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

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 9/28/26 4:38 AM, Zhang Zheng wrote:
> The legacy SBI console extension is planned for deprecation. Replace the
> RISC-V early printk backend with direct access to 8250 UART MMIO.
Won't it better just to start to add support of newer SBI? It provides 
pretty good support + faster as it supports passing of string instead of 
one char.
Also, I think it will lead to lesser changes.

~ Oleksii

> 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