Re: [PATCH 0/2] acpi, x86: Add SPCR table support

From: Prarit Bhargava <prarit@redhat.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: linux-acpi@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-pm@vger.kernel.org,
	linux-serial@vger.kernel.org,
	Bhupesh Sharma <bhsharma@redhat.com>,
	Lv Zheng <lv.zheng@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	x86@kernel.org, Jonathan Corbet <corbet@lwn.net>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>
Subject: Re: [PATCH 0/2] acpi, x86: Add SPCR table support
Date: Fri, 8 Dec 2017 09:29:54 -0500	[thread overview]
Message-ID: <f078ada7-1e30-41cc-28dc-077ed2a5986d@redhat.com> (raw)
In-Reply-To: <20171208062925.64ov23u7oxmdfzyk@gmail.com>

On 12/08/2017 01:29 AM, Ingo Molnar wrote:
> 
> * Prarit Bhargava <prarit@redhat.com> wrote:
> 
>> The SPCR (Serial Port Console Redirection) Table provides information
>> about the configuration of serial port.  This information can be used
>> to configure the early console.
> 
> s/about the configuration of serial port
>  /about the configuration of the serial port
> 
>> SPCR support was added for arm64 and is made available across all arches
>> in this patchset.  The first patch adds a weak per-arch configuration function
>> and moves the SPCR code into ACPI.  The second patch adds support to x86.
>>
>> The existing behaviour on arm64 is maintained.  If the SPCR exists the
>> earlycon and console are automatically configured.
> 
> s/arm64
>  /ARM64
> 
> which is easier to read and it's also the prevalent spelling:
> 
>  triton:~/tip> for N in $(git grep -ih arm64 arch/arm64/ | sed 's/[[:punct:]]/ /g'); do echo $N | grep -iw arm64; done | sort | uniq -c
>      412 arm64
>        1 Arm64
>      854 ARM64
> 
>> The existing default behaviour on x86 is also maintained.  If no console or
>> earlycon parameter is defined and the SPCR exists , the serial port is not
>> configured.  If the earlycon parameter is used both the early console
>> and the console are configured using the data from the SPCR.
> 
> s/exists , the
>  /exists, the
> 
> But, the logic to not use the SPCR looks confusing to me.
> 
> The SPCR is only present if the user has explicitly configured a serial console 
> for that machine, either in the firmware, or remotely via IPMI, correct? I.e. SPCR 
> will not be spuriously present by default on systems that have a serial console 
> but the user never expressed any interest for them, right?

If I disable "Serial Port Console Debug" in my BIOS I still see the SPCR configured:

[root@prarit-lab ~]# dmesg | grep SPCR
[    0.000000] ACPI: SPCR 0x0000000069031000 000050 (v01
00000000      00000000)

AFAICT the SPCR is always enabled on some systems.

> 
> If so then we should pick up that serial console configuration and activate it, 
> regardless of any kernel boot options!

I'm worried about someone who doesn't want a console on ttyS0 suddenly ending up
with one.  The SPCR could contain incorrect data, etc.

I originally wanted this on by default, but the chances of breaking someone's
setup seems significant doesn't it?  Maybe I'm being too cautious.  Anyone else
want to weigh in?  I'm not ignoring your idea Ingo, I'm just worried about being
yelled at by a user :) because I broke their default console setup.

P.