linux-kernel - Re: [PATCH] ACPI: Do not enable ACPI SPCR console by default on arm64

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Date: Fri, 21 Jun 2024 12:37:50 +0800
From: Liu Wei <liuwei09@...tc.cn>
To: prarit@...hat.com,
	Liu Wei <liuwei09@...tc.cn>,
	sudeep.holla@....com,
	Will Deacon <will@...nel.org>
Cc: catalin.marinas@....com,
	guohanjun@...wei.com,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	lpieralisi@...nel.org,
	rafael@...nel.org
Subject: Re: [PATCH] ACPI: Do not enable ACPI SPCR console by default on arm64

From: Prarit Bhargava <prarit@...hat.com>

> On 6/18/24 23:43, Liu Wei wrote:
> > From: Sudeep Holla <sudeep.holla@....com>
> > 
> > On Tue, Jun 18, 2024 at 04:29:24PM +0100, Will Deacon wrote:
> >>> On Thu, May 30, 2024 at 09:06:17AM -0400, Prarit Bhargava wrote:
> >>>> On 5/29/24 21:53, Liu Wei wrote:
> >>>>> Consistency with x86 and loongarch, don't enable ACPI SPCR console
> >>>>> by default on arm64
> >>>>>
> >>>>> Signed-off-by: Liu Wei <liuwei09@...tc.cn>
> >>>>> ---
> >>>>>    arch/arm64/kernel/acpi.c | 3 ++-
> >>>>>    1 file changed, 2 insertions(+), 1 deletion(-)
> >>>>>
> >>>>> diff --git a/arch/arm64/kernel/acpi.c b/arch/arm64/kernel/acpi.c
> >>>>> index dba8fcec7f33..1deda3e5a0d2 100644
> >>>>> --- a/arch/arm64/kernel/acpi.c
> >>>>> +++ b/arch/arm64/kernel/acpi.c
> >>>>> @@ -227,7 +227,8 @@ void __init acpi_boot_table_init(void)
> >>>>>    		if (earlycon_acpi_spcr_enable)
> >>>>>    			early_init_dt_scan_chosen_stdout();
> >>>>>    	} else {
> >>>>> -		acpi_parse_spcr(earlycon_acpi_spcr_enable, true);
> >>>>> +		/* Do not enable ACPI SPCR console by default */
> >>>>> +		acpi_parse_spcr(earlycon_acpi_spcr_enable, false);
> >>>>>    		if (IS_ENABLED(CONFIG_ACPI_BGRT))
> >>>>>    			acpi_table_parse(ACPI_SIG_BGRT, acpi_parse_bgrt);
> >>>>>    	}
> >>>>
> >>>> It's been a while, and the status of arm hardware may have changed. IIRC the
> >>>> choice to force enable this is that most arm hardware is headless and this
> >>>> was a _required_ option for booting.
> >>>>
> >>>> I'm not sure if that's still the case as it's been a long time.
> >>>>
> >>>> Can anyone from the ARM community provide an approval here?
> >>>
> >>> I don't have a strong opinion either way, but adding the Arm ACPI folks
> >>> in case they care.
> >>>
> >>> Having said that, if the only rationale for this patch is consistency
> >>> with other architectures, then I think I'd lean towards leaving the
> >>> behaviour as-is so we don't give users a nasty surprise on their next
> >>> kernel upgrade.
> >>>
> >>
> >> +1, I am concerned about breaking existing behaviour on the platforms
> >> in the wild. Also many platforms running ACPI would have already used
> >> console cmdline parameter if SPCR is not their choice for the console.
> >> So I don't see the need to align with other arch default behaviour here,
> >> we can revisit if someone shouts with a real reason as why cmdline option
> >> is not viable.
> > 
> > For varying privacy and security reasons, sometimes we would like to
> > completely silence the serial console output, and only enable it through
> > cmdline when needed.
> > 
> 
> FWIW, I understand that concern but the feedback you're receiving is 
> that there are many existing systems that depend on this behavior. 
> Breaking users for your concerns is not a good idea.
> 
> Perhaps a solution is to add yet-another-config-option, or add a kernel 
> parameter to disable the SPCR console on ARM?
> 
> Something like "acpi=nospcr"?

Thank you for your suggestion, I will send the new version later.

Best wishes,
Liu wei
> 
> P.
> 
> > On ARM, it is difficult because SPCR is enabled by default.
> > 
> > Thanks for your patience,
> > Liu Wei
> > 
> >>
> >> --
> >> Regards,
> >> Sudeep
> >