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

[Zhang Zheng <zhangzheng@iscas.ac.cn>]

On Mon, Sep 28, 2026 at 09:38:14AM +0200, Oleksii Kurochko wrote:
> 
> 
> 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.
> 

Using the newer SBI DBCN extension would indeed result in a smaller change. At
the time, I mainly followed the direction suggested by the TODO and referred
directly to the arm64 implementation, from which most of the code was adapted.

> Also, I think it will lead to lesser changes.
> 

If falling back to SBI console putchar is acceptable, DBCN should work
well here.

> ~ 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