[PATCH v1 0/1] [RISC-V] replace SBI early console with 8250 UART

[Zhang Zheng <zhangzheng@xxxxxxxxxxx>]

The legacy SBI console extension is planned for deprecation. Replace the
RISC-V early printk backend with direct access to 8250 UART MMIO.

This also addresses truncated output observed on SpacemiT K3 when
switching from early printk to dtuart. Direct UART access lets early
printk flush the UART after each string before the dtuart driver takes
over and clear it's FIFO, which cause the truncated uart output.

The truncated uart looks like:

  (XEN) Command line: dtuart=/soc/uart@d4017000:115200/921600 console=dtuart 
sync_console
  (XEN) Domain heap initialised
  (XEN) Booting using Device Tree
  (XEN) /home/zz/dev/git/xen_stuff/xen/xen/arch/riscv/cpufeature.c:710: SSTC 
detected; supported for Xen use, but not for guests
  (XEN) Looking for dtuart at "/soc/uart@d40 __  __            _  _    ____  
_____                    _        _     _
   \ \/ /___ _ __   | || |  |___ \|___ /    _   _ _ __  ___| |_ __ _| |__ | | 
___
    \  // _ \ '_ \  | || |_   __) | |_ \ __| | | | '_ \/ __| __/ _` | '_ \| |/ 
_ \
    /  \  __/ | | | |__   _| / __/ ___) |__| |_| | | | \__ \ || (_| | |_) | |  
__/
   /_/\_\___|_| |_|    |_|(_)_____|____/    \__,_|_| 
|_|___/\__\__,_|_.__/|_|\___|

  (XEN) Xen version 4.23-unstable (zz@) (riscv64-linux-gnu-gcc (GCC) 15.1.0) 
debug=y ubsan=y Sat Sep 26 14:50:31 CST 2026
  (XEN) Latest ChangeSet: Fri Sep 18 10:11:27 2026 +0800 git:77544a69ea

Zheng Zhang (1):
  xen/riscv: replace SBI early console with 8250 UAR

 xen/arch/riscv/Kconfig.debug              | 53 ++++++++++++++++++++--
 xen/arch/riscv/configs/tiny64_defconfig   |  2 +
 xen/arch/riscv/early_printk.c             | 37 +++++++---------
 xen/arch/riscv/include/asm/early_printk.h | 12 +++++
 xen/arch/riscv/include/asm/fixmap.h       |  6 ++-
 xen/arch/riscv/include/asm/sbi.h          |  7 ---
 xen/arch/riscv/mm.c                       | 41 +++++++++++++----
 xen/arch/riscv/riscv64/Makefile           |  1 +
 xen/arch/riscv/riscv64/debug-8250.inc     | 51 +++++++++++++++++++++
 xen/arch/riscv/riscv64/debug.S            | 54 +++++++++++++++++++++++
 xen/arch/riscv/sbi.c                      |  5 ---
 11 files changed, 223 insertions(+), 46 deletions(-)
 create mode 100644 xen/arch/riscv/riscv64/debug-8250.inc
 create mode 100644 xen/arch/riscv/riscv64/debug.S

base-commit: 4c1aba82cff95ab11f7fe4af8965a06703537288
-- 
2.54.0