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

[Jeffrey Hugo]

On 12/8/2017 7:29 AM, Prarit Bhargava wrote:
> On 12/08/2017 01:29 AM, Ingo Molnar wrote:
> > * Prarit Bhargava <prarit@xxxxxxxxxx> 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.