Re: [PATCH V2 0/3] acpi: Add machine option to disable SPCR table

[Li Chen]

Hi Bibo,

 ---- On Wed, 23 Apr 2025 16:49:09 +0800  bibo mao <maobibo@loongson.cn> wrote 
--- 
 > 
 > 
 > On 2025/4/23 下午4:36, Li Chen wrote:
 > > Hi Bibo,
 > > 
 > >   ---- On Wed, 23 Apr 2025 14:27:36 +0800  bibo mao <maobibo@loongson.cn> 
 > > wrote ---
 > >   >
 > >   >
 > >   > On 2025/4/22 下午8:03, Li Chen wrote:
 > >   > > This series introduces a new machine option, spcr=on|off, allowing 
 > > users
 > >   > > to disable the ACPI SPCR (Serial Port Console Redirection) table.
 > >   > > By default, SPCR is enabled. Disabling it can help ensure that the 
 > > guest > console behavior is determined solely by kernel command-line 
 > > parameters
 > >   > Hi Li,
 > >   >
 > >   > SPCR only provides serial port HW description information.
 > >   >
 > >   > However how to use is determined by Linux kernel, Can you describe the
 > >   > detailed scenario which is unintended serial console configurations
 > >   > imposed by firmware?
 > > 
 > > Yes, it is decided by the kernel, more specifically, by how 
 > > architecture-specific code utilizes acpi_parse_spcr. For example, in the 
 > > 5.10 arm64 kernel
 > > https://elixir.bootlin.com/linux/v5.10.236/source/arch/arm64/kernel/acpi.c#L236,
 > >   it passes true to acpi_parse_spcr's param enable_console.
 > > 
 > > So, SPCR consoles are respected by Arm64 Linux by default. Therefore, even 
 > > without a console=ttyAMA0 configuration, ttyAMA0 is still added as a 
 > > preferred console
 > > and used by printk.
 > About serial port, there is ACPI SPCR table and DSDT table. About how to 
 > use, it is decided by kernel, so I think that it probably kernel usage 
 > issue.
 > 
 > However I have no opposing view with your patch, adding option 
 > "spcr=on|off" sounds good.


Thanks, I totally agree with you. BTW, newer kernels introduce the nospcr 
command-line argument, allowing users to control if respect SPCR or not:
https://elixir.bootlin.com/linux/v6.15-rc3/source/arch/arm64/kernel/acpi.c#L254

Regards,
Li