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

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 9/28/26 10:40 AM, Zhang Zheng wrote:
> 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.
I think TODO was written at that way because I missed DBCN (or it wasn't 
ratified at that moment). Considering that it is ratified now ...
> > Also, I think it will lead to lesser changes.
> If falling back to SBI console putchar is acceptable, DBCN should work
> well here.
... I prefer to use SBI functionality (in the current case DBCN) as 
maximum as possible so if there is no any reason (and I don't see at the 
moment it) which doesn't allow us to use DBCN here, I would prefer to 
use DBCN.
If you still want to in the way suggested by TODO then some parts which 
are common between Arm and RISC-V should be moved to common instead of 
introducing them in RISC-V specific folder.
Let me know which approach you are going to follow. If the current one 
you already introduced then I will start to review it, if you will go 
SBI-way then I prefer to wait for your v2.
~ 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