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

[Oleksii Kurochko <oleksii.kurochko@gmail.com>]

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
>