
* a7 - Overwritten
*
*/
/* 8250-compatible UART transmit registers */
#define EARLY_UART_8250_TX ( 0 << EARLY_UART_REG_SHIFT )
#define EARLY_UART_8250_LSR ( 5 << EARLY_UART_REG_SHIFT )
#define EARLY_UART_8250_LSR_THRE 0x20
.macro print_char_uart_8250
li a7, EARLY_UART_REG_BASE
sb a0, EARLY_UART_8250_TX(a7)
uart_wait_\@:
lbu a1, EARLY_UART_8250_LSR(a7)
andi a1, a1, EARLY_UART_8250_LSR_THRE
beqz a1, uart_wait_\@
.endm
/*****************************************************************************
*
* Print message to debug console
@@ -159,6 +236,7 @@ print_message_alt:
lbu a0, (t1)
addi t1, t1, 1
beqz a0, 2f
print_char_uart
print_char_dbcn
beqz a0, 1b
lbu a0, -1(t1)
@@ -250,6 +328,7 @@ print_hex_value_alt:
1: /* Print each digit in turn */
srli a0, t1, ( __riscv_xlen - 4 )
nibble_to_ascii a0
print_char_uart
print_char_dbcn
beqz a0, 2f
srli a0, t1, ( __riscv_xlen - 4 )