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SubjectRe: [PATCH 0/2] acpi, x86: Add SPCR table support
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On 12/8/2017 7:29 AM, Prarit Bhargava wrote:
>
>
> 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.

"Serial Port Console Debug" sounds like DBG2 to me, although I am unsure
of the specific platform you are referencing.

>
>>
>> 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.
>

SPCR is always present on QDF2400 (ARM64 platform). Firmware will
automatically update the SPCR table to point to the correct console (IE
Serial-over-LAN or SOL if configured via IPMI). Unless a user
specifically chooses to override the SPCR via "console=", we expect that
a console will be present based on the data in SPCR.

--
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.

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